Optimizing Air-Conditioner EER SEER Efficiencies Optimizing Evaporator Input

with Darrell Udelhoven Darrell Udelhoven >Darrell Cynergy Home HVAC Energy Raters Listen While Reading


Please Respond on my Blog; let me know if you can't post: Ballpark Checking A/C Performance

Condenser Temp-Splits Knowing Operating Static Pressures | Room T-stat Differential Selecting Correct CFM Fan Speeds | Gurgling sounds at TEV | Oil Furnace blower Graph Design Engineering | Determining which metering device TXV or Fixed Orifice | 50% Load HomeAir Leaks | R410a_Evacuation
CFM & FPM Velocity Sizing MERV 8 Pleaded Filters
1” deep M-8 filter’s rated at 175 fpm medium and 350 fpm high; A 16X20 1" MERV 8, Rated @ only 780-CFM

*HEAT PUMP DIAGNOSTICS

*Basics Featuring the Testo 556 - Video of a very thorough Air Conditioning Testing setup 2/26/11

A lot more HVAC Videos at bottom of pages
Why Look at your Ducts
(Leakage up-to 33% of cooling cost) when replacing your AC System; a must viewing:
http://www.youtube.com/watch?v=IV0Rwv5gco4

California Research Report on EER SEER  pdf - download 07/23/08, SEER Payback - cannot be accurately represented!
HVAC-TALK General Discussions: Importance of Two-Way Communications with HVAC Customers
    - with Darrell Udelhoven - HVAC RETIRED - udarrell

Always begin with a thorough Home Energy AUDIT that shows all the options for lowering the heat-gain & heat-loss, then after reducing air infiltration, etc., have a room by room manual J heatload calc performed. A reduction in equipment sizing will usually greatly improve the duct system performance.

This is where the greatest savings in both heating & cooling will accrue; this will help in the down-sizing of equipment.      

"The 'proper heatload' on the evaporator coil must be established"
Check and thoroughly seal all the ductwork! For efficient operation, always check the return air temperature at the blower & at the Return Air Grille(s) to know whether it is drawing hot air from the attic or garage areas.

What I stated above, that ought to be done is far more important than SEER rating; as that will determine the SEER achieved & the energy savings.

Then do a manual J room by room heat-gain calc with the option shown so you can do everything possible to reduce all sources of heat-gain & heat-loss, greatly reducing both heat & cooling BTUH equipment sizing.

Before you do anything else, educate yourself enough to "ensure that you request the proper things be done in the proper order of sequence." Checking ductwork & Airflow Checking Static Pressures  is Critically Important. As is knowing the operating feet per minute (FPM) velocity, the CFM & BTUH to each room along with the Total CFM airflow & BTUH.


First, before doing anything else check and thoroughly seal all the ductwork! For efficient operation, always check the return air temperature at the blower & at the Return Air Grille(s)

INTRODUCTION TO TOTAL COOLING PERFORMANCE;
How improper installation of a 3-ton system can become a 1.5-ton system of actual delivered cooling, or heat transfer to the outdoors! (SURPRISE!)

The second thing you must do & know "before charging any system" is to establish the proper airflow and heatload through the evaporator coil.
It is impossible to accurately charge a Cap-Tube, Piston-Flow-Rator, or (TXV) Thermostatic Expansion Valve refrigerant fed evaporator coil, without "an adequate an evenly
distributed heat-load passing through all the coil's circuits."

Some liquid refrigerant must be supplied throughout the total length of the evaporator coils along with an adequate  heat-load to vaporize it in order to achieve the coils full
BTU/hr capacity. If an insufficient heat-load is supplied a TXV will shut down the flow of refrigerant, a Piston-Flow-Rator will tend to overload the coil with liquid, with
the potential of compressor slugging.

You must know & record the operating feet per minute (FPM) velocity & the CFM to each room & the Total CFM airflow!

You know the scenario: an oversized unit and high humidity with room TH set at 68 or 70-F degrees (clammy cold) too large a piston metering device matched to a
48,000-BTU/hr evaporator coil, --only  heat evaporation/absorbing 26,000-btu/hr. (Check the condenser Temp/split.)

Simple easy anyone can do ways to check the performance of your central air conditioner so, if needed, you can call an Energy Efficiency HVAC Technician

If U want me to run a ballpark analysis of how your system is performing in respect to its 'Nominal Rated Btuh' we need at least the following numbers:

Performance Data Collection – Best Time to collect data is Late afternoon around 4:30 pm, when attic is HOT; also when outdoor temps are around 85; 95; 105F or, anywhere in between.

 *All U need is a good thermometer (digital reading in tenths preferable) & and indoor Humidity Gauge

1) Helpful; Tonnage & SEER of Unit & outdoor condenser model number: __________________

2) TXV or, orifice metering device? _______. Only if U know…

3) Outdoor condenser’s discharge-air-temperature ______-F

Subtract Outdoor air temperature: _______

Outdoor Condenser Air-Temp-Split _______ 

4) Need the ‘Indoor’ percent of relative humidity - away from Supply-Air outlets ______

5) Indoor Return-Air Temperature ______

Subtract Indoor Supply-Air Temperature ______ -F

 Indoor temperature-split _______-F 

Need the above information for troubleshooting & performance analysis.

Example below:

A Goodman 2-Ton 13-SEER condenser, 800-cfm indoor airflow; 80-F indoor dry bulb & 50% relative humidity; Indoor temp-split 18 to 19-F.

@ 85-F outdoors; 103.9-F - 85-F outdoors or around an 18.9-F temp-split; 

@ Indoor 75-F & 50% RH condenser temp-split is only around 14.9-F.

In summer an all electric farm home TWO Half-Ton Window A/Cs & basement large dehumidifier:

June 30, 2012; Darrell’s meter; 62610 – 62140= used 470-KWh * .0985= $46.295
================================

========================== ===============
A Major Oil Furnace Airflow Problem Fix
Regal & Hallmark & nearly all Oil Furnaces - Installation manuals
http://www.boyertownfurnace.com/ProductDocuments/index.aspx
Download the installation & service manuals from ABOVE LINK BELOW LINK MAY NOT WORK!
http://www.boyertownfurnace.com/ProductDocuments/HallmarkONLYManual042909.pdf 
To find the information below; Use within the pdf search:  at least 6” above
Or use down arrow to P-8 & scroll down a-ways...

"If the oil furnace is used in connection with summer air conditioning the evaporator coil must be installed at least 6” above the oil furnace
for proper airflow. Distances less than 6” will result in decreased airflow." 
Make sure outlet supply takeoffs are NOT blocked by the coil. In all cases, refer to the manufacturers’ data for static pressure losses to
ensure the total system static pressure does not exceed 0
.5” WC.
===================================================
Air Conditioning Performance Diagnosis using listed (CT) Condensing Temperatures
Using Goodman 16-SEER "Expanded Performance Data"

  Find the correct (CT) Condensing Temperature with the following known mfg’ers data.

Outdoor Ambient Temp (OAT) 85-F; IDB 75-F; IWB 63-F or 50%-RH.
Listed pressure is 316-psig, or 99-F CT; that is 99-F -85-F is a 14-F SPLIT.

The delta T or temp-split should be within a 10-psig range or, +/- 2-F degrees; 97 or 101-F.

The mfg’ers Supply Outlet should be able to provide Contractors & Techs with those performance data charts. Goodman has their “Expanded Performance Data” on the Internet.

BTUH = CFM *X's 4.5 @ sea level or 4.35 @ 1000-feet *X's RA-SA wet bulb enthalpy difference from enthalpy Chart <-Click

*** The above will ballpark the operating delivered BTUH of your system. Other of my pages show you how to get the CFM.

-------------------------
Around 2005, my brother had a 12-SEER 1.5-Ton Heil condenser installed with a 2-Ton evaporator with a TXV metering device, it was erroneously installed directly on top of the Thermo Pride OL11 Oil furnace.

I checked the airflow with a low cost anemometer, it was below 300-cfm, I also checked the subcooling & it was only 1-F, indicating low on refrigerant.
My electronic leak detector says there is a leak in the evaporator area. With my new digital pocket thermometer I checked the supply & Return wet bulb temps & found the enthalpy change on my chart to get the extremely low BTUH performance of his A/C unit.

Dons 1.5 Ton Heil AC Data Chart Condenser is ejecting HALF the heat of its Nominal Rating; evap-coil way under heatloaded!

The required  FIX: Pump much of the R-22 down into the condenser, it's a Scroll compressor, then close service valves & recover the R-22 from the lines & E-Coil, remove coil & repair leak, reinstall at least 6" above the furnace preferably on a transition rather than rails. Purge with nitrogen then flow dry nitrogen at under 3-PSIG, then evacuate to 500-microns & see if it holds under 1000-microns.

Replace quarter HP belt drive blower motor with a third HP motor. Check airflow & adjust RPMs, in his situation, until 700-CFM is achieved.


Open service valves & charge until subcooling reaches around 12-F, look up target subcooling for that unit. Also, check superheat to see if TXV is holding within its requisite perimeters. Then recheck its BTUH performance output compared to company charts at those conditions.
-------------------------------

Especially if your system is oversized or there are a lot of low AC load days use an adjustable differential room TH.
TH Differential
: Some have cycles per hour settings, however...

Especially if your system is oversized or there are a lot of lighter AC load days, we need an adjustable differential room t stat.

With the addition of fans to move air when system is off; this is what I want in a room t stat.

TH Differential: Differential is defined as the difference between the cut-in and cut-out points as measured at the thermostat. I would want one with at least half degree increments,
up to at least 3-F differential.

For example, if the thermostat turns the Cooling Equipment ON at 78-F & OFF at 75-F or a 3 degree differential setting; heating mode, on at 70 degrees F
and turns the heating equipment off at 74 degrees F, or 4 degrees F differential, and also uses an adjustable heat anticipator. 

Some have half degree increment settings over several degrees of differential spread.

Cooling anticipators use to be fixed settings by mfg'ers, heating anticipators were adjustable.

For example, in Rockford, IL a 2,400 sq./ft home with 600-sq./ft of window area, those using wrong headed 600
-sq./ft per-ton, it figures to take 4 tons or so to cool it. However, a 2-Ton Unit moving 1,000-CFM of air (or 500-cfm per ton of cooling), even at 95 degrees with a blazing sun heat outside and very high humidity the 2-ton cooling equipment system still cycles! It is very comfortable at around 75-F and 50% Relative Humidity or less. 

On mild days with a high humidity an adjustable differential t-stat would be very helpful. The comfort zone at lower humidities with adequate air movement covers a wide range of temps.

At 50% Relative Humidity the Human Comfort Zone is max 79-F to min 73-F, or a 6 degree differential. That would allow an oversized unit to produce longer cycles higher SEER performance & get humidity in a comfort zone.

There's a 2400 sq./ft home in Lancaster (SW WI) and one I know of in Ohio, cooling the homes to very comfortable levels using 2-ton A/C systems.

---------------------------------------

The most common cause of low suction pressure and low btu/hr output is an insufficient heat-load on the evaporator coil. Before you do anything else, always make certain that at normal room TH settings, an optimal heat-load is going through the evaporator coil, and also that the coil and blower wheel blades are clean. (Check Condenser coils and fins too.)

The evaporator coil works the opposite of the condenser, here refrigerant liquid is converted to gas, absorbing heat from the air in the process. 

When the liquid refrigerant reaches the evaporator its pressure has been reduced, dissipating its heat content and making it much cooler than the fan air flowing around it. This causes the refrigerant to absorb heat from the warm air and reach its low boiling point rapidly. The refrigerant then vaporizes, absorbing the maximum amount of heat.
---------------------------
An Affordable Test Instrument You Need!
All I had was the Sling Psychrometer & spinning it was a bit time consuming, but I used it religiously, it is information you need. 

The Testo 605-H2 Humidity Stick (wet bulb), displays relative humidity, air temperature and wet bulb temperature.

It is very affordable & because of its potential to help deliver tons of other data everyone should have one!

For more information on it:
http://www.amazon.com/Testo-605-H2-H.../dp/B000774B6A

The other test data you need is the system's CFM airflow through the evaporator coil, then with software I have you can peg the BTUH the operating unit is delivering under those conditions.
Add to that a low cost Magnehelic gauge to read static pressures to compare with mfg'ers blower performance charts; plus a velocity meter & you have a ballparked CFM to plug into for the BTUH.

We could easily provide a detailed psychrometric print out of exactly what the operating system is delivering including condensate lbs/hr, & actual sensible & latent cooling BTUH & Ratio, every data detail imaginable.

Think about what that would mean to you & those you serve. - Darrell

--------------------------------------------
Determining which metering device TXV or Fixed Orifice the system has without physically looking

If you do not absolutely know whether the metering device is a TXV, or a fixed orifice device or cap tube. 

Hook up your manifold gauges, block off considerable condenser air intake for a short time.
If the suction pressure starts rising, you have a piston, or a cap tube.
If only the high side goes up, you have a TXV.

Have things with you in your van or truck to block-off the condenser air for a short time.
Check every time you are not certain what metering device it has.
There will be a lot of guessing in the future.

Do this procedure on known metering devices to observe the difference.
Report back to me how well it works for you.

In some situations, that could save you from cutting a hole in the plenum.

Squirrel cage wheels with forward curved blades on residential systems
unload when discharge air is blocked off too much & will overload
when there is no static pressure.
 
There is a preferable ESP range for each Air Handler blower design, that ought to be listed on the blower; they vary at the point of serious unloading.
 If you amp-probe check enough of those blower motors, if the amp draw is too low according to its rating, you can begin to tell that the External Static Pressures (ESP) is too high.
Additionally, mfg'ers could list the amp draw at various design ESP numbers, then we could amp-probe & know if it was too far above the amp rating, a duct maybe off,
if amp reading is too low, it is time to check all static pressures & delivered CFM to each room.

I lot of us used to set a nearly empty R-22 cylinder on top of a condenser to warm it a little. Back then fan motors had more HP
& higher amp draws, therefore it didn't seem to cause any harm, just more noise.

Back in the 1960's & 1970's there were a far number of TXV metering devices & some table top condensers' that had the fan underneath blowing up through the coils.
Well, where there were cottonwood trees, nearby clothes dryer lint vents, or a lot of leaves or other debris under the unit, the fan motors would be blocked overload & burnout.

I don't understand the engineering genius of that moronic design.

However, on hot days & a heat-loaded E-Coil,
You could move your wrist over the condenser from outlet up to inlet, & tell if the liquid was taking up too much area of the coils; an overcharged system. - udarrell
--------------------
Always get the CFM airflow correct, first, if it is a piston or cap tube, use the superheat method to charge it.
If it is a TXV, subcooling is the way to charge it, but check the Superheat to verify the TXV is holding within its known specs
====================
Quick Check for Sizing Units to enough Airflow
Actually, even on service calls where there are cooling problems the ductwork should have a quick Manual D performed.

Then take the ESP static pressure & compare to blower graph or chart, also take the FPM duct velocity.

Then do a quick estimate of airflow per equipment tonnage.

To find area of a round duct; Duct diam is 7"; 7"X7"= 49-sq.ins., X's .7854 = 38.04845-sq.ins divided/ by 144= 0.2672541-sq.ft. area X's FPM Velocity 600-FPM = 160.35246-CFM X30 =
4,810.5738 each 7" run X's 6 branch runs = 28,863-BTUH, or airflow for 2.4-ton.

(12,000-BTUH /400-cfm per-ton = 30-BTU per cfm ratio | / 450 = 26.666-BTUH per-cfm)

That would also be good for 2-ton; at 550-FPM velocity X's 0.2672541= 147-CFM X 30 = 4,410-BTUH each run X 6-runs = airflow for 26,460-BTUH.

*Never sell units requiring more airflow than the duct system will support! - Darrell
=======================

Selecting Correct CFM Fan Speeds


An adequate CFM of warm enough air is essential to the evaporation process in the coil. The amount of heat exchanged depends upon a temperature differential of the air and the refrigerant. An optimally high air temperature and Cubic Feet per-Minute (CFM) airflow through the cooling coil will allow rapid heat transfer between the air and the refrigerant --resulting in more rapid boiling and more total heat absorption per-minute (more BTU/hr) of run-time. 

An unbalanced airflow through the evaporator coil (DX coil) circuits can cause a large reduction in heat absorption capacity. The non heat loaded vapor or ultra cold liquid will cause a TEV to shut down the flow of refrigerant to the coil. Superheat charging will be inaccurate when the coil is fed by flow rator pistons. Total BTUH capacity could drop 15 to 30% or more. An 18000-BTUH unit losing 30% of design capacity would be delivering only 12600-BTUH.

Design Engineering and Installation Objectives should be focused towards achieving the most efficient and effective means toward  a conditioned space that is within the "Human Comfort Zone, and within an affordable investment 'payback' period." 

Summer Comfort Zone

Relative Humidity

Maximum Comfortable Temperature

Minimum Comfortable Temperature

60%

78.5oF

 72.5oF

50%

79o

73oF

40%

79.5oF

  73.5oF

30%

80oF

74oF


The above comfort zone was found to be acceptable to 90% of test subjects drawn from a range of age groups and genders, with work and life-styles involving varying levels of activity and clothing. An air conditioning system that establishes and maintains indoor conditions within this zone will provide thermal comfort. It will produce a neutral sensation, occupants will feel neither too hot nor too cold. Above chart and findings From: Home Energy Magazine Online September/October 1996) Sizing Air Conditioners: If Bigger Is Not Better, What Is?  by John Proctor and Peggy Albright  Toward Optimal Occupant Comfort 

If you over pay for over capacity equipment, --you will be paying more every month and will not be as comfortable as you would sizing it right to also achieve the appropriate humidity levels!

SEER
7 SEER or less
8 SEER
9 SEER
10 SEER
12 SEER
13 SEER
'Max' condenser air temp  delta-T
30
25
24
+23
21
<less
Max temp spread 'across' E-Coil
20
22
24
26
>more
>more
'Max' SA/Return Entering Air Delta-T
33
30
26
23
19
<less
The Supply Air & the Entering Return Air delta-T, (< less than, symbol)- tends towards less & less as the SEER goes higher,
therefore, dehumidification could become more difficult at the highest SEER levels. The EER & SEER levels widen, as SEER sky rockets.

====================
My Scan of My ThermoPride OL 11 "Oil Furnace" Graphed Blower-Curve-Chart (Same as my brother's Oil furnace)
Thermopride OL 11 Graph ipg image - Thank you Dave Staso, CA. for the better expandable image!
"After it loads Right click "Show Original Images" - Move cursor arrow over graph - Click + when 'over graph' for expanded image," then print on the highest quality setting.
Notice at 700-RPM with a quarter 1/4HP motor, I checked his actual airflow at less than 300-CFM (no appreciable duct air leaks). the graph shows 5.24" SP.

Now, we switch to a 1/3HP motor @800-RPM, the graph shows 6.85" SP & only 400-CFM, not nearly enough airflow for 1.5-Ton of cooling. Therefore we have to raise the evaporator coil 6" above the furnace on rails & then check the airflow. Eliminating that restriction using a 1/3HP motor, hopefully that will be adequate at
+800-rpm & say +5.5" SP & +700-CFM. Formula: SP2= (SP2/SP1)2 X's SP1

Every manufacturer should furnish blower curve charts with their units and also put them on the Internet for service tech's to download and print. Also, air conditioning codes should be updated in respect to proper sizing of the duct work which must include all the pressure inducing factors when sizing the supply and return ducts. Also, illustrate best furnace to evaporator coil transitions, especially on oil furnaces!  You should always keep the ESP to 0.5" or mfg'ers listing.

The evaporator must be mounted 4 to 6 inches above this model oil furnace to achieve adequate airflow!

Below is an example of this problem with a (Thermo Pride OL 11 oil furnace).

The low airflow probable cause is "an unbalanced airflow heatload through the evaporator coil, along with what is known as "static regain," due to the evaporator coil being too close to the large oil furnace heat exchanger.

Those oil furnaces have a very large heat exchanger that goes to near the top of the furnace, --due to a low basement ceiling the DX coil sets perhaps illegally close to the heat exchanger causing an airflow restriction, and a few of the coil's circuits to be unevenly heatloaded. Since the liquid refrigerant is not completely evaporated it will cause the outlet line that the TEV sensor bulb is on, to be too cold and the TEV will shut-down the refrigerant flow, which greatly reduces the BTUH capacity of the DX coil and the system.

On piston refrigerant control systems, they may flood back liquid which could damage the compressor, unless the system is way under-charge. Thermo Pride could install airflow turning vanes just above the heat exchanger to funnel the air directly into the DX coil, instead of most of the airflow hitting the bottom of the DX's drain pan causing extreme back-pressure/turbulence and an imbalanced DX coil circuitry heatload!

Static regain explained: every time the velocity is reduced there is a conversion to static pressure. In this case it not only loses all of the velocity airflow energy due to hitting the evaporator drain pan, but also skyrockets static pressure, greatly reducing the blower's ability to deliver the required CFM!

The required main trunk Supply Air velocity and static pressure is lost between the heat exchanger and the evaporator drain pan, and therefore there is insufficient velocity and static pressure at the SA diffusers to deliver the throw and requisite CFM!

Flooded or Starved Evaporator Coils 

Changing the state of the refrigerant in the evaporator coils is as important as the air flow over the coils. Liquid refrigerant supplied to the coils by the expansion valve expands to a vapor as it absorbs heat from the air. Some liquid refrigerant must be supplied throughout the total length of the evaporator coils for full capacity.
-----------------------
Gurgling sounds at TEV: Low evaporator heat-loads lead to reduced liquid line mass and increased evaporator mass could be due to airflow problems. Eliminate low evaporator heat-loads before looking into adjusting the refrigerant charge.
Gurgling and pulsation noises at the expansion device can be caused by low charge, and/or non-condensibles and moisture in the system. Unbalanced airflow through the various distributor circuits of the evaporator coil will cause the TEV to close down refrigerant flow starving the coil. Piston-flow-rators will make it impossible to properly charge the system and cooling will be greatly compromised unless you eliminate the cause!


On every Rheem condenser cover it lists "non-condensibles and or moisture" as causes for a gurgling or pulsating noise at the expansion device. The entire evaporator circuits, may not become active for various reasons, - "the entire coil must become fully active for efficient performance."

A starved evaporator coil is a condition in which not enough refrigerant has been supplied through the total coil length. Therefore, expansion of the refrigerant has not occurred through the whole coil length, resulting in too-low a heat exchange and lowered BTU/hr capacity operation.

The purpose of these recommendations is to provide liquid refrigerant at the expansion device and provide efficient operation. Hopefully, this will aid your research.  If I can be of additional assistance, contact me.
-----------------------------------------

Too many do not properly purge & evacuate contaminated central air conditioning systems.

The Triple Evacuation Method is normally done on refrigeration systems, R-410a systems require it on central air conditioning systems:

First, remove any valve cores with a special  valve core remover this will speed up the evacuation time. Back service valves two turns off their back seat.

1) Re-claim unit charge (Recover all the refrigerant)

2) Charge system to 150-PSIG with dry nitrogen and leak test

3) On contaminated systems replace the filter dryers. Then Repair all leak(s)

4) Evacuate system to 500-microns valve off & see if it holds 500-microns for ten minutes, if it holds for 15-minutes, break the vacuum with dry nitrogen

5) Evacuate system to a deeper 350-microns, valve off vac pump, & again break the vacuum with dry nitrogen

6) Evacuate system to 350-microns and & then Check to see if it holds for a few minutes. (Recharge with fresh clean refrigerant)

7) Check to see if the Supply and Return air ducts were correctly sized & sealed by the original installer.

If a vacuum pump will not evacuate a system below 1500 microns there is a problem with the pump itself, a leak in the system, or moisture in the system. Moisture is most likely because water vaporizes at 1500 microns.


Many HVAC contractors will consider this excessive time & effort for contaminated residential air conditioning systems, however it is a must for low temp applications & the new A/C refrigerants.

The “micron” is a metric unit of measure for distance. The micron is a unit of linear measure; one micron equals 1/25,400ths of an inch. Modern high capacity vacuum pumps help speed up the evacuation process.

When a system has been evacuated below 500 microns, the pump is valved-off with the micron gauge connected, if the vacuum rises to 1500 microns and stops, there is moisture remaining in the system. If it rises above 1500 microns & continues to rise there is a leak. You should allow at least 15 minutes after the pump has been shut off an accurate micron gauge reading. When a system will not evacuate below 1500 microns there is either a lot of water or there is a system leak.
=============================================================== 

A flooded evaporator is the opposite of the starved coil. Using a Flow-rator Piston refrigerant control, too much refrigerant is passed through the evaporator coils, resulting in unevaporated liquid passing onto the suction line and an under capacity system.
http://www.udarrell.com/air_temperature_drop_evaporator.jpg
Air Temperature Drop Through Evaporator Coil (1987 Period)
"Indoor temperature and humidity load variations graph."
Refrigeration & Air-Conditioning (ARI) Second Edition,
Page 624, © 1987

Measuring Low Airflow

I normally would measure the airflow with a flow hood, also called a capture hood. You should normally have around 400 CFM (Cubic Feet per Minute) per ton of cooling. Half of the systems I measure have [a mere] 200 CFM per ton, OR LESS. This will be aggravated by a dirty air filter, Some Restrictive high efficiency air filter's or grilles closed in rooms that you are not using. Normally, do not turn the thermostat down below 70º  [74º 76º -better] degrees. A/C Tech guru, 'Stretch' A/C OWNERS: Measuring the air temperature rise across the outdoor condenser coils is the easiest check point to determine the total amount of latent and sensible BTUH of heat your air-conditioner is actually transferring to the outside. You will enjoy doing it, doing it could lead to making changes that could considerably improve your Air Conditioning System's performance, thus improving your total comfort while in most cases greatly reducing your cooling bills. SEE CHARTS BELOW.

By way of background: ARI introduced the Energy Efficiency Ratio "EER" in 1975. This was an "HVAC" industry instituted and policed way "to determine the relative efficiencies" of one unit to another in the cooling mode. "EER" was determined by dividing the published steady state capacity by the published steady sate power input at 80°F dB/ 67°F Wb indoor and 95°F dB outdoor. This was quite objective yet unrealistic with respect to system "real world" operating conditions. 

The SEER of a system is determined by multiplying the steady state Energy Efficiency Ratio (EER) measured at conditions of 82°F outdoor temperature, 80°F dB/ 67°F Wb indoor entering air temperature by the (run time) Part Load Factor (PLF) of the system. A major factor NOT considered, is the actual part loading factor of the indoor evaporator cooling coil, that greatly reduces the unit's listed btuh capacity and SEER efficiency level.

Optimizing the "Evaporator HeatLoad" will Optimize the Condenser BTUH HeatLoad Output from your Home

Most evaporator coils are under-loaded when operating at the normal room temp setting!

The airflow should be adjusted to fully load the evaporator coil at the normal room temperature setting! This airflow adjustment will optimize your air conditioner's BTUH and SEER performance. Most air conditioner's have an underloaded evaporator coil at the room temp thermostat setting, where the vast majority of its run time will take place! In 8 foot ceilings, Return Air (RA) should always come from the warmer ceiling air areas.

On TXV metered systems the Subcooling should be within +/- 2-F of the mfg’ers installation instructions.
==========================

Air Conditioning Performance Diagnosis using listed (CT) Condensing Temperatures

Using Goodman 16-SEER "Expanded Performance Data"

What is the correct (CT) Condensing Temperature with the following known mfg’ers data?

Outdoor Ambient Temp (OAT) 85-F; IDB 75-F; IWB 63-F or 50%-RH.
Listed pressure is 316-psig, or 99-F CT; that is 99-F -85-F is a 14-F SPLIT.

The delta T or temp-split should be within a 10-psig range or, +/- 2-F degrees; 97 or 101-F.

The mfg’ers Supply Outlet should be able to provide Contractors & Techs with those performance data charts. Goodman has their “Expanded Performance Data” on the Internet.

======================================================

In an oil furnace installation, a high static pressure can be partially due to the evaporator coil being installed too close to the big round heat exchanger. If you have room, a reducer transition should be stalled to funnel the air into the aperture opening of the A coil.

Installing the coil on to of the furnace can cause high turbulence and back pressure, which combined with inadequate, (along with floor level intake returns) ducting coupled with long runs and other problems could increase the static pressure so high that your blower motor's HP will not move enough heat loaded room air across the heat absorbing evaporator coils and fins to fully heat load the outside condenser coil.

Floor level supply and return air quadruples the problem. External static pressures need to be kept within or below 0.5" Water Column for efficient operation of the blower wheel design and motor HP.


Too low an air flow, or SA and RA air at the floor level, can greatly reduce the capacity of your AC unit. In the case of a thermostatic expansion valve (TEV) (liquid refrigerant metering device); it will simply shut down the flow of liquid refrigerant into the evaporator coil to keep it at the TEV's usual 10ºF Super-Heat setting.

A flow rator type metering device will continue to feed to much refrigerant into the coil which can cause it to drop below freezing temperatures which will block air flow and also can flood liquid back to the compressor causing severe damages to it. Additionally, the refrigerant charge will not be accurate unless it is weighed into the system. There can be NO accurate measure of Superheat without an optimally balanced heat load through all the evaporator coil circuits.

Take the condenser entering air temp and leaving air temp, subtract for the temp-split. As a double verification: You can use the high-side (SCT) Saturated Condensing Temperature minus the outdoors-ambient temperature; the difference gives you the condenser temperature-rise or Delta-T temperature/split. There is NO excuse for not utilizing this important diagnostic check. Always use an accurate volt meter and amprobe to make sure you are not overloading the compressor's Wattage Service Factor and check the compressor discharge line to see that it is under 225-F.

On wrap-around condenser coil top air discharge condensers' --first, check the condenser entering air dry-bulb temp., and the condenser dry-bulb discharge air temp., while moving the TH around in the air stream. (This will usually be around a 18 to 24 degree condenser temperature split rise. Older units run a higher temperature split.)

Techs should get the condenser air flow data in CFM from the manufacturer's data. Because all the heat discharged by the condenser air flow also includes the converted latent heat of the evaporator's absorbed condensation heat, you can determine the total BTUH of heat exhausted by the AC condenser and thus determine if it is getting anywhere near its BTUH rating. You also need to add the additive heat of the condenser's compressor and fan motor. The indoor blower motor is also a heat contributing factor, not figured in this formula.

You can make up the charts for 10, 12, and 14 SEER units for specific makes. One chart might include many different makes. The 14 SEER is a whole different bucket of bolts, as it uses a larger condenser and a very high CFM for a lower temp-split.

For the uninitiated, Delta-T is the difference between the air temperature entering and leaving the outdoor AC condensing unit. This is a good diagnostic check because it measures the latent heat of condensation as well as the sensible heat absorbed by the vaporizing refrigerant in the indoor evaporator coil. I'm betting when you find out approximately how many BTUH that the AC system is actually transferring outside, you may be shocked. Many new Packaged Units have a very high condenser CFM airflow and a LOW TEMPERATURE SPLIT! Very high SEER units have oversized condenser coils  and very low temp-splits! To get the gross BTUH Heatload the Evaporator (DX) Coil is absorbing (which includes both latent, sensible heat) (These are ARI Formulas) There are many ways to figure the amount of heat the evaporator is transferring to the condenser. 

First, determine the Gross Rated BTUH the condenser is ejecting. 
Condenser’s Gross Btuh = Condenser’s rated CFM X’s Temp Split X’s 0.88
Brother’s Example: Heil, 1.5-ton, with 2-ton DX (evaporator) coil with a TEV refrigerant control, -Condenser Rated at 18,400-BTUH, with a 13-SEER rating.

1400-cfm X’s (13-temp rise X’s 1.08) = 19,656-Gross BTUH heat ejected, subtract the 6,562.5-btuh motor heat additive = only 13,093-NET BTUH transferred from the evaporator (DX) Coil to the condenser, compared to a net heat transfer rating of 18,400-btuh! A loss of 6,307-btuh or over half a ton loss, or over a one-third loss of heat transfer! A one ton condenser would have done almost as much! As the rooms cool it is only a 12-F temp-split or 11,582-btuh output! The actual lack of an adequate DX coil heatload would only require a small one-ton condenser!

CONDENSER TEMP-SPLITS - My Brother's Heil 12-SEER Condensing Unit 
1.5-Ton - Rated at 18,400-BTUH,  Condenser fan CFM 1400 (Total Cond. Watts 2221 X's power Factors 0.88 X's= 1887 X's = 1954.48 * 3.413 = 6,670-BTUH Motor Heat additive +18400= Motor Power "Rated Gross Heat Ejection" is 25,070-BTUH / 1400 = 17.9-F Temp Rise Cond/Split. The condenser only gets a 10 to 13-F temp-rise-split, depending on the heat load in the house. Supply air and return air are both at the floor level recirculating the coldest air in the room to the DX coil, the evaporator is NOT being supplied with an adequate temp split heat load or, an unbalanced heatload on the DX coil's circuitry. 

The probable cause is "an unbalanced airflow heatload through the evaporator coil. "It's a (Thermo Pride OL 11 oil furnace). Those oil furnaces have a very large round heat exchanger that goes to near the top of the furnace, --due to a low basement ceiling the DX coil sets perhaps illegally close to the heat exchanger causing a few of the coil's circuits to be under heatloaded. Since the liquid refrigerant is not completely evaporated it will cause the outlet line that the TEV sensor bulb is on to be too cold and the TEV will shut-down the flow, which greatly reduces the BTUH capacity of the DX coil and the system. On piston refrigerant control systems, they may flood back liquid which could damage the compressor, unless the system is way under-charge. Thermo Pride could install airflow turning vanes just above the heat exchanger to funnel the air directly into the DX coil, instead of most of the airflow hitting the bottom of the DX's drain pan causing extreme turbulence back-pressure and an imbalanced DX coil circuitry heatload!

The chart split listed below is at Condenser Design conditions: Indoor Return Air 80-F dry bulb 67-F Wet Bulb or 50% Relative Humidity as you go up to 99% RH the condenser split could increase by up to 6-F; down as much as 4-F at a low humidity of 55-F Wet Bulb.

Do your own figuring based on this formula. Motor BTU/hr additive = Watts X's PF x's 3.413 for Btu/Watts additive added to rated BTUH, divided by condenser fan CFM X's 1.08 =  condenser Temp-Split. Get the Motor Power Factors (PF) of the compressor and fan motor from the manufacturers. (A 0.80 factor could be close.) Some of the 10-SEER temp-split figures need correcting, will do ASAP. Most Splits rounded off.

Formula for finding CFM Airflow from Velocity in FPM
If you can measure the air velocity coming from a known size duct or open area of a SA register, here is a rough ballpark formula to get the CFM:

CFM = (velocity in (FPM) Feet per Minute times the Square Footage of the duct area). To convert sq.ins. multiply by 0.00694 for sq.ft., or divide sq.ins. by 144.

Converting square duct
inches to round duct size, Figuring the Square Inches of Round Ducts, an 8" x 8" duct = 64-sq.ins. x .7854 = 50.26 sq. ins. You round off to 50 sq. ins. for an 8" duct.  Or, simply getting the square inches of round ducts: a 7" duct; 7" x 7" = 49 x .7854 = 38.48-sq.ins. or divide / by 144 = .2672222-sq.ft. X's a velocity of 500-fpm = 133.6-cubic feet per minute delivered to the room; 133.6-cfm x 30 = 4,008-BTUH.


Sized for in the chart below - BTU/hr per CFM figures "are figured for heatpumps at 450-CFM per ton of cooling."
Use 800 to 900-FPM MAINS' VEL. Use an optimum of 500-FPM VEL for Supply Branch Runs | Air speed Face of Return. 

Air Filter Rack Sizing
I realize this will never happen if you use the furnace filter size; however, Air Filter Rack Sizing for efficient operation - Size Gross Return Air filter grille area for 200-sq. ins. per ton. For a 5-Ton system, that would mean Two filter racks 25X20's each, I would go with Two 30X18" RA filter racks for 1080-sq.ins for a 5-Ton system.

Do your own figuring based on this formula. Get the Motor Power Factors (PF) of the compressor and fan motor from the manufacturers.
CONDENSER TEMP-SPLITS 12-SEER units - Comfortmaker® | Heil® | Temp Star®  - used 0.88 Motor Power Factors
ARI Conditions are: 95ºF-OAT; 80ºF-IDB; 67ºF-IWB or 50%RH | ARI Conditions; 95-OAT; 80ºF-IDB; 63-IWB  or around 50%RH | &Try 85ºF-OAT | Outdoor Ambient Temperature (OAT) 1.5-Ton 18,000  21-F Split Cond. CFM 1400 WATTS 1536
*** 1.5-Ton
is from actual published DATA - Only ARI Rating Conditions

1.5-Ton 18,000 @ 95ºF OAT; Indoors 75-IDB; 63ºF-IWB or near 50%RH; @ 600-CFM; Outdoor Ambient Temperature (OAT); 18ºF condenser split | @ 85ºF OAT; 67-IWB or 66.5%RH; +20ºF cond. split.
To figure this; units pressure chart, the Temps, instead of IWB the %RH, & CFM, For users, No gauges required, to check if your A/C is near specs!
However, the temperatures & indoor humidity make a big difference in the condenser split.  (Airflow & proper heat-load on evaporator!)

Brother Don's perfect combination Heil, Model: HAC218AKA1, Scroll compressor 12-SEER, matched with a 2-Ton evaporator with a TXV metering device,  1.5-Ton 18,400-BTUH @ 95ºF (OAT) Outdoor Ambient Temperature; Indoors: 75-(IDB) Indoor Dry Bulb; 63ºF-(IWB) Indoor Wet Bulb, or near 50%RH; @ 600-CFM; ; "+17ºF Condenser-Temp-Split" | @ 85ºF OAT; 67-IWB or 66.5%RH; "+20ºF cond. split."

Take the both the indoor Supply Air & Return Air DB, WB or %RH , too! If you have an accurate airflow CFM, I can Ballpark the BTUH your A/C  or Heat Pump is delivering in the cooling mode.
12-SEER Rated Condensers
1.5-Ton  18,000 Outdoor temp 95F; 80-F IDB, @ 67-F IWB or 50% RH; ARI Conditions = 21-F
Condenser Air-Temp-Split.  Don's @10-F to 12-F Split - needs critical ID Airflow attention!
2-Ton  24,800  24-F Temp-S  Cond. CFM 1400     WATTS 2659 
(All below ARI Conditions)
2.5-T  30,200  21-F Temp-S   Cond. CFM 2000     WATTS 3404
3-Ton  35,600  18-F Temp-S  Cond. CFM 2800     WATTS 4117
3.5 T  42,500  21-F Temp-S   Cond. CFM 2800     WATTS 4554
4-T    48,500  19.5-F Split       Cond. CFM 3400     WATTS 4761
5-Ton  59,000  25-F Temp-S  Cond. CFM 3400     WATTS 6969
==========================================================

http://www.udarrell.com/air_return_latent_condenser_split.jpg
Page 618, Refrigeration & Air-Conditioning (ARI) Second Edition, © 1987
Those lower SEER units had higher condenser splits than 12-SEER and higher units.
Sorry, I defiled the graph.
90-db outdoor, 80-db indoors with 67 wet bulb/50% RH
represents the condenser splits shown above.

http://www.udarrell.com/air_temperature_drop_evaporator.jpg
Air Temperature Drop Through Evaporator Coil (1987 Period)
"Indoor temperature and humidity latent load variations graph."
Refrigeration & Air-Conditioning (ARI) Second Edition,
Page 624, © 1987

-------------
As you put more total heat-load through the evaporator coil, up to its capacity ceiling, both total capacity and efficiency increase for optimal Btu/hr, EER, and SEER.

You can achieve a higher latent ratio in high humidity climates through design changes, evaporator, and TXV metering device selection, set at 6º F for optimum reduction of the temperature of the DX Coil, and reducing airflow after the unit gets the humidity near 50%. At the higher humidity levels, the unit will operate closer to its Btu/hr rating set at 400-cfm/ton. Using modern control systems, this would be easy to achieve.

When we use to have an oversized compressor in relationship to the evaporator's capacity, it could get that DX coil very cold in a hurry and keep it that way.

Specially engineer-designed DX Coils could optimize total latent capacity at the highest to below 50% relative humidity levels. Areas of Florida, Louisiana and even Green Bay, WI need unit systems designed for optimal latent heat operating capacities.


Do your own figuring based on this formula. Get the Motor Power Factors (PF) of the compressor and fan motor from the manufacturers.
CONDENSER TEMP-SPLITS 12-SEER units - Comfortmaker® | Heil® | Temp Star®  - used 0.88 Motor Power Factors
ARI Conditions are: 95ºF-OAT; 80ºF-IDB; 67ºF-IWB or 50%RH | ARI Conditions; 95-OAT; 80ºF-IDB; 63-IWB  or around 50%RH | &Try 85ºF-OAT | Outdoor Ambient Temperature (OAT) 1.5-Ton 18,000  21-F Split Cond. CFM 1400 WATTS 1536
*** 1.5-Ton
is from actual published DATA - Only ARI Rating Conditions

1.5-Ton 18,000 @ 95ºF OAT; Indoors 75-IDB; 63ºF-IWB or near 50%RH; @ 600-CFM; Outdoor Ambient Temperature (OAT); 18ºF condenser split | @ 85ºF OAT; 67-IWB or 66.5%RH; +20ºF cond. split.
To figure this; units pressure chart, the Temps, instead of IWB the %RH, & CFM, For users, No gauges required, to check if your A/C is near specs!
However, the temperatures & indoor humidity make a big difference in the condenser split.  (Airflow & proper heat-load on evaporator!)

Take the both the indoor Supply Air & Return Air DB, WB or %RH , too! If you have an accurate airflow CFM, I can Ballpark the BTUH your A/C  or Heat Pump is delivering in the cooling mode.
==========================================================

http://www.udarrell.com/air_return_latent_condenser_split.jpg
Page 618, Refrigeration & Air-Conditioning (ARI) Second Edition, © 1987
Those lower SEER units had higher condenser splits than 12-SEER and higher units.
Sorry, I defiled the graph.
90-db outdoor, 80-db indoors with 67 wet bulb/50% RH
represents the condenser splits shown above.

http://www.udarrell.com/air_temperature_drop_evaporator.jpg
Air Temperature Drop Through Evaporator Coil (1987 Period)
"Indoor temperature and humidity latent load variations graph."
Refrigeration & Air-Conditioning (ARI) Second Edition,
Page 624, © 1987

-------------
As you put more total heat-load through the evaporator coil, up to its capacity ceiling, both total capacity and efficiency increase for optimal Btu/hr, EER, and SEER.

You can achieve a higher latent ratio in high humidity climates through design changes, evaporator, and TXV metering device selection, set at 6º F for optimum reduction of the temperature of the DX Coil, and reducing airflow after the unit gets the humidity near 50%. At the higher humidity levels, the unit will operate closer to its Btu/hr rating set at 400-cfm/ton. Using modern control systems, this would be easy to achieve.

When we use to have an oversized compressor in relationship to the evaporator's capacity, it could get that DX coil very cold in a hurry and keep it that way.

Specially engineer-designed DX Coils could optimize total latent capacity at the highest to below 50% relative humidity levels. Areas of Florida, Louisiana and even Green Bay, WI need unit systems designed for optimal latent heat operating capacities.

--------------
 
Typical matched units from major manufacturers have Sensible Heat Ratios (SHR) in the 68% to 80% range (or 32% to 20% Latent) when it is 95-F outside and 75-F with 50% relative humidity inside. Proper mixing of the air and proper distribution to individual rooms is critical for comfort.
 The condenser fan speeds are slower on several of the 10-SEER Tonnage Models. We are only trying to get a figure to go by for a comparison. When new condensers and Evaporator coils "are installed on older air handlers" the new, or old, evaporator coils are usually under heat-loaded. (Always, check voltage and amp draw!)
The Base Spec sheets 12-SEER part no. 421 41 33301 03, Feb 2001. These are the Comfortmaker® units, which are nearly identical to Heil® units. I used the first rating on each tonnage class. While the "Performance Cooling Data" is listed at a 95-F outside ambient temperature, you can adjust the indoor airflow to get the Nominal BTUH Rating at the customer's normal indoor stat' temp' setting and the most outside temperature/degree operating hours.

Take the "listed watts" of the compressor and Condenser fan and multiply that wattage by 0.85 X's 3.413 to get the BTUH heat additive of the motor then add the listed BTUH of the condenser to it, and then divide by the condenser fan's CFM.

By using the various units' "base specification sheet data" from the dealer, you can determine if it is operating near its BTUH capacity rating. Some packaged units run a very high condenser discharge CFM airflow!

Some "Condenser Makes" will have different temp-splits. The 2-ton 10-SEER, Janitrol; GMC; Goodman; with the U-29 E-Coil delivers less btuh, or 23000-btuh, I subtracted a reasonable amount from the total of the wattage and come up with 19 to 20-F temp-split. That is "if" its CFM is 1400, --get the figures on the "different Makes." The figures are used to provide an idea of what the condenser temp-split should be for use by the unit's owner and the service tech.

With a properly sized system and proper evaporator airflow you will have consistent optimal nominal capacity heat absorption and removal coupled with the requisite longer run-time cycles. I believe that optimal efficiencies, with variable latent/sensible heat loads, could be effectively achieved through the use of computerized control system components.

For efficiencies sake, do this immediately. Measure the "Return Air" duct/chase area. If it's a round duct measure the inside diameter, I'll give you the sq.ins.

For "Return Air" Sizing:

Take your air conditioner's btuh and divide it by 120, (or dividing 24,000-btuh by 150 will give you 160 sq.ins., close to a 0.10" return air duct Static Pressure drop) to get the amount of free air square inches for the Return Air duct system. 

A 4-ton condensing unit, 48,000 btuh would need 400 sq.ins. or two 16" rd. ducts. A 5 ton 60,000 btuh calls for 500 sq. ins., or two 18" rd ducts for 510 sq.ins. A two ton 24,000 btuh takes 200 sq. ins., or one 16" rd. duct. For a 18000-btuh or 180 sq. ins., go with the 200 sq.in. 16" rd. duct. This will permit building more pressure at the supply air diffuser grilles providing more throw across the room. Before you make all the recheck tests, it is very important that your condenser coil and evaporator coil and indoor blower wheel be truly clean.

This AC system would have to be sized to the combined latent and sensible heat load targets (i.e., 75F/50RH) and the cubic foot volume of air changes that we would like per hour. This would need to be performed accurately to achieve the requisite run time, and CFM airflow through the cooling coil, to achieve our combined comfort zone and unit efficiency goals.

Proper duct sizing and location is important. If you have a high ceiling supply air and return air ducts should be at the floor level so you can take advantage of air stratification. There is no need to cool the air above the occupant height level.

When U have lighter load  conditions: To achieve a proper evaporator heat load level with a floor level SA/RA system, an increase to 475-CFM per ton of cooling capacity may be necessary. If the conditioned space is extreme hot it might be wise to shut down some Supply Air ducts and partially cover the Return Air grilles so the condenser doesn't become overloaded."

I'm talking about any heatload that might be excessive at 475-cfm per ton of cooling. The combination of being real hot outdoors & hot indoors can lead to an overloaded condenser.  Also, in some cases we are using a larger tonnage cooling coil.

Most older homes need reduced ambient air infiltration and more effective use of vapor barriers coupled with adequate insulation. Windows are special areas to work on. My upstairs windows around the pulley wheels allowed air to blow in unrestricted from the attic area winter and summer.

All air conditioning condenser manufacturers' should publish the CFM and normal temperature rise range across the condenser coil, so that the service tech's can measure the heat transferred from the evaporator coil. Most high efficiency units will have temperature degree rises between 18 and 25ºF. Older lower SEER condensers can have temperature rises up to 30 degrees.

Such temperature rise data provides a guide to the actual heat transfer by the evaporator coil to the outdoor condenser coil, and therefore also, whether the proper design amount of (cubic feet per minute)  CFM of indoor air/per ton of cooling BTU/HR, is passing through the heat absorbing cooling coil.

Let's say you had 0.20-IWC without evaporator coil," then due to the large oil furnace heat exchanger near the bottom of the E-Coil you add the 0.20 IWC, the coil adds 0.20 IWC for a total of 0.70IWC.  That is without adding everything else! Additionally, the pressure & velocity drop is where you do not want it.  You want all of the pressure & velocity possible at the diffusers to get the requisite CFM & throw!

Component
Pressure Drops IWC
Cooling coil
0.15 to 0.50-IWC
Pleated filters
0.10-in. to 0.45-in.
Electrostatic filters.
0.20-in. to 0.80-in.
Grilles and registers
0.02-in. to 0.15-in.
Transitions, Boots
0.05-in. to 0.35-in.
Elbows, - Use Turning Vanes!
0.01-in. to 0.10-in.
100-ft. duct length
0.05-in. to 0.20-in.
Disposable filters
0.05-in. to 0.30-in.

Check the static pressure with a wet coil then check the Units Blower Curve Chart to see if you are getting 375 to 450 CFM per ton of cooling, depending on the humidity removal needs. Is your motor horsepower, blower wheel size and blower RPM up to the task?

Subtract 50 cfm from the cfm derived from the formula for the wet cooling coil cfm. It may not be getting the requisite  375 to 450 cfm per ton of cooling, especially if there is a low demand for heating in your area and a high demand for cooling tonnage, or the cooling coil is too close to the oil furnace's heat exchanger causing a restriction and a lot of turbulence back pressure, running static pressures as high as 0.75 or higher water column static pressures (view blower curve chart).

Owner's of AC systems should check the return air filters frequently and the blower wheel squirrel cage curved blades for dust and lint accumulation. If the blower wheel and motor are dirty the evaporator fins and coils will need to be checked on the air entry side, all components must be cleaned to regain btu/hr heat transfer efficiency to original specifications. It will cost you big-time on your cooling costs if you don't keep the blower wheel blades and the cooling coil clean. Also, the blower motor is air cooled and will overheat and burn out prematurely.

The condenser coils and fins must also be clean. Never use household detergents, most detergents have an oil base that will insulate coils and fins which will reduce the evaporator & condenser coil's heat absorbing efficiencies. Use only the proper AC coil cleaning fluids for for cleaning the indoor and outdoor air conditioning heat transfer coils. High pressure water can be used but you must never bend the coil's heat transfer fins, so keep the stream perfectly straight with the fins.

Typically, a system is designed where the appropriate fixed sized metering device bridges (or matches) condenser capacity to evaporator capacity as dictated by the compressor and a specified CFM at a specific temperature/humidity heatload point. With a TEV refrigerant control to the evaporator coils, the CFM range can be any workable CFM from 350 to 450 CFM per ton of condenser cooling capacity. Variable speed blower fan motors "that will provide those 350 to 450 CFM/ton, would be ideal,"  or adjustable speed belt drive blowers allow the technician to provide the evaporator coil with an optimal heat load at normal indoor  temperature and humidity levels. If these factors are ignored and are out of the required specifications your new unit won't deliver the btu/hr or the SEER you paid for. Your new 12 or 14 SEER may be delivering only 8 or 9 SEER.

Subject: What design for lower duct static and lower blower motor HP?
Remember that many oil furnaces have a large round heat exchanger in the center up to near the top, and if the evaporator coil is set too close this can cause extreme air turbulence and back pressure which could be a huge factor in running the static pressure way up!

Need for Low Flow Resistance Residential Duct Systems
Beyond improving evaporator airflow, reducing fan power and duct leakage are two further reasons to promote proper duct design with a lower external pressure drop than those encountered in many research studies.

For instance a duct system moving 800 cfm with a pressure drop similar to that measured in a study (0.63 IWC  without coil and 0.83 with the evaporator coil) would result in a power draw of 347-Watts. However, a duct system with a total pressure drop of only 0.20 IWC, or 0.40 IWC with the evaporator coil would produce a power demand of only 167 Watts -- a fan power reduction of 52%. If the compressor electrical demand was 1800 W to produce 24,000 Btu/hr (7032 Watts) of cooling at the coil (not including fan energy), the improvement would alter EER from 10.63 to 11.91 Btu/W -- a 10% net increase in cooling efficiency and capacity.

One HP = 746 watts: with S.P. @0.83" | 347 watts / 746 = 0.465 HP or a Half HP Motor | with S.P. @0.4 | 167 watts / 746 = 0.2238 or 0.25, or a quarter HP motor. Properly sized and laid out ducting is critically important to performance.

The larger AC units are usually short changed on return air filtering area! Figure the sq. in. of your furnace's return air filter. Furnace free area return air filter area should be sized for the largest AC unit it will handle!

The heat-load is determined by the amount of heat the evaporator coil is absorbing from the conditioned areas' --air flowing through it at an optimal CFM heat-load level will properly load the compressor and condenser. Let's take a closer look at the effects of a low heat load on an evaporator coil with a evaporator temperature controlling TEV refrigerant control.

The lower heat-load will cause the temperature sensor bulb to reduce the flow of liquid refrigerant into the evaporator coils resulting in a lower than normal suction pressure which reduces the volumetric capacity of the compressor, and liquid refrigerant will begin to back up in the condenser coils which also reduces its capacity.

This means that your entire cooling system, (which includes the ductwork design), would NOT be delivering the unit's rated BTUH. Lack of an adequate airflow heatload through the evaporator coil will reduce the BTUH transfer of heat by the evaporator,  therefore to the compressor and condenser and on to the outside air.

Under very light heat-load conditions the subcooling might appear close to Normal. However, the BTU capacity of the system would be lowered as would the SEER rating because the total amp draw of the system does not drop enough from a fully heat loaded BTU design capacity to come close to the resultant inefficiency.

A fixed orifice would begin to flood the evaporator with liquid refrigerant reducing its capacity, because there wouldn't be a sufficient heat-load to vaporize it. Liquid refrigerant could flood back to the compressor causing irreparable damage.

The relationship between head pressure variation with liquid subcooling and suction superheat is not the same with TEV/TXV when compared to fixed orifice. With a fixed orifice, the relationship is immediately obvious to experienced tech's.

When condenser dT (temperature difference) is very low, a static pressure fan-curve-graphic chart check-up, is required procedure. "On older retro-systems" ("break out your Magnehelic / manometers") this procedure is essential "before attempting to charge a TEV system or fixed orifice on older systems" --that may have serious airflow heat-load mismatches.

Additionally, a liquid line sight glass near the evaporator coil is a help, in that you can recover refrigerant until it begins to bubble. Then add charge according to the manufacturer's Return Air ºF match to the wet bulb / dry bulb listed Temp. difference figures, —Superheat charging table, while also monitoring liquid line Subcooling.

Service techs' put your Magnehelic gauge and Digital Micromanometer to good use to measure the static pressure and then get and apply the blower curve charts on each system you are working on, then you know you're getting the proper evaporator airflow temperature and heat-load to meet the customer's desired humidity and temperature comfort zone.  It is always very good practice to measure the external static pressure on all systems; you can do this with a simple magnehelic gauge or with a digital micromanometer. In any case, static pressures above 0.45 IWC should be investigated and reduced if at all possible.

Service techs' use your sling-psychrometers' and do the job right.

With an Insufficient Heat Load on the Evaporator COIL:
Suction will be LOW.
Super-Heat LOW - with fixed orifice or flow-rater control (TEV 10ºF)
Head-pressure - LOW
Subcooling - LOW
Compressor Amps- LOW
With the TEV, liquid would probably still back up some in the condenser coils reducing BTU capacity, which it doesn't need as much of with the light evaporator load.

Air Infiltration sources DTI Corp Catalog
Air Infiltration DTI Corp

Insulation & Weatherization Costs Forum
============================

Knowing the operating static pressure is a first order essential to revealing the operating CFM. If ductwork retrofitting doesn't solve the problem; Blower wheel RPM and blower motor Horse Power may need to be increased to achieve the optimal CFM to achieve your Unit's rated nominal BTUH and Energy Efficiency Rating. (80% don't !) There ought to be a code requiring every manufacturer of an airhandler or furnace to provide capped taps ahead of the evaporator coil and ahead of the blower for easy static pressure testing access.

Read the pressure on the gauge, and record the reading on the supply side, then on the return side. Use a (+) sign before the positive or supply side reading to show where it was taken, and a (-) sign before the negative or return side reading.

Add the two pressures. Disregard the positive and negative signs before the pressures, because both negative and positive pressures affect the fan as a force, so they must be added together to determine the total resistance the fan has to overcome. For example a +.35" I.W.C. plus a -.25" I.W.C. equals a total static pressure reading of .60" I.W.C.

Record the pressure readings on a diagnostic report or on your service ticket. Write the pressures on the cooling coil for future reference and use. Any future changes in static pressure reveals a change in the system that should be addressed.

Finding the New Static Pressure:

  • SP2 = (rpm2/rpm 1)2 X SP1 = SP2

  • Required fan motor horsepower (hp) varies as to the cube of the rpm speed:
    hp2 = (rpm2/rpm1)3 x hp1 = hp2

    CFM Fan delivery varies directly as to the fan RPM speed:
  • cfm2 = (rpm2/rpm1) X cfm1 = cfm2
  • A few calculations and presto, a matched airflow with your systems' heat absorbing coil capacities, delivering near its BTUH, EER, and SEER ratings at normal room temperature settings! (Most don't)

    You will need a good service tech to make the proper tests, and perform the proper adjustments. Utilizing numerous other energy savings techniques, you'll save tons!

    Below, PDF File: ThermoPride Blower-Curve-Chart - Click Print , Click on Properties, Click on Graphics, Slide Setting to the Darkest Setting, click OK, or blower curve lines won't show up on the printed copy! (for Techs)
    My scan of my doctored Thermopride OL 11
    http://www.udarrell.com/Blower_Curve_Graph.tif

     SL11B.pdf  PDF File: Blower curve lines show in (blasted) yellow, use darkest printing page settings to get readable lines! Every manufacturer should furnish blower curve charts with their units and put them on the Internet for service tech's to download and print. Also, air conditioning codes should be updated in respect to proper sizing of the duct work which must include all the pressure inducing factors when sizing the supply and return ducts.

    TEL ASP FR Chart Graph  Loads slow using dailup - Save both the pdf to a quick access PC folder for review
    Designing or Redesigning Duct Systems Chart  Print

    Variable Speed Motors and Static Pressure

    DISCLAIMER:
    I assume NO responsibility for the USE of any information I post on any of my Web pages, in E-Mails or News Groups.
    All HVAC/R work should always be done by a licensed Contractor & properly licensed Techs!
    This information is only placed on these pages primarily for your understanding & communication with contractors & techs.
    This information is also for the edification of Contractors and Techs.
    Never attempt anything that you are NOT competent to do in a SAFE manner!
    I am NOT liable for your screw-ups, you are liable for what you do! - Darrell Udelhoven

      Federal Refrigerant Licensed - Retired Licensed Tech & Contractor

     
    Please write me if you have anything you'd like to contribute! - Darrell 

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    - EMPOWERMENT COMMUNICATIONS
    *Video checking static ESP *Video 2 checking static ESP View! I Got DSL 11/2010

    Air Flow CFM of a very large Supply Air Register using a Testo 410 Vane Anemometer  Basic...

    *Video measuring airflow Velocity W/ anemometer on a Return Air grille
     
    He programmed it in, because when I did the math using .90% for the grille I got 336.57863-FPM Vel *X 2.376562-SF free-air-area= 799.9-CFM, or 2-Ton of airflow.
    I'd use close to .50% for the free-area of a clean FILTER, & .90% factor for open grille area.

    *Basics Featuring the Testo 556 - Video of a very thorough Air Conditioning Testing setup 2/26/11

    Realtime HVAC A/C BTUH Performance Output (Part 1 of 2) YouTube Video

    *Realtime HVAC A/C BTUH Performance Output (Part 2 of 2)

    -----------------------------------------------------------------------------------------------
    Darrell Udelhoven - udarrell
    Empowerment Communications
    Covering The Real Political Issues
    Posted: 05/14/05; Edited: 01/08/13
    Darrell, Darrell Bloomington, Lancaster, Grant County, SW WI