Air Conditioning System Superheat & Sub-Cooling Procedures for Optimal Charging

     - with Darrell Udelhoven - HVAC Contractor & Technician since mid-1970's - RETIRED

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

Best ROI Investment or Lose the Investment money annually "Home Energy Efficiency Pays You."
More authenticated survey test corroboration; the surveyed HVAC systems were systems that they had complied with their program’s energy standards and "all had received substantial incentive payments," but delivered an average of only 63% of their Rated Btu/Hr to the homes & 50% of the required airflow. http://bit.ly/1adKKG6 
70% of homes in California are operating at 50% capacity. - California Energy Commission


Local Contractor Locater Map - Find These Contractor PROs in your area
HVAC-TALK General Discussions: Importance of Two-Way Communications with HVAC Customers
* Customers A Simple A/C Check you can do!
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
  *Selecting Air Filters - Sizing the Filtering Area - Critically important!
Low Priced Affordable Infrared Camera

Comparative Tests on Filter Pressure Drops   - Important Criteria!
At 300-fpm, the 4" deep Merv 7 has the same pressure drop at .10 of an inch, as the 1" fiber glass filter; all filter areas were 24X24.

A New Brand of Comfort and Efficiency Excellent linked articles by 'Doc'
Jan 2, 2013 1:33 PM, By Rob 'Doc' Falke

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

*Checking BTUH Performance of your Air Conditioner
AIRFLOW DIAGNOSTICS and REPAIR PDF  Excellent & thorough pdf Slide Show Views
* HEAT PUMP DIAGNOSTICS
COMFORT INSTITUTE Consumer Protection Division Found 04/14/12 What I've been saying for years
Do a Search for a Comfort Institute HVAC Contractor 'Member' in your zip code

Home Energy Audits - Rochester, NY Area Real Energy Cost Savings - Ted Kidd
company logo is a triskelMO Home Energy Audits LLC
 
Everything You need to know about HOME INSPECTIONS
First, Anchor link, Determining which metering device TXV or Fixed Orifice -
without looking
at the A-Coil| SUBCOOLING|
*Video Checking Static ESP*Video 2 Checking Static ESP View!Got DSL 11/2010
*Instructive HVAC Videos at bottom of this page!

*Basics Featuring BTUH Testo 556 - *Video | Testo 550 Features

Home Energy Audits - Rochester, NY Area Real Energy Cost Savings - Ted Kidd

*Video measuring airflow Velocity - Vane anemometer on a Return Air grille
I'd use close to .50% for the free-area of a clean FILTER, & .90% factor for open grille area.
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.

Description of 416 Testo Telescoping Vane Anemometer
Testo 605-H2 Fast Accurate Wet Bulb

The Testo 416 Airflow Test Checking airflow must be performed before charging a system

Why Home Energy Efficiency Auditing is important to our future Jeremy Rifkin Video
===================

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.

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

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

Air flow CFM through a clean evaporator coil must be correct before balancing a charge!

Importance of figuring static pressure & Amp draw changes in respect to CFM air flow changes:

SP2= 2-Ton {800-CFM new / 650-CFM old}2 = 1.2307692*1.2307692=1.5147928*old SP= 0.98"SP new;way too high!
SP2= 5-Ton A/C {2000-CFM new / 1650-CFM old}2 = 1.21212*1.21212= 1.4692363*old SP  .60" = 0.88" new;  way too high.
SP2= 4-Ton  {1600-CFM new / 1350-CFM old}2 = 1.85185*1.85185= 1.4046634*65" old SP= 0.913" new SP2; way too high!

It is always better to add SA branch runs & more RA area, plus "a lot more RA filter area."

Required fan motor horsepower (hp) varies as to the cube of the rpm speed:

CFM Fan delivery varies directly as to the fan RPM speed:

Duct retro-work can solve the problem, increasing blower HP alone won't usually work well! A few calculations plus retro-work 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)


HVAC TALK HVAC Talk Community Ask me & other Experts FREE - Save Money

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 the above 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.


Only after you have verified that all the coils are clean & the airflow is right-on,
can you begin to check the system's charge using Subcooling method with a Superheat check.

Always check the actual airflow CFM 'before' checking the charge, - get that Right First!

There is a local TXV 1.5-Ton system that has very low airflow, actually less than 200-cfm per-ton of cooling,
they're only checking the suction pressure & saying the charge & everything is okay!

By the way, the furnace is a 112,000-btuh output & requires near 1200-CFM;
it is a CRIME to do such an A/C installation!

That system has a TXV & shows; 98-F condenser saturation temp & 97-F liquid line temp near E-Coil,
a mere 1-F Subcooling, it's undercharged even with a mere 200-cfm per-ton cooling load! Unbelievable, -
but it's happening out there!


Check the
Superheat & the Subcooling as outlined below and always compare to the charging instructions that are with the equipment as some use the
Approach Method & other methods may vary the operating figures & Target figures vary somewhat from Super Heat & Sub Cooling methods!

Those varied methods will usually be close to Super-Heat (SH) & Sub-Cooling (SC) results. I would always use the SH & SC method in conjunction with the
mfg'ers method to trouble shoot  the refrigerant system.

Let's say with a TXV according to the Target Super Heat & your collected data indicates a starved evaporator coil & a normal or slightly off Subcooling,  even if you are using a Mfg'ers Approach Method do NOT automatically believe that it is undercharged.

One company is reportedly having problems with TXVs starving Evaporator Coils, or it could be a partially plugged TXV strainer/screen or other restriction.

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A new easy to use Super Heat & Sub Cooling test instrument:
http://www.ueitest.com/ssm1/

Optimize Evaporator BTU/hr Heat Input  1st always - "Optimize evaporator airflow heatload."

To Determine Super Heat (SH):
*** First, the airflow through the evaporator has to be absolutely correct blower wheel blades & evaporator clean!

1.  Take the Suction Saturation Temperature (SST) reading from your manifold gauge.

2.  Then take the Suction Line Temperature (SLT) as close to the condensing section just before the serve valve.
Actually it is better to take it close to the TXVs power element, but that is inconvenient.

3.  Take the difference between the above readings (Suction Line Temp 'minus' Gauge Saturation temp reading) = Superheat

4.  When ambient air temperature (Outside air temperature) is 85 degrees or above the Superheat should be 8-12 degrees. 
A/C Thermostatic Expansion Valves (TEV / TXV) should usually be set for a minimum of 7-F Degrees Superheat.

Some Heat Pumps with TXV's are set at 7 to 9-F Super Heat or less because they have Suction Line Accumulators to store any spill-over liquid, which protects the compressor.

For TXV Subcooling, take the pressure of the liquid line note the gauge saturation temperature.
Compare it to the actual temperature obtained near the same point the pressure was obtained.
Thermostatic Expansion Valves (TEV / TXV) check with mfg'er Goodman concerning Superheat;
SH varies by mfg'er Goodman TXV's are normally set between 7 to 9 or, 8 to 12-F; Goodman TXVs are also adjustable. Always check Both SH & SC!
Heat pumps with accumulators down to 7-F or less Super Heat.

This linked page is strictly a SUPERHEAT TABLE  Print these Tables & use them!

Print this Two linked pdf pages: Target Super Heat Chart and this Target Temperature Split for Airflow Chart

Here is a formula for getting the Super Heat Target - Within normal perimeters:
((IWB) Indoor Wet Bulb X's 3 - 80 -  (OAT) Outdoor Ambient Temp) / divided by 2 = Superheat Target

5.  If Superheat is low then the evaporator is flooding. Note:   Do NOT adjust charge YET.

6.  If Superheat is high then the evaporator is starving. Note:   Do NOT adjust charge YET!

7.  Do not adjust charge UNTIL Sub-Cooling is checked.


Note:  When charging a system using Superheat, you are charging the unit to the amount of air (CFM) and total heat load that is crossing the evaporator coil
(Or, the amount of latent and sensible heat load being absorbed by the evaporator coil).


Note:   Do not adjust charge based on Superheat on systems with Thermal Expansion Valves (TXV, TEV's), (use Liquid Line Sub-Cooling & Superheat to check the system.
TEV’s control the superheat; you should check the superheat to see if the TEV is working properly.

Use the manufacturers listed & Subcooling numbers, Goodman Superheat Thermostatic Expansion Valves (TEV / TXV) SH varies by mfg'er is normally set between 7 to 9 or, 8 to 12-F; Goodman TXVs are also adjustable. Always check Both SH & SC!

Brands vary;
Some 14 SEER Goodman "TXV" A/Cs wants 7 to 9 degrees of Superheat and 5 to 9 degrees of Subcooling.

To Determine Liquid Line Sub-Cooling (SC):
* First, the airflow through a clean evaporator should always be checked & be correct!
1.  Take the high side pressure and convert it to temperature using chart or gauge readings.

2.  Then take the temperature of the liquid line as close to the condenser as possible..

3.  Take the difference between the above readings. (Saturation Temp – Liquid Line Temp.).
Note: liquid line temperature at the evaporator should be within 2 degrees of liquid line temperature at condensing unit.
If not, could be a restriction or line set too long.

4.  Sub-Cooling with a TXV, Goodman's around 7F +-2F degrees, always check with the mfg’ers for correct SC

5.  Then using the information from Superheat and Sub-Cooling we can have some idea where to look for problems.
Example:
Suction Line Temp is -- 60 degrees @ condenser
Gauge Suction Pressure is --76-psig -- 45 degrees, Read Gauge Suction Saturation Temperature (SST)
65 degrees Suction Linee – 45 degrees = 10-F degrees Superheat - Adjust charge to the mfg'ers SH & SC Settings

Liquid Pressure is  -- 226-psig -- 110-F, Read Gauge - Liquid Saturation Temperature (LST)
Liquid Line Temp (LLT) is -- 105-F
110-F – 101-F = 9-F Sub-Cooling
- Adjust Refrigerant charge to the mfg'ers SC settings; see that SH is within target range.
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"

  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; I.E.; 95-F @ 97-F or 99-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.
(1) Unit owners get a low-cost digital flat-headed pocket Thermometer to use flat on the piping;
these test instruments will PAY big returns!

(2)Get a low cost Testo Tester & ballpark figure actual BTUH & EER - the information on it:
Testo 605-H2 Fast Accurate Wet Bulb

(3)Home owners; -a very low cost anemometer to get airflow FPM Velocities, Info U Need:
http://www.amazon.com/Crosse-Technol.../dp/B0002WZRKE  Search Hart&Cooley for the Sq.FT., or Ak free-areas of diffusers,etc.


Get a low cost Testo Tester & ballpark figure actual BTUH & EER - the information on it:
Testo 605-H2 Fast Accurate Wet Bulb

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This should be helpful, as you can use this info to determine the actual realtime BTUH output of your A/C system:
Take the Supply Air & Return Air wet bulb temperatures & interpose them on the enthalpy chart linked below.

Duct system CFM X* 4.5 @sea-level, or use X* 4.35 if 1000' above sea-level,
X* change in enthalpy = BTUH (Ballpark) Operating Performance.
"U Must Right Click Link & open in New Tab," look-up wet bulb enthalpy figures on chart," & figure enthalpy change.
Wet Bulb Enthalpy Chart

Rules of Thumb for Duct Systems  - Hart&Cooley
DESIGN AND INSTALLATION OF RESIDENTIAL FLEXIBLE DUCTWORK SYSTEMS
http://www.dca.state.ga.us/development/constructioncodes/publications/1ONE.pdf

Look at the ducting, if it is not to code; make hard copies of this code & give it to whoever does the ducting work
Make sure they redo it right!
Never have flex duct interiors commercially cleaned, I just viewed Home Inspection photos showing the interior damaged & insulation plugging the duct.
Home Inspectors warn people because the duct cleaner's tell them it won't damage the ducts. Some HI's look into the boot areas for clues of problems...

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Identifying your registers/diffusers & their (Ak) sq.ft. area, so you can multiply the FPM Velocity times the Ak to get the (CFM) Cubic Feet per Minute airflow from that register.
Have or do a manual J heat-gain calc for each room. If a room calls for 3,000-BTUH; first divide 12,000-BTUH by the CFM PER TON you want to use.

I.E., Wet coil, 12,000/400=30-BTUH per each CFM; Wet coil 12,000/425=28.235294; 3000/28.235-= 106.25-CFM; 
Dry coil, 12,000/450= 26.6666-BTUH; 3500 / 26.6666= 131.25-CFM
If register/diffuser has the same (Ak) free-airflow-area, as the duct run!

Room calls for 3,500-BTUH, using 450-cfm per/ton dry coil or 26.6666-BTUH per CFM= 131.25-CFM.
I.E., 6" rd duct .6*6=36*.7854=28.2744sq.ins/144=0.19635-sq.ft.; 131.25-cfm / 0.19635-sq.ft= or 668.4-fpm velocity.
  • CFM airflow must always be correct before charging a system!
  • Importance of figuring static pressure & Amp draw changes in respect to CFM air flow changes:

  • SP2 (Static Pressure new) = {CFM-new / CFM-old}2 *X old SP
SP2= 2-Ton {800-CFM / 650-CFM}2 = 1.2307692*1.2307692=1.5147928* old SP= 0.98"SP new;way too high!
SP2= 5-Ton A/C {2000-CFM / 1650-CFM}2 = 1.21212 * 1.21212= 1.4692363 * old SP  .60" = 0.88" new; way too high.
SP2= 4-Ton  {1600-CFM / 1350-CFM}2 = 1.85185*1.85185= 1.4046634*.65" old SP= 0.913" new SP2;way too high!

It is always better to add SA branch runs & more RA area, plus "a lot more RA filter area."
  •  SP2 ={(rpm2/rpm 1}2 X SP1 = SP2

  • Fan amp-draw increases as to the cube of the cfm increase
  • Amp2 = Amp1 (cfm2/cfm1)3 cubed *x Amp1= Amp2

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
Duct retro-work can solve the problem, increasing blower HP alone won't usually work well! A few calculations plus retro-work 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)
  • 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
Duct retro-work can solve the problem, increasing blower HP alone won't usually work!
A few calculations
plus retro-work 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 could ballpark the CFM using the static test & a air handler graph. You could measure the CFM delivered to each room with a hood Alnor Balometer,
it's usually the best instrument to use, but not cheap. Measuring the air velocity is a bit tricky because you have to use the diffuser data which you don't always have available.

A rough ballpark formula to get the CFM: CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area, you have to have & use the diffuser data & get velocity there -for operating conditions.) Taking the manifold gage head pressure & gage condensing temp, is important data. Coupled with a TH condenser temp-reading, if the condenser gage pres/temp is too high compared to the TH reading, there may be non-condensibles in the system.

Also, there is a legitimate formula I use to determine the operating BTUH it is delivering at all the data taken. All the mfg'ers ought to list the condenser temp-split (it varies with EER & SEER) just like they list the indoor split, it is valuable trouble shooting info.

You can also use the condenser temp-split (it contains both Latent & sensible heat) combined with the indoor data to plot the indoor CFM. I was never good at math, but those equations have to balance, & they do work!
==================
The A/C user need not know all the tech info, all they need to know are a very few basic ways to identify that the system is not functioning correctly so they can call for a pro-tech to properly trouble shoot the system.

Now the reality is that you don't need a manifold gauge, - anyone can Ballpark check the charge in respect to the indoor airflow with only a little low cost MA-Line digital thermometer using some tubing insulation on the sensor probe.
http://www.ma-line.com/HTML/L1_productmenu.html 
You can't order it there.

However First, you check the discharge air temp off the outdoor condenser, (that is the "Condensing Temperature (CT)," the higher the SEER Rating the lower the outdoor normal temp/split above the outdoor temperature will be.)
Then you either check the small liquid line tubing temp outdoors where there is a tubing bend, or indoors where it bends to enter the plenum. 

Indoors could be inaccurate as the tubing temp could vary higher or lower depending on temp conditions the line is conducting.

Subtract the small liquid line temp from the CT & you have the Subcooling temp which varies some from mfg'ers, but will be Ballparke 8 to 12-F Subcooled.

The condenser temp (CT) above the outdoor temp tells you how much indoor sensible & latent heat, & the 3 motor heats it is ejecting. The indoor humidity level has the biggest effect, the higher the humidity the hight the split.

Now, indoors the higher the humidity the lower the temp/split between Supply-Air & Return-Air.

At 50% indoor humidity the split should be Ballpark, around 18 - 20-F.

If blower wheel blades & all coils are relatively clean & the split is well above 20-F, you have low airflow with a low heatload through the evaporator coil.

If the indoor split is way below 18-F at 50% RH, you either have way too much airflow or some problems in the functioning refrigerant system.

To solve any of these problems you need to call a knowledgeable HVAC TECH! - Darrell U

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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.

============================
It is important to understand that "equipment ratings are only the 'potential efficiency' of that component of the system under perfect conditions." Over half of the system’s efficiency depends on correct equipment sizing run-time, on the duct system sizing, i.e., on the quality of the complete field-installation!

Especially if your system is oversized or there are a lot of low AC load days use an adjustable differential room TH.
TH Differential
: Differential is defined as the difference between the cut-in and cut-out points as measured at the thermostat under specified operating conditions. For example, if the thermostat turns the COOLING EQUIPMENT on at 78-F & OFF at 75-F that is a 3-F degree differential setting. This is a good way to control high humidity problems & also improve SEER performance; plus longer more efficient heating cycles.


What you want & need is right sized equipment operating at its optimal ratings within varying conditions, for your optimal comfort and savings.

*CONDENSING TEMP CT Diagnosis *

| Lennox TXV Subcooling Approach | R410a Evacuation |
 
Differential RM T- stat Improves SEER |*** Sizing Units to Adequate Airflow New İ
CHECK RETURN AIR for HOT AIR LEAKS  | Gurgling sounds at TEV  | Trouble Shooting TXV Valves  Affordable Test Instrument You Need 

The duct system airflow must be checked & correct,
before performing any charging procedures!

My ordered procedure that must be followed to achieve an optimally charged efficient operating air conditioner.

Do not leave out any of the steps and always do these procedures in the order sequence illustrated.

First, check that there are NO air leaks in the Supply and Return Air duct system.
Next, Check to see if Indoor Squirrel Cage Blower wheel blades
are free of lint or other build-up & Filter.
Before checking the refrigerant charge always check the actual air flow with an anemometer & apply the math shown below!

Check for a dirty lint clogged Evaporator Coil fins then check the Condenser Coil fins, check both coils on the air entering sides as well as between the fins, -- clean if needed.

Take a look at the ductwork for proper sizing and for leaks, Check the External Static Pressure (ESP), check the indoor Cubic Feet per Minute (CFM) Airflow, then outdoor condenser discha
rge air Temperature split (delta-T), then indoor Delta-T, then
after 15 minutes of run time before any charging adjustments are made.  On smaller tonnage equipment & in most climates that are not overly humid, I like 425 to 450-CFM per/ton of cooling on a wet coil.

In summer, Air conditioner & heat pump user's, this is a "Simple Easy Safe Way" to Check an A/C's Thermostatic Expansion Valve  (TXV) metering system's refrigerant charge & any A/C's  Performance:
Record the outdoor temperature, then "Take a thermometer & check the air temperature coming off the outdoor condenser."

Locate the small uninsulated copper line where it makes a bend, outdoors or indoors, so you can put a digital thermometer flat on it & insulate the TH with something (piece of the tubing insulation) read the temperature & subtract it from the outdoor condenser discharge air temp.

E.g., condenser temp 110, line temp 100-F that is 10-F subcooling & 10 to 12-F is a normally charge system. If it's considerably less than 10-F it could be low on refrigerant or need a Tech to check it; too far above 12-F it's overcharged, call a Tech, could also be a restriction in the lines.

You should own a low cost digital pocket thermometer:
Now the reality is that you don't need a manifold gauge, - anyone can Ballpark check the charge in respect to the indoor airflow with only a little low cost MA-Line digital thermometer using some tubing insulation on the sensor probe. An A/C supply Outlet might sell you one or inquire at hardware stores! Saves money on needless A/C Service Calls!
http://www.ma-line.com/HTML/L1_productmenu.html

Take the outdoor temperature & subtract it from the outdoor condenser's discharge air temperature.
This temperature is standard for different SEER Rated units.
An older 12-SEER unit, with 50%  INDOOR Relative humidity ballpark should be 18 to 20-F temp split.
A 14-SEER will be around a 14 to 16-F temp/split. if too far above or below those temps, call for an A/C Tech.


Formula for finding CFM Airflow
If you can measure the air velocity coming from a duct, 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)
Example,
16" Rd duct 201-sq.ins. X's 0.00694 = 1.39494-sq.ft. X's Velocity of 800-fpm = 1,116-CFM
Times 1000-FPM = 1395-CFM.
Branch ducts: 7" Rd duct 38.48-sq. ins. X's 0.00694 = 0.2670512-sq.ft. X's 500-fpm=133.5-cfm
However, times a velocity of 600-FPM
X's 0.00694 = 160-CFM, the velocity is a big room CFM & BTUH number changer.
-----------------------------------------------------------------------
Quick Check for Sizing Units to enough Airflow

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TROUBLE  SHOOTING  TXV  VALVE SYSTEMS 

Bulb location: Some Mfg’s have there preferences, but a good rule of thumb is 10 or 2 O’Clock, away from headers and heat exchangers, on a smooth clean surface. 

Also, make sure the cap tube is on top (horizontal or vertical and never upside down).

Pressure drop: TXVs like to have at least 100-psi pressure drop across them to operate correctly. A solid column of liquid (at the valve) is also a requirement.

Flood back: Always make sure you have "the correct CFM airflow" (clean coils,"
Clean fan blades
& fans running on correct speeds and in the right direction) before you try adjusting a valve.

No flow: A plugged screen is rare in air conditioning, but happens often in refrigeration.
I have seen the
external equalizer tube leak through in liquid form and give the bulb a false reading (which causes hunting more so than no flow).
-------------------------------
TXV or filter driers partially plugged up stream from Service Gauge Port LOW SUCTION Pressure, if downstream from service port partially plugged Compressor Inlet screen high suction pressure.

In HVAC-TALK  These Posts Illustrate and Discuss these Test Results


Lennox TXV Subcooling - Approach Method -  pdf P- 8
http://www.davelennox.com/pdfs/installation_maintenance/Lennox-12ACB-IOM.pdf

Always check both SH & SC for trouble shooting comparison to normal parameters!
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Even on service calls where there are cooling problems the ductwork should have a quick Manual D performed.

Take the ESP static pressure & compare to blower graph or chart, also take the FPM duct velocity.
For Room Return Air balancing, i.e., -.01" IWC = approximately -2.48 Pascals, which is a more precise easier incremental scale to read. 
One inch water column (IWC) equals rounded to > 250 pascals,
0.5" IWC is about 125 pascals; 0.25" WC = 62.5 pascals; 0.125 = 31.25 pa.;
1 / 250 pascals =0.0040322 *X's  -2.50 pascals = -0.01003657696655" IWC or make it 
- 0.01" IWC for low Return Air room pressure differentials; - use pascals.

Here are some examples of residential or light commercial in Inches Water Column (IWC) Pressure Drops. Check the engineering data as you design, and you may be surprised.
This is mainly only the example of the Device pressure losses.


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 udarrell




First, before any operating performance feedback data is recorded, the following tests & corrections must be performed. We need to make certain that the airflow is checked to be within the proper parameters and that the ductwork is properly sized and sealed with return air grilles in every room at the ceiling level. In addition, if possible for the cooling mode, Supply Air Diffusers should also be at the ceiling level.

Accurate tests should also be made, to determine whether proper airflow CFM is being delivered, as well as into each room.

The manufacturers could be of great help in this respect, if they would list the Delta-T of the condensing unit at different BTUH load output levels, also at various outdoor ambient temperatures.

I believe that with adequate test data feedback the subcooling could be targeted within plus or minus 1 or 2 degrees Fahrenheit, which would be a two to four degree differential.

That would be more accurate & concise a temperature target than present subcooling temperature targeting methodologies.

Additionally, I would consider using the Lennox Approach Method to help select the Subcooling Method.

The Lennox Approach Method subtracts the Outdoor Ambient Temperature from the Liquid Line Temperature (LLT), whereas, the subcooling temperature targeting method subtracts the Liquid Line Temperature near the evaporator from the Condenser Saturation Temperature (CST). I do not see why the Lennox Approach Method would not help pinpoint the subcooling target on other systems. The Indoor Heatload has to be part of the equation; there are other factors to incorporate as well.

Possible Diagnosis using Super-Heat and Sub-Cooling:
If Superheat is high and Sub-Cooling is low:
Charge must be adjusted. System is Undercharged.


If superheat is low and sub-cooling is high: 
Charge must be adjusted. System is Overcharged.


If Superheat is very high and Sub-Cooling is a little high: 
Could have blockage in coil, TXV strainer screen - settings, etc., orifice, filter dryers etc.


If Super-Heat is low and Sub-Cooling is low:  
Piston orifice could be too big, or some put in in backwards, there is no orifice in the unit or the orifice is stuck and refrigerant is bypassing it.


To Determine Delta T  (Td) (Temperature difference across the coil):
1.  While unit is running take the temperature of the air in the supply plenum near the coil (approx. 12 inches.)
2.  Then, while the unit is still running, take the temperature of the air in the return plenum near the unit.
3.  Then take the difference between the above readings.
4.  Depending on indoor humidity & airflow volume; should be around 14-20 degrees.  Use linked Chart above!
5.  If to low then coil might not be seated in pan correctly - air bypassing cooling coil. (Assuming superheat and Sub-Cooling are OK.)
6. A TXV's normal
Superheat setting is 7 to 12-F, between 7-F to 9-F on heat pumps with an accumulator.  There must be a full liquid stream to the TXV!

With a TXV metering device if Superheat is too high say, 16-F or above — look for, suction line restriction, hot gas discharge line restriction, filter dyer, downstream of suction service port, or compressor screen restriction or inefficient compressor.
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What are the proper methods to determine operating superheat, sub-cooling?

Superheat at the evaporator should be checked as close to the end of the coil as possible (preferably near the expansion valve thermal bulb). Convert this to saturation temperature and compare it to the actual temperature obtained near the thermal bulb.
Take the suction pressure at the service valve and convert it to saturation temperature. Compare this to the actual temperature obtained approximately six inches out on the suction line.

Subcooling should be checked as close to the condenser as possible & then as close to the TXV as possible noting the difference.
============
With a TXV metering device if Superheat is too high say,  20-F or above — look for, suction line restriction, plugged cap tube/orifice or, TXV screen plugged./liquid line restriction, hot gas discharge line restriction, filter dyer restriction, downstream of suction service port, or compressor screen restriction or inefficient compressor.
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What are the proper methods to determine operating superheat, sub-cooling?

Superheat at the evaporator should be checked as close to the end of the coil as possible (preferably near the expansion valve thermal bulb). Convert this to saturation temperature and compare it to the actual temperature obtained near the thermal bulb.
Take the suction pressure at the service valve and convert it to saturation temperature. Compare this to the actual temperature obtained approximately six inches out on the suction line.

Subcooling should be checked as close to the condenser as possible & then as close to the TXV as possible noting the difference.
============

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 suction pressure changes it's a fixed piston or 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 may 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 or too little SP.
 
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 fair 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
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 - pulsation noises in Liquid Line at the expansion device can be caused by low evaporator circuit heat-loads, 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! "Put your ear on the liquid line at the evaporator coil."

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."

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.
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Check Return Air (RA) at grille & at entry of blower for heat gain, due to hot Return Air leaks.
Where airhandlers' set over Return Air Chambers check for air leaks through the sheet rock & down the wall studs from  the attic - this is a fairly common condition that will overload the AC system!

DISCLAIMER:
Any of the HVAC companies I list on any of my web pages have nothing to do with the information I post on any of my Web pages nor do I assume any responsibility for how anyone uses that information.
All HVAC/R work should always be done by a licensed Contractor! This information is only placed on these pages for your understanding & communication with contractors & techs.

This information is for the edification of contractors and techs. I am NOT liable for your screw-ups, you are liable for what you do! - Darrell Udelhoven (U-dl-hoven)

Darrell's Refrigeration Heating and Air Conditioning - Federal Refrigerant Licensed - Retired HVAC Contractor

Suction Low-Side
Suction-PSIG -
Normal to High
SUCTION
Super-Heat -
HIGH
 High-Side
Head-Pres. -
LOW
Liquid-Line
SUB-Cooling -
NORMAL
Cond.-Unit
Amp-Draw -
LOW

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