Air Conditioning System Sizing for Optimal EER & SEER Efficiency

The Proper Sizing of Equipment, and Duct Sealing with Proper Airflow CFM will greatly reduce cooling costs!

    - with Darrell Udelhoven > udarrell

Free Load-Calc  
This Free online calc will help you determine equipment sizing & point-out areas that need efficiency retro-work - Once you calculate the page it saves the inputs for up to 24 minutes or, until you change inputs or close your browser.
You can
easily reduce infiltration rates yourself, therefore, I’d use 0.4 ACH (I'd Add the suggested (Air Changes per Hour) CFM into the ‘Fresh Air Recommended ‘line-slot, or it won’t figure the Infiltration & fresh air Btuh. After you get it right click on the SIZING link then WE can compare the unit's performance numbers to meet sizing numbers. A fifth grader can do this stuff...

You can experiment with changing the design temperatures in both heat & cooling, (or start-over showing the New reduced Btu/hr SIZING) also to see whether the equipment exceeds, 'in each' of the 3 cooling categories; Total Btuh, Sensible Btuh & Latent Btuh.

Using Goodman Manufacturing or other Expanded Cooling Data; e.g., using a 2-Ton 13-SEER, R410A, @75F 50%RH Indoors; "outdoor 90F," 'interpolate' between Charts 85F & 90F ambient, @800-CFM Btu/hr is 21,550, S/T is .765 * 21,550 is16,486-Btuh Sensible; 21,550 - 16,486 is 5,064 Latent. That's the simple close enough math...A fifth grader can do this stuff...

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

The critical importance of selecting the proper equipment components for your climate zone anchor link

Cynergy Home Energy Raters Listen While Reading | RETURN-AIR FILTER GRILLE SIZING

*Basics Featuring the Testo 556 -*Video of a very thorough Air Conditioning BTUH performance test

A lot of HVAC Instructional Videos at the bottom of this page!  |* HEAT PUMP DIAGNOSTICS 
Cynergy Home HVAC Energy Raters Listen While ReadingDesign Engineering Objectives |TH Differential|Equipment Ratings|
Sizing Units to Adequate Airflow |
50% Load- Home Air Leaks | Energy Star ACCA pdf 
Determining Metering Device TXV / Fixed Orifice |
Affordable Test Instruments Techs Must Own & Use
|Check** CONDENSER TEMP DIAGNOSTICS | Customers Simple Check! Is your A/C 30 to 50% Oversized, but NOT COOLING?
Government initiated Energy Audits - CNN audio| Oil Furnace Airfow Problems HVAC-Talk FORUM @#10 | Local Contractor Locater Map - Talk with These Forum Contractor PROs in your area
HVAC-TALK General Discussions: Importance of Two-Way Communications with HVAC Customers

To achieve optimal efficiency & the highest operating SEER Rating longer runtimes are essential.
It takes a lot of amps during startup, & it takes more than 5 minutes to reach optimal cooling performance.

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. A fifth grader can know this stuff...

"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. (Common Sense...)

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

Get a low cost Testo Tester & ballpark figure actual BTUH & EER - the information on it:
Home Owners, very low cost anemometer to get airflow FPM Velocities, I'd get it:

This should be helpful.

CFM X change in enthalpy X 4.5 = BTUH (Ballpark) Operating Performance & EER
"U Must Right Click Link & open in New Tab"

The critical importance of selecting the proper equipment components for your climate zone

It is essential to understand why the local weather environment dictates what SEER level air conditioning equipment you should choose.
In choosing equipment and its SEER level, it is important to understand the design engineering behind its functional capabilities.
First, when the engineers designed for higher Seer levels, they increased the volume and the BTU per hour capacity of the condenser coils and the evaporator coils; however, they reduced the BTU per hour capacity of the compressor.

The volumetric capacity of that smaller compressor depends on the absolute suction and discharge pressures under which the compressor is operating. The higher SEER units also have a much larger quantity of refrigerant charge than the older 10 & 12-SEER units had.

The lower volumetric capacity ratio of the compressor to the higher coil capacities only works well in an 82°F laboratory weather environment with a 50% relative humidity level,which is never a stable operating condition in the real world environment.

When you select the high Seer units in a climate where you have high outdoor sensible temperatures along with a high humidity, the temperature pressure ratio of the evaporator coil skyrockets as does the condenser coil pressures and temperatures, therefore the smaller capacity compressor in its relationship to the coils becomes overloaded.

Here comes the engineering caveat, if you are in a high temperature high humidity climate zone the evaporator pressure temperature ratio will be so high that there will be very little condensation of the moisture in the air. Additionally, always select an evaporator coil with a TXV thermostatic expansion valve refrigerant metering device as it will keep the coil colder at varying lower outdoor temperature conditions.

Also, buy a digital programmable room T stat that also has a cycles per hour (CPH) or a swing setting from 1 to 9 so you can reduce the number of short cycles while increasing the runtime of each cycle. This can really increase SEER performance & help a lot to control humidity problems in the summer cooling mode.

Additionally, the volumetric capacity of the smaller compressor will not be able to handle the increased volume of vapor. Added to this the condensing pressure will be much higher with also reduces the volumetric capacity of the compressor which is rated at less than the BTU per hour of both coils.

The higher SEER level you select above the 13 or 14-SEER levels the ratio between the lower BTU per hour capacity of the compressor compared to the evaporator and condenser coils becomes worse. Therefore, if you are located in a hot or, a hot and humid climate from an engineering standpoint and a performance standpoint, I do not believe it is a wise decision to go to extremely high SEER rated equipment.

There maybe rare exceptions to the above statements, if a variable speed compressor and variable speed blower motors are used. When these components are used, you should make sure that the contractor proves to you, before you buy the equipment, that the combination of components will work properly in your climate zone.

A Major "Oil Furnace" Low Airflow Problem - Fix
Regal & Hallmark & nearly all Oil Furnaces - Installation manuals
Download the installation & service manuals from ABOVE LINK BELOW LINK MAY NOT WORK!
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.

Wet Bulb Enthalpy Chart

The North Country Oil furnace A/C or heat pump scenario:

Here is just one scenario; the small one story home with a basement requires only 14,000-BTUH of cooling but it has a 112,000-BTUH Oil furnace with a belt-drive quarter HP blower motor.

Three things have to be done right; first, the evaporator coil has to be sized to flow at least 1250-cfm that requires a 3-ton coil.

Second, the evaporator coil has to be mounted at least 6” above the Oil furnace to eliminate an airflow restriction between it and the super large heat exchanger near the top of the furnace.

Third, the belt drive motor has to be replaced by a multi-speed direct-drive blower motor that will deliver the correct 1250-cfm for heating & 600 to 675-cfm airflow for cooling.

I have witnessed a 2-ton evaporator coil being installed directly on top of the Oil furnace & the quarter HP direct drive motor left in place.

Can you cite the horrendous problems this creates?

Think through what you’re doing & the consequences before doing it!

Required fan motor HP varies as to the cube of the rpm blower speed.

Also, at 700-rpm & .2" SP for heating my Thermo Pride OL 11 with its quarter Hp motor will deliver 1200-CFM;  add a cooling coil, & at .5 SP it will deliver only 400-CFM.

Keeping the total static pressure as low as possible and within mfg'ers ESP requirements for air conditioning is the first requirement in an efficient system design.

BTW, what is the average pressure drop across the new +90 high efficiency furnace condensers? That pressure drop should be published by all of the companies!


Rules of Thumb for Duct Systems  - Hart&Cooley

Optimizing efficiencies: 
We could cut residential heating and cooling equipment size in America by 30% to 50% if Contractor's would perform honest Manual J calculations, and provided full credit for every load reducing element or detail they could do prior to equipment sizing, prior to doing the initial load calculation audit.

Air infiltration rate, can be half the load, & should be checked & reduced. Ductwork & airflow must be checked & optimized for full nominal BTUH performance.

Additionally, load reduction remedial actions should be provided as options toward further reducing Air Conditioning and heating equipment sizing.

Then undersize equipment just a little, while optimizing the ductwork & thus reducing blower motor HP & its heat, while optimizing airflow through the evaporator coil.

The comfort level is never as good with short cycling oversized units; & it is very hard on equipment.

Smaller units draw less electricity; I use a Half-Ton window unit for nearly 700-sq.ft., it uses around 500-watts or less, my brother has a 1.5-Ton central unit & the indoor blower draws as many watts as my entire window unit. 

Down sizing has many advantages; the existing duct system & air handler will function more efficiently helping to achieve the unit's nominal tonnage capacity with less than peak heatload conditions. That improves EER & SEER performance.

Contractors' should use the equipment manufacturers blower data and the Manual D procedures to find the room cubic feet per minute (CFM) airflow values and then use published performance data to select the appropriate sized supply air outlet, type and size, for each room. There also should be a low resistance return air path for every room that has a supply outlet, door undercuts are borderline acceptable.

Manual D procedures should be used to size all the duct runs, and systems should comply with ASHRAE standards; completely seal all runs located in an unconditioned space and insulate these runs to preferably R-8.

Contractor's should Certify the work they have done, i.e., —measured all air flows, balanced the air distribution system and then used certified protocols to check and balance the refrigerant charge. After all the installation work has been done, the Operating Performance Standard Data of the operating System should be Certified by the contractor. This should Include the static pressure readings, CFM of the system's airflow, air temperature rise across the condensing coils, SA/RA dry bulbs & wet bulbs, the entire performance data. Provide your customers with more than they paid for, and you will have more solid referrals and more business.

The North Country Oil furnace A/C or heat pump scenario:

Here is just one scenario; the small one story home with a basement requires only 14,000-BTUH of cooling but it has a 112,000-BTUH Oil furnace with a belt-drive quarter HP blower motor.

Three things have to be done right; first, the evaporator coil has to be sized to flow at least 1250-cfm that requires a 3-ton coil.

Second, the evaporator coil has to be mounted at least 6” above the Oil furnace to eliminate an airflow restriction between it and the super large heat exchanger near the top of the furnace.

Third, the belt drive motor has to be replaced by a multi-speed direct-drive blower motor that will deliver the correct 1250-cfm for heating & 600 to 675-cfm airflow for cooling.

I have witnessed a 2-ton evaporator coil being installed directly on top of the Oil furnace & the quarter HP direct drive motor left in place.

Can you cite the horrendous problems this creates?

Think through what you’re doing & the consequences before doing it!

Quick Check for Sizing Units to enough Airflow
Home owners, 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/diffuser velocity.

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.
E.G., Wet coil, 12,000/400-cfm per/ton=30-BTUH per each CFM; Wet coil 12,000/425-cfm per/ton=28.235294; 3000/28.235-= 106.25-CFM.
E.G., 6" rd duct .6*6=36*.7854=
28.2744sq.ins/144=0.19635-sq.ft.;using 450-cfm per/ton dry coil: 131.25-cfm / 0.19635-sq.ft= or 668.4-fpm 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 udarrell

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 550-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. 

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

Don't have supply air diffuser engineering data: free air area of Hart & Cooley, one, 2 & 3-way diffusers is about 50% of the sq.ft. area of the diffuser's listed size. Multiply that figure by FPM velocity to get the CFM.  Also, search, Lima & J&J Register for their (Ak) numbers.

Google search Hart & Cooley, also this pdf might help you select the right diffuser for the particular application, & list (Ak) free sq.ft., area of the diffusers:

Recommended Return-Air main velocities should be 600-fpm or less, on Branch Return Air ducts try for 550-fpm or less velocities.
Keep air velocities through the RA Filter(s) as low as possible.

Typical disposable 1-inch capacity return air filter is 2 cfm per square inch, I USE 1.5-CFM PER SQ.IN.)
of gross filter area. Recommended filter velocity is 300-fpm, lower is better.  See the math below.

 Velocities higher than 500 fpm will decrease filter performance. Increase flow resistance, and possibly
blow off collections of dirt. Measure Velocity 1” from RA grille face.

Average Free Air area of most Return Air grilles about 75%.
Never sell units requiring more airflow than the duct system will support! - Darrell udarrell

========================= HVAC-TALK - Forum - Approximately 9/27/10

You're going to gasp at what I suggest will greatly improve your (3-Ton) Return Air filtering situation, & blower efficiency...

A filter grille, with a clean low resistant filter in accordance with Manual D, should initially have 300-fpm velocity.

To achieve 300-fpm velocity it will need 600-sq.ins.,/ by 144 or 4.1666-sq.ft of free air area.

The formula is 300-fpm X's 4.1666= 1250-CFM or 3-Ton of airflow.

The problem is that the filter Grille & the filter, both, reduce the free air area!

Therefore, when you figure the area needed, they say, 2-CFM per, I'd use 1.5-CFM per, of the actual grilled area, not the outside measurements. 
1200-CFM / 1.5 = 800-sq.ins., or two return filter grilles. You could even use two 500 or more, filter grille racks; sounds far out huh, but it will work just fine.

Hart&Cooley usually just show the RA Grille sizing, Not the filter grille, plus the the filter's Ak sq.ft., open area! 
That always calls for increased sizing for more Efficient Filtering & Return Airflow Efficiency.  ha...

You will never get too much RA filter area, the more the better, because as the filter loads the velocity will go above 500-fpm velocity where the filter begins to allow too much debris blow-by.

Filter box depth sizing

Having a large filter/grille area is of little value if there is insufficient filter box depth, so that the RA duct is way too close to the filter/grille.

Therefore, the depth of the filter/grille box &, when room permits, its funnel shape to the 14” duct collar is also critically important. Figuring 650-CFM each RA duct run, that's 608-FPM Velocity. A 16" return duct flowing 650-CFM would produce 466-fpm velocity.
CFM / fpm velocity = sq.ft. area.

Filter mfg'ers ought to be required to list the sq.ft., free-air-area of their new filters! So, what is the free-air-area of the filter you are using?

Figure two 14” duct runs, if jammed too close to the filter, each duct collar opening would only be 154-sq.ins., which explains why the filter box depth is critically important to an unimpeded return airflow. - Udarrell
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. Also by 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. says A/C Tech guru, 'Stretch'

First, before doing anything else, check for correct duct sizing, and thoroughly seal and properly insulate all the ductwork! Always check the ESP & Airflow, record the results!

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













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!

7 SEER or less
'Max' condenser air temp 'delta-T'
24 +23
ave. <less
Max temp drop 'across' E-Coil
ave. >more
ave. >more
'Max' SA/Return Entering Air 'Delta-T'
ave. <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.

California Research Report on EER SEER  pdf - download 07/23/08, SEER Payback Savings cannot be accurately represented!

AHIR - SAVE ENERGY - CALCULATORS  Find Your Best Payback Investment Return

When a typical HVAC contractor quotes the efficiency of the Air Conditioning equipment's SEER & Btu/hr, and leads you to believe the new equipment will automatically deliver that SEER efficiency & Btu/hr rating, think again. Typically, --installed equipment only operates at 55% to 70% of rated capacity. Oversized equipment is the worst combination there is because the duct system airflow and heatload on the cooling coil are often way off what is required!

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 toward adequate run-time, on the duct system sizing, i.e., on the quality of the complete field-installation!

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

If all contractor's would do the above, coupled with installing equipment sized according Manual J loads (with no safety factor), along with Manual S selection procedures, comfort would go up, humidity control would improve, and installation and operating costs would be much lower.

Utility demand loads could be cut by at least a third, or even up to a half. Energy loads would be significantly reduced, reducing our nation's energy usage. The return on the time and effort invested on this higher quality level of work would be tremendous - for customers, the community and the nation.

Unfortunately, most HVAC contractors don't use these procedures to size equipment and design duct systems. It's estimated that only 10% of heating and cooling equipment sizing decisions are based on some type of Manual J calculation and that less than 1% of the jobs are based on an aggressive accurate implementation of these recommended design procedures.

Many if not most contractors are designing new and replacement systems that feature oversized equipment, "improperly sized supply outlets" and duct runs that are too small, too leaky and inadequately insulated.

The Manual J gives appropriate answers if you use an “aggressive” set of assumptions. However, most HVAC contractors tend to skew input data to make the calculations match their favorite rules of thumb. Follow the manual J rules and you will get a reasonable margin of safety. However, after skewing the numbers, many contractors throw in an extra half ton or more of A/C to feel safe. No wonder a large percentage of equipment is considerably oversized. Also, the airflow is usually so compromised on the oversized units that it isn't putting out many more btuh than properly sized equipment would be, but it's an energy waster and is costing and arm and leg to operate.

TH Differential: I know some have cycles per hour settings, etc.
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.

Okay, with 8 foot ceilings, 19200 cu ft air volume at 1000-cfm that's 60000 cu ft per hour, or just over 3 air changes per hour, with the added long run times to reduce humidity. Would you rather have the costly oversized 4 Ton Unit?

Perhaps the Manual J could be improved; however, if used with "integrity" it can deliver good results. However, contractors that don't want to size according to an honest manual J calculation simply change some of the inputs to make the procedure deliver answers they feel safe with, and, --they are never challenged.

First, you need a tight, well insulated building with good windows. Then you need to make sure the person performing the load calculations uses accurate information, and doesn't “fudge or skew the numbers." After they have the equipment sizing answer, "they must be certain the ductwork is properly sized, sealed and well insulated." It's not rocket science and it is time all Contractors are held to a new set of codes and ethical standards.

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 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. Some high-end new condenser's have a TH that opens the control voltage circuit when the discharge line becomes too hot.

Some HVAC Forum Talk - Go there
Originally Posted by udarrell View Post
This (forum thread) scenario ought to be a learning lesson for ALL OF US.

In order to make sure we are doing it right, "everything has to be done in the proper sequence."

Before we as contractors sell a customer equipment, we need to know what went into the construction of the house, without accurate information a manual J will NOT be accurate & could be way off base.

If there are too many things wrong with the construction, "those things have to be fixed before any load calcs or equipment sizing is done."

Energy Conservation & its Efficient Use, is not simply about selling high SEER Rated equipment! For many applications, the reality could be, that a 13 to 15-SEER system could be a much better choice than a 20 to 23-SEER system.

If we as contractors' want satisfied customers, we better speak the truth to them, & put the focus on the only sequence that has the potential to work toward optimal comfort, coupled with efficient performance, along with a good choice toward a decent return on their valuable dollars invested!

CONDENSER TEMP-SPLITS - My Brother's Oil Furnace with a 2-ton coil (mounted directly on top of the Oil furnace) for an 1.5-Ton 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 0.85 X's= 1887 X's * 3.413 = 6,443-BTUH Motor Heat additive +18400= Motor Power "Rated Gross Heat Ejection" is 24,843-BTUH / 1400= 17.7-F  = 17.5-F Temp Rise Cond/Split. His condenser only gets a 10 to 12 temp rise split, the evaporator is under heat-loaded due to low airflow.

My Scan of the Thermo Pride 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

The new Goodman 13-SEER 1.5-Ton Condenser, 2-Ton Evaporator:
At 675-cfm 450-per/ton cooling | 85-F ODB | 63-IWB | 52% RH | 20-F ID Delta T | 18,600-Btuh
201-psig 100-F = 15-F cond. temp split - larger coil areas | 80-psig suction

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 TXV sensor bulb is on to be too cold and the TXV 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-charged. 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 turbulence back-pressure and an imbalanced DX coil circuitry heatload!

Many DX coils are delivering half to a ton less btuh than the rating of the condenser. Either remedy the problems or install a smaller condenser sized to the achievable DX coil's heat transfer limits.

What is included or excluded in the capacity ratings in respect to motor heat Btu/hr that does nothing to reduce the total heatload of the conditioned air. Motor heat is a factor to be dealt with, perhaps more so on the smaller units.

Finding out which metering device 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

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 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.
Normally plus or minus 3-degrees is acceptable for Subcooling, i.e., SC Target of 12 +/- 3 = 15 or 9-F SC.
Affordable Test Instruments Techs Must Own & Use!
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:

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 what the operating system is delivering in BTUH, including condensate lbs/hr, & actual sensible & latent cooling BTUH & Ratio, every data detail imaginable. - Darrell

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.

====================================================== ======================================================
Do your own figuring based on this formula. Get the Motor Power Factors (PF) of the compressor and fan motor from the manufacturers.

12-SEER units - Comfortmaker® | Heil® | Temp Star® others - used 0.88 Motor Power Factors
ARI Conditions are: 95ºF-OAT; 80ºF-IDB; 67ºF-IWB or 50%RH | TVA conditions; 95-OAT; 75ºFIDB; 63-IWB  or around 50%RH | Try 85ºF-OAT
Heil 12-SEER 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; 18ºF condenser split | @ 85ºF OAT; 67-IWB or 66.5%RH; +20ºF cond. split. | 
Outdoor Ambient Temperature = (OAT)
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 differenence in the condenser split.  (Airflow & proper 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.
1.5-Ton/18,400 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 - Low ID Airflow!
1.5-Ton 18,000 17ºF Temp/Split    Cond.CFM 1400 @75-F IDB & only 50% RH, 600-CFM; 85ºF Oudoor Ambient Temp (OAT)
2-Ton  24,800  23-F T.-Split     Cond. CFM 1400     WATTS 2659 
(All ARI Conditions)
2.5-T  30,200  20-F Temp-S     Cond. CFM 2000     WATTS 3404
3-Ton  35,600  17-F Temp-S    Cond. CFM 2800     WATTS 4107
3.5 T  42,500  19-F Temp-S    Cond. CFM 2800     WATTS 4554
4-Ton  48,500  18.5-F Temp-S  Cond. CFM 3400   WATTS 4761
5-Ton  59,000  23-F Temp-S    Cond. CFM 3400    WATTS 6969

The new Goodman 13-SEER 1.5-Ton Condenser, 2-Ton Evaporator:
At 675-cfm 450-per/ton cooling | 85-F OAT | 63-IWB or 52% RH | 20-F Indoor Door Temp-split | 18,600-Btuh
201-psig 100-F = 15-F cond. temp split - smaller capacity compressor to larger coil areas | 80-psig suction

================================================================= IE Browser's
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. (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

Like to lower your Air Conditioning Utility Bills?

Look at all the ways you can to reduce the heat and humidity load: caulk, weather-strip, insulate, radiant barriers, to keep the heat and humidity outside.

Have a manual J heat-load calc done, and in humid climates, consider slightly undersizing a replacement condenser.

If you have an older furnace, there may not be enough airflow through the evaporator coil.

Have all of the ductwork, diffusers, return-air grilles and filter areas checked and properly sized, the ductwork must be properly sealed.

In cold climate, consider replacing the furnace with a new high efficiency condenser furnace.

Then replace the central A/C system, Go for a scroll compressor & a TXV refrigerant control on the evaporator.

By slightly undersizing the condenser and making certain you have an optimal and balanced heatload on the cooling coil during normal run-time conditions, —the unit will have a much greater BTU/hr heat transfer output during average run-time conditions!

With a properly sized system and 450-CFM per/ton of cooling going through the evaporator coil, you will have consistent optimal nominal-capacity heat absorption, coupled with longer run-time cycles for optimal humidity control.

I believe that optimal efficiencies, with variable latent/sensible ratio heatloads, could be effectively achieved through the use of computerized control system components.

Air Temperature Drop Through Evaporator Coil (1987 Period)
Indoor temperature and humidity load variations graph.
Refrigeration & Air-Conditioning (ARI) Second Edition,
Page 624, © 1987

Getting it right makes all the difference in the world.

This Energy Star ACCA pdf is interesting & confirms what many of us have been preaching, including educating the potential customers!

Scroll down to page 5 Market Conditions, & study to learn how we can all profit, customers included...

There are more ACCA links but this is a good starter document.

Air Infiltration sources DTI Corp Catalog
DTI Corp

Proper Sizing of Residential Heating & Air Conditioning Ductwork Systems 

Insulation & Weatherization Costs Forum

I do NOT assume any responsibility for how anyone uses the information on my Web pages.
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 what you do, you are liable for what you do!  - Darrell Udelhoven

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


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

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  • ac-trouble-shooting-chart.html

  • AC Superheat and Subcooling   

  • The Air Side of Air Conditioning - Static Pressure
  • Excessive Airflow coupled with an Excessive Charge will greatly reduce AC capacity

  • Air Conditioning SEER Levels & Evaporator Air Flow - Are you losing 15 to 40% under SEER Rating?


  • SEER Payback is NOT being Properly Represented to the Consumers
  • Determining Your Air Conditioner's Actual BTUH Capacity Output 

  • Proper Sizing of Residential Heating & Air Conditioning Ductwork Systems

  • Air Conditioning Engineered for Latent Heat Removal  For high humidity climates
  • Air Conditioning Contractors  Discussion Sizing SEER EER Latent Heat
  • An Air Conditioning and Heating Efficiency Check Up - Contractor

  • Air Conditioning Maximum Efficiency - Check-Up  Get your A/C optimized for efficiency

  • Air Conditioning - Latent Heat Removal  Comfort-Zone Efficiency

  • FINDING the LATENT HEAT of CONDENSATION of Your Air Conditioner

  • Optimizing Room Air Conditioner's EER     
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    *Video checking static ESP *Video 2 checking static ESP View Videos! Got DSL 11/2010

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

    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
    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.

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

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



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