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 can when doing the calculation audit. Additionally, load reduction remedial actions should be provided as options toward further reducing Air Conditioning and heating equipment sizing.

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 airflow, air temperature rise across the condensing coils, and the entire performance data. Provide your customers with more than they paid for and you will have more business and solid referrals.

Measuring Low Airflow

First, before doing anything else check the sizing, and thoroughly seal and properly insulate all the ductwork!

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%	79oF	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!

EER	7 EER or less	8 EER	9 EER	10 EER	12 EER	13 EER
'Max' condenser air temp 'delta-T'	30	27	24	15 to 25	14 to 24	ave. less
Max temp drop 'across' E-Coil	20	22	24	26	ave. more	ave. more
'Max' SA/Return Entering Air 'Delta-T'	35	32	29	27	ave. less	ave. less

The Supply Air & the Entering Return Air delta-T, - tends towards less & less as the EER goes higher,
therefore, dehumidification could become more difficult at the highest EER levels. The EER & SEER levels widen, as SEER sky rockets.

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.

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 76-F that is a 2 degree differential setting; heating equipment on at 70 degrees F and turns the heating equipment off at 74 degrees F, then the differential is 4 degrees F. Some have half degree increment settings over several degrees of differential spread.

For example, in Rockford, IL a 2,400 sq./ft home with 600 sq./ft of window area, 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.

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 varification: 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.

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 0.85 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 appears to be under heat-loaded or, an unbalanced heatload on the DX coil's circuits.

  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.

If you do not know whether the metering device is a TXV or orifice? 
Hook up your manifold gauges, block off considerable condenser air intake. 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.

If it is a piston, get the CFM airflow  correct first, then 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 specs.  

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

1.5-Ton 18,000  21-F Split       Cond. CFM 1400      WATTS 1536 1.5-Ton is from actual published DATA - ARI Rating Conditions
2-Ton  24,800  23-F T.-Split     Cond. CFM 1400     WATTS 2659
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 ODB | 63-IWB | 52% RH | 20-F ID Delta T | 18,600-Btuh
201-psig 100-F = 15-F cond. temp split - smaller capacity compressor to larger coil areas | 80-psig suction
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http://www.udarrell.com/air_return_latent_condenser_split.jpg 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!)