To achieve optimal efficiency & the highest operating SEER Rating long runtimes are essential.

It takes a lot of amps during startup & it takes around 5 minute to reach optimal cooling performance.

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, when 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 MTR 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 900-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.
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Quick Check for Sizing Units to enough Airflow

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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)
. 
Never sell units requiring more airflow than the duct system will support! - Darrell udarrell
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Measuring Low Airflow

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
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!

SEER	7 SEER or less	8 SEER	9 SEER	10 SEER	12 SEER	13 SEER
'Max' condenser air temp 'delta-T'	30	25	24	+23	21	ave. <less
Max temp drop 'across' E-Coil	20	22	24	26	ave. >more	ave. >more
'Max' SA/Return Entering Air 'Delta-T'	33	30	26	23	19	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
http://hvac-talk.com/vbb/showthread.php?t=175142&page=8
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!
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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 absolutely know whether the metering device is a TXV, or a fixed orifice device, or cap tube. 

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

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

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

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

Squirrel cage wheels with forward curved blades on residential systems
unload when discharge air is blocked off too much & will overload
when there is no static pressure.
 
There is a preferable ESP range for each Air Handler blower design, that ought to be listed on the blower; they vary at the point of serious unloading.
 If you amp-probe check enough of those blower motors, if the amp draw is too low according to its rating, you can begin to tell that the 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
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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.  

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® 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.
12-SEER
1.5-Ton 18,000 18ºF T.Split    Cond.CFM 1400 @ 50% RH, 600-CFM; 85ºF 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 ODB | 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
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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!)