Knowledge Center

Bigger is Not Better: Sizing Air Conditioners Properly

It is generally accepted that “the right way” to specify an air conditioning system is to calculate the loads and select a piece of equipment that will provide comfort to the customer in a wide variety of conditions. Unfortunately this is rarely practiced.

A colleague of ours (we will call him Bill) approached us at a conference seeking advice on selecting an air conditioner for his renovated home. Our recommendations included, “Be sure that the cooling load is calculated and that the air conditioner is sized to that load.” When Bill attempted to follow these instructions, only one of the four contractors would submit a sizing calculation (two others just wanted to know how many square feet there were in the house). Bill hired the contractor who did the calculation and installed a high-efficiency four-ton unit. Is this a success story? Not really.

The contractor calculated a total cooling load of 37,580 Btus per hour at 105�F outside and 70�F inside. While the cooling load he calculated could have been met by a three-and-a-half ton air conditioner, the contractor convinced Bill to buy a four-ton unit “because then you will always have plenty of cooling.”

Bill’s air conditioner short-cycles (runs for shorter periods of time than it should) even during the hottest weather and removes very little moisture from the air. What went wrong? Four things:

  • The design temperature for the air is 97F. The contractor increased the outside design temperature by 8F.
  • The recommended design indoor temperature is 75F. The indoor temperature was lowered by 5F. The temperature “fudges” increased the inside to outside differential by 59%.
  • The contractor increased the calculated load by 20% as a safety factor.
  • The equipment selected was a half-ton larger than the next highest available size to meet the load he calculated.

A two-and-a-half ton air conditioner would have been perfect for Bill’s house. Instead he paid more for an extra one-and-a-half tons of cooling. In addition to costing more to buy, Bill’s air conditioner will use more energy than a properly sized system, raising his utility bills. It won’t dehumidify the air as well as a smaller system would, and chances are that Bill will be less comfortable. The utility, which gave Bill a rebate for his purchase, will also lose, since the oversized unit aggravates summer peak-load requirements.

Selecting the Right Air Conditioner for the Job

Before one can design an efficient and effective air conditioning system, the load must first be calculated using established techniques. The Air Conditioning Contractors of America (ACCA) conducted an industry study of residential cooling load calculations and developed Manual J to estimate these loads. Manual J was adopted by ACCA and the Air-Conditioning and Refrigeration Institute (ARI), and is the standard method of sizing loads for residences.

ACCA has also produced Manual S for selecting equipment and Manual D for duct design (revised in January 1995). Manual S provides a method to select air conditioners based on the estimated sensible and latent load calculated for the particular house in the local climate.

If mistakes are made in the load calculations or the sizing method is flawed or incorrect inputs are used, the equipment will be incorrectly sized and will not perform as it should. Field studies have shown that most equipment is substantially oversized compared to Manual J specifications. In the Model Energy Communities Project, Pacific Gas and Electric Company (PG&E) found that 53% of the air conditioners checked were a ton (12,000 Btu/h) or more oversized and a study by Pacific Northwest Laboratories found a third of the air conditioners to be a ton or more oversized.

Because of the efficiency penalty of oversized air conditioners and because oversized air conditioners contribute substantially to utility demand peaks, in 1994, PG&E commissioned a study by Proctor Engineering Group to compare common load calculations and sizing methodologies to Manual J calculated values.

What is “Proper” AC Sizing?

Since optimum efficiency is achieved at continuous running, it is important that the air conditioner be sized to achieve the longest run times possible. Manual J specifies use of the 2.5% design temperature as developed by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE). For instance, a 2.5% summer design temperature of 100�F for Fresno, California, means that the temperature generally only exceeds 100�F for 73 hours in the season (0.025 x 2,928 hours in the months of June through September). A theoretical perfectly-sized air conditioner will run continuously during those 73 hours. During the rest of the time the air conditioner will cycle and operate at less than its potential efficiency.

A properly sized air conditioner should provide maximum value to the customer as well as a reasonable profit and further customer referrals for the contractor. If an air conditioner is cycling even at four in the afternoon on the hottest days, it is a sure sign it is oversized. Incidentally, if the air conditioner is running continuously on hot days, it doesn’t necessarily mean that it is the right size. It is more likely that the system is oversized and has one of three big problems: leaky ducts, improper charge, or low air flow across the coil.

Oversizing: Causes and Effect

Customers depend on the expertise of contractors in selecting air conditioners. Yet contractors generally size air conditioners at least a half-ton larger than necessary and often oversize by a ton or more. Even the most conscientious contractor is driven to avoid call-backs (or even lawsuits). An oversized air conditioner can mask problems from duct leaks, improper flow across the coils, and improper charge. Unfortunately, many customers think that “bigger is better,” so in a competitive situation, the contractor proposing the proper size unit may lose the bid. Contractors are hesitant to adopt an unfamiliar method of sizing when the methods they have developed over the years have served them well: “I’ve done it this way for 30 years and I’ve never had a complaint.” It is no surprise then that air conditioners are oversized; however, the advantages of a properly sized air conditioner are so large that these barriers need to be overcome. Customers pay a price for oversized air conditioners, and in many climates, lose comfort as well.

A properly sized air conditioner costs the customer less. Bill’s air conditioner cost him more money because it was too big. The contractor had the opportunity to discuss the value of the air conditioner based on the delivered efficiency and offer Bill equipment at a lower cost. He missed the opportunity.

Short Cycles

Air conditioners are very inefficient when they first start operation. It is far better for the air conditioner to run longer cycles than shorter ones. The efficiency of the typical air conditioner increases the longer it runs. If the on-time of an air conditioner is only 5 minutes the efficiency (EER) is 6.2. If a properly sized air conditioner half the size were used instead, the same amount of cooling would take place in about 9 minutes, and the efficiency would rise to 6.9. This represents a savings of 10% for the customer. Most of the cooling season the cooling loads are well below the capacity of properly sized air conditioners, and for oversized units the short cycling is a substantial problem. Because of the short cycles, Bill’s high-efficiency air conditioner is less efficient.

Moisture Buildup

The ability of the air conditioner to remove moisture (latent capacity) is lowest at the beginning of the air conditioner cycle. The moisture removed from the indoor air is dependent upon the indoor coil temperature being below the dew-point temperature of the air. The moisture then wets the indoor coil and, should the unit run long enough, will begin to flow off the coil and be removed out of the condensate drain. For short cycles, the coil does not have time to operate at the low temperature and when the unit stops, the moisture on the coil evaporated back into the indoor air. Thus, in humid climates, a properly sized air conditioner will do a far better job of removing moisture from the air than oversized units. Bill’s oversized air conditioner could not remove enough moisture from the air, so his house was cold and clammy.

Noisy Operation

The speed of the air blowing through the supply registers and the air being drawn into the return grille affects an air conditioner’s performance. If the air speed is too high, it will be noisy and uncomfortable, and the return grille filter effectiveness will be reduced. The speed through the grilles depends on the size of the air conditioner (a larger unit has more air flow and higher air speed) and the area of the grille (a smaller grille causes higher air speed). With a properly sized air conditioner, it is easier to have sufficient supply and return grille area to keep the air speed low and the noise at a minimum. Common complaints about oversized air conditioners are that they blast frigid air and that they are noisy. A properly sized air conditioner, with proper ductwork and grilles, will provide longer cycles, more consistent temperatures, and better mixing of the house air.

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