Sizing Residential Heating and Air Conditioning Systems

Older space conditioning systems (more than 10 years old) are often unreliable and much less efficient than a modern system. When it's time for a new replacement, choosing one of the correct size (heating and/or cooling output) is critical to getting the best efficiency, comfort, and lowest maintenance and operating costs over the life of the new system. Some national surveys have determined that well over half of all HVAC contractors do not size heating and cooling systems correctly.

The most common sizing mistake is in over-sizing. This is not only makes the new system cost more to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Over-sized air conditioners (and heat pumps) do not run long enough to dehumidify the air. This results in the "clammy" feeling and unhealthy mold growth in many air conditioned houses. Over-sized heating equipment often creates uncomfortable and large temperature swings in the house.

It is the installer/contractor's job to perform the correct sizing calculation for the building. However, many poorly trained installers only check the "nameplate" (a metal tag that tells you the Btu per hour, output among other things) of the existing system and sell you one just like it, or even worse, one that's larger. This is a not a correct sizing method and not in your best interests! Before the era of tightly constructed homes, it was not uncommon to install furnaces and air conditioners that had two to four times the needed capacity. Also, since many people like to try to conserve fuel and make their homes more comfortable by installing new windows, caulking, and weatherstipping, and adding more insulation, going by the nameplate will most certainly result in the wrong size being installed. Making improvements to the way the house holds energy in allows you to install a smaller system while still being comfortable, as well as saving large amounts of energy.

Correct sizing requires a great many more items (i.e., level of insulation; size, type and location of windows; air infiltration, how many people live there, the local climate, etc.) than simply reading the nameplate of the existing unit.

Building owners should insist upon a correct system sizing calculation before signing a contract. This service is often offered at little or no cost to homeowners by gas and electric utilities, major heating equipment manufacturers, and conscientious heating and air conditioning contractors. Manual J, published by the Air Conditioning Contractors of America (ACCA), is the most common method in use. Many user-friendly computer software packages or worksheets can simplify the calculation procedure. You should make sure that the procedure used by the contractor follows Manual J or one of the approved standards in the Bibliography below.

Many factors effect a home's heating or cooling load. A good estimator will measure walls, ceilings, floor space, and windows for the accurate determination of room volumes. Also, a good estimate takes into account the R-value of the home's insulation, windows, and building materials. A close estimate of the building's air leakage is necessary. A blower door test is the best way to measure air leakage.

A good estimate will also include an inspection of the size, condition (how well joints are sealed and the ducts are insulated), and location of the distribution ducts. The placement of supply and return registers, should be appropriate for the system type and size. The orientation of the house also effects heat gain and heat loss through windows. Overhangs can reduce solar gain through windows. Make sure the contractor uses the correct design outdoor temperature and humidity for your area. Using a higher summer design temperature results in over-sizing air conditioners. Underestimating the latent (humidity) load (energy used by the air conditioner to remove moisture from the air) results in undersized air conditioners.

Any bid should include an agreement to provide written calculations (listing the procedures and standards that will be followed), equipment and installation warranties, a payment schedule, and a firm completion date. When the contractor is finished, get a copy of their calculations, assumptions, and the computer output or finished worksheet. This is your only proof that they did the job right.

Here is a list of "quickie" methods some contractors may use. They are also somewhat useful for a very rough idea as to what you need to buy. NEVER use any of these to determine the final size.

1. The contractor walks in the house, looks at the existing unit, and recommends that the replacement unit be the same size, or larger. This obviously does not take into account any improvements made to the house or mistakes made in sizing the original unit.

2. The contractor asks you how many square feet of living space there are in your house, then tells you what size unit you need. This is called "sizing by square footage" and is the most commonly used inaccurate method of sizing. A typical value used for air conditioners is one ton (12,000 Btu/hour) per 500 square feet (46 m2). This does not take into account differences among house orientation, design, construction, and energy efficiency or intended use of the system.

3. You may get different answers from different contractors who use the previous technique. In that case, they may have a different "rule of thumb," or one of them may be using the "lowest cost" method. This involves adjusting the square footage rule so that whatever the contractor has in their warehouse becomes the right size for you. Since the "in-stock " unit costs the contractor (but not necessarily you) less to install, this becomes the "lowest cost" method.

4. Another poor method involves a prepared chart such as the one below, which is for heating systems. You use the chart in the following way. First, determine the floor area of all the heated rooms, and the levels of insulation in the floors, walls, and ceilings. Next, find the category (under description) that best describes the home. Then, multiply both the upper and lower values for heat loss in Btu per hour per square foot (from the table) by the floor area of the home to roughly estimate the required heating range.

Home Type or Characteristics ..... Heat loss (Btu/hr/ft2)
1) No insulation in walls, ceilings, or floors; no storm windows; windows and doors fit loosely .... 90 to 110
2) R-11 insulation in walls and ceilings; no insulation in floors over crawl spaces; no storm windows; doors and windows fit fairly tight. ..... 50 to 70
3) R-19 insulation in walls, R-30 in ceilings, and R-11 in floors; tight-fitting storm windows or double pane windows. ..... 29 to 35
4) Superinsulated house with R-24 wall insulation, R-40 in ceilings, and R-19 in floor; tight-fitting storm windows or double pane windows; vapor barrier sealed carefully during construction. ..... 21 to 25
5) Earth-sheltered house with little exposure; well insulated. ..... 10 to 13

For example, if a home's energy-saving features are best described by category 2, and the home has a heated space of 1,500 square feet (139.35 m2), then the design heating load is roughly 75,000 to 105,000 Btu/hour (18,900 to 26,460 kilocalories/hour) (1,500 X 50 and 1,500 X 70). Although a chart like this looks official, not all houses fit the profile given. There is also no accounting for the thermostat temperature setting, the location of the house, the shape of the house, or many other factors.

To save some time the above methods are often used for a first "guess" or rough estimate. If so, then it should be plainly stated to you that this is the case. However, DO NOT USE THESE ESTIMATES for the final sizing.

The following publications provide additional information about load calculations and sizing heating or air conditioning systems. The publications are based upon standards approved by professional organizations. This bibliography was reviewed in December 1999.

ANSI/AHAM RAC-1-1992, Room Air Conditioners, Association of Home Appliance Manufacturers (AHAM), 1992. Available from AHAM, 20 North Wacker Drive, Chicago, IL 60606, (312) 984-5800 x315. 25 pp., $7.50.

ASHRAE Standard 90.2-1993: Energy Efficient Design of New Low-Rise Residential Buildings, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, 1993. Available from ASHRAE (see Source List below). 107 pp., $84.00.

Cooling and Heating Load Calculation Manual, GRP 138, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1992. Available from ASHRAE (see Source List below). 209 pp., $80.00.

Heat Loss Calculation Guide No. H-22, (1st ed.), Hydronics Institute, 1989. Available from Hydronics Institute, 35 Russo Place, P.O. Box 218, Berkeley Heights, NJ 07922, (908) 464-8200. 63 pp., $14.00 plus shipping.

Residential Duct Systems, Manual D, (2nd ed.), Air Conditioning Contractors of America, 1995. Available from ACCA (see Source List below). 200 pp., $30.00.

Residential Equipment Selection Manual, Manual S, Air Conditioning Contractors of America, (2nd ed). Available from ACCA (see Source List below). 115 pp., $40.00.

Residential Load Calculation, Manual J, (7th ed.), Air Conditioning Contractors of America, 1988. Available from ACCA (see Source List below). 126 pp., $30.00.