The most common sizing mistake is in oversizing. This 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. Oversized heating equipment also often creates uncomfortable and large temperature swings in the house. Oversized air conditioners (and heat pumps) do not run long enough to dehumidify the air, which results in the "clammy" feeling and unhealthy mold growth in many air-conditioned houses.

It is the installer/contractor’s job to perform the correct sizing calculation for the building. However, many installers only check the "nameplate" (the label on the unit that has 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 necessary capacity. Also, since many people have added new windows, caulking, weatherstripping, and insulation to their homes, going by the nameplate is likely to result in an oversized system. Making improvements such as these to reduce heat loss in the winter and heat gain in the summer should allow you to install a smaller systems while still being comfortable, as well as saving large amounts of energy.

Correct system sizing requires consideration of a many more factors than simply reading the nameplate of the existing unit. Key factors for correctly sizing a heating and cooling system include:

  • The local climate
  • Size, shape, and orientation of the house
  • Insulation levels
  • Window area, location, and type
  • Air infiltration rates
  • The number and ages of occupants
  • Occupant comfort preferences
  • The types and efficiencies of lights and major home appliances (which give off heat).

Correctly Sizing Heating and Air Conditioning Systems
Building owners should insist that contractors use a correct 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 in the United States. There are also many user-friendly computer software packages or worksheets that 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 affect a home’s heating or cooling requirement or "load." A good estimator will measure walls, ceilings, floor space, and windows to determine the room volumes, and will assess the R-value of the home’s insulation, windows, and building materials. A close estimate of the building’s air leakage is also 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 of seals on joints and insulation, and location of the distribution ducts in forced air systems. The placement of supply and return registers, should be appropriate for the system type and size.

The orientation of the house also affects 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 oversizing air conditioners.

Sizing Heaters and Air Conditioners: Quick but Inaccurate Methods
The following are some of the "quickie" methods some contractors may use to size a system. They are also somewhat useful for very rough sizing. NEVER use any of these to determine the final size.

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.

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, insulation levels, design, construction, and energy efficiency or intended use of the system. You may get different answers from different contractors who use this 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.

Another rough method for sizing heating systems involves a prepared chart such as the one below. 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 that best describes the house. Then, multiply both the higher and lower numbers for heat loss in Btu per hour per square foot (from the table) by the floor area of the home to give you a rough range for the heating load.

Home Type or Characteristics—Heat loss (Btu/hr/ft2)

No insulation in walls, ceilings, or floors; no storm windows; windows and doors fit loosely 90 to 110
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
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
"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
Earth-sheltered house with little exposure; well insulated. 10 to 13

For example, if a home’s energy-saving features are best described by #2, and the home has a heated space of 1,500 square feet, then 1,500 x 50 and 1,500 x 70 is the heating load range. Roughly 75,000 to 105,000 Btu/hour (18,900 to 26,460 kilocalories/hour.)

Although a chart like this looks official, not all houses fit the profile given. There is also no accounting for the other factors mentioned above.

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. The final bid should be based on the results of the use of a procedure such as Manual J.

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 printout or finished worksheet. This is your only proof that they did the job right.

Credit: U.S. Department of Energy