Ask Focus on Energy

This can be tricky, read on.

The heating or cooling equipment should be sized according to the home's heat gain and loss (the amount of heat entering or exiting your home). Key factors for correctly sizing a heating and cooling system include the following:
• 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).
ACCA (Air Conditioning Contractors of America) offers Manual J, the HVAC industry's most accepted way to calculate a home's heat gains and losses. Manual J uses a number of factors, including wall area, window area, insulation values and exposure to determine the heating and cooling needs of a home. If you are getting different equipment sizes on your bids from contractors, talk to them and ask how the equipment size was determined.
Unfortunately the calculation takes some time so contractors aren't likely to include it with a free estimate. Most contractors will size the equipment based on the existing furnace, use a rule of thumb, or make some assumptions about the home and then use software (probably based on Manual J) to determine a size.

To make matters more confusing two contractors could come up with the same btu (British Thermal Unit - a measure of heat) requirement and still quote two different sizes. Furnaces will come in either 20,000 btu or 25,000 btu sizes. This means that contractors will quote different sizes because that's where your home falls in their lineup. In your case the 60,000 btu unit is larger than your home needs, but the contractor was not comfortable with next smaller unit in their line (probably 40,000 btu). The second contractor felt comfortable that their 45,000 btu unit was big enough. Two different sizes, both the right size.
 

Published Thursday, November 11, 2010