The Importance of Properly Sizing a New Furnace
Be Sure to Get the Most Out of Your Heating System
Nov 4, 2009
A new furnace costs a lot to buy and install. The cost to run a furnace can also get steep, especially during colder weather, so you want your heating system to operate efficiently as possible at all times. You also depend on your furnace to provide warmth when required throughout the home. Again, you pay for this comfort.
To increase the likelihood you'll be satisfied with this important purchase, be sure to choose a furnace that is carefully sized to match the unique heating requirements of your home. A greatly oversized furnace cycles frequently and won't operate at peak efficiency, while an undersized unit will not deliver enough heat on really cold days.
In addition, you wouldn't want to spend extra money on a larger furnace, if you didn't need to.
Heat Loss Calculation
To properly size a furnace, a heat loss calculation must first be performed on the home--typically by the heating contractor. This analysis takes into account: house age; area of heated space; window efficiencies and total size, and; insulation depth in attic, walls and basement.
The analysis indicates how much energy, measured in British Thermal Units (BTUs), is required to adequately heat the home during extremely cold temperatures. Coincidentally, furnaces are sized in BTUs and come in increments of 10,000 BTUs.
An air depressurization test also determines a home's heating requirements, but limited heating companies offer this test.
The Right Furnace Size
Gas furnaces have a BTU input and output number. The closer these numbers are together, the more energy-efficient the heating system is.
The input number reflects the amount of energy going into the heating system, while the output number shows the maximum amount of energy distributed throughout the home in the form of heated air.
To obtain the right size, the BTU output of the new furnace should match the BTU number (to the nearest 10,000) determined through the heat loss calculation.
Choose Your Heating Contractor Wisely
Unfortunately, not all heating contractors perform a heat loss calculation. In fact, some even base the new furnace size solely on the existing system, which can be problematic. (In the past, heating systems were commonly oversized, but operated at lower efficiencies anyway. The cost of energy also was much cheaper than it is today, so wasting energy wasn't much of a concern.)
As a final word of advice, when scoping out a furnace contractor, choose one that will perform a heat loss calculation on your home.
The furnace is the heart of your home and if it's too big or small, you may be greatly disappointed with your purchase.
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