What Size Furnace Do I Need and How is it Determined?
Choosing a new furnace for your home means evaluating a number of features, and at the top of that list should be selecting the right furnace size. Even the most efficient furnace model, if too big or too small, will not function properly. You can approximate the correct size on your own, but be sure to discuss how to calculate furnace size accurately with your HVAC technician.
Why does furnace size matter?
The size of the furnace is critical for performance. Size impacts the furnace’s efficiency, cost and lifespan, as well as its ability to provide comfort in your home. Understand that over the life of the furnace, bigger does not necessarily mean more powerful (as in providing more heat) and smaller doesn’t necessarily mean less expensive. The most important thing is to choose the right size.
If your furnace is too big…
An oversized furnace is more likely to heat your home unevenly. It will turn on when it gets cold and heat up some areas of the home too quickly. When those rooms become uncomfortably hot, the heater will shut off, leaving other areas of the home still cold. That frequent cycling is less efficient and raises your energy costs.
If your furnace is too small…
Similarly, a furnace that is too small will be unable to adequately heat your entire home, leaving some areas cold and causing the system to run longer or turn on frequently. The extra wear and tear will possibly lead to more breakdowns and repair and may also shorten the lifespan of the furnace.
How to determine the right size furnace for your home
There are two ways to determine the correct size furnace for your home.
1. Approximate Calculation
Determine the square footage. This may be written somewhere in your purchase documents, or you can measure it yourself. Next, determine the base BTU. The British Thermal Unit is the amount of energy used to cool one square foot of your home, approximately 25 BTUs. Multiply the number of square feet by 25 to get the base BTU measurement. Lastly, if your home has high ceilings (over 8 feet) increase the BTU amount by 25%.
2. The Professional Method (Manual J)
Your HVAC technician can obtain a more precise measurement using Manual J calculations, which consider a number of factors in addition to square footage. These include the number of people in the home; the climate; the number, placement and orientation of windows; the amount of insulation in the walls and more.