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Why You Shouldn't Use BTU Per Square Foot When Sizing Your HVAC System

Posted by Gregg on Apr 18, 2017 12:10:19 PM

BTU per square footAt Stay Comfy we’re often asked about the correlation between a home’s size (square footage) and BTUs. In particular, we’re asked whether or not the home’s square footage should be used to determine the size of the HVAC system to install. Keith, our resident expert, says an emphatic NO! Here’s why he says not to use “BTU per square foot” as a way to determine the size of your HVAC system.

Why “BTU Per Square Foot” Can’t Be Accurate

Keith says the way to best understand why “BTU per square foot” can’t be accurate is to use a real-life example:

  • Imagine a first-floor room that’s 10 x 15 feet. It has one 10-foot exterior wall and three interior walls, one small window facing north, an air conditioned room above it, and let’s say it’s a bedroom.
  • Now consider another 10 x 15-foot room. This one has two exposed walls and two interior walls and an insulated ceiling overhead. It has four large windows—two facing south and two facing west, one exterior glass patio door, and a vaulted ceiling that averages 12-feet in height. Let’s say it’s a recreation room.

Both rooms are 150 square feet. But what about the energy it will take to cool or heat these identically-sized rooms? Let’s compare:

  • The bedroom will have very little heat gain (the absorption of external heat, primarily from the sun). The only exposure to outdoor heat is one wall, 10 x 8 feet, or 80 square feet. One small window facing north will have no solar gain and very little transmission gain (heat gain based on the temperature difference and R value of the window).

  • The recreation room has 25 feet of exposed wall with a 12-foot height that equals 300 square feet of outdoor exposure. Add to that 150 square feet of ceiling, four large windows facing south and west (for a lot of solar heat gain), and the transmission loss of the large window area. The patio door is essentially another very large window adding to both solar gain and transmission loss. In addition, a rec room may have electrical or electronic devices that generate heat, which adds internal gains to the total. Plus, infiltration can be a big heat gain. Infiltration is the amount of outside air entering through the cracks and crevices of the doors and windows. And since the rec room has a lot of windows and a patio door, there will be a significant addition to the heating/cooling load.

As you can tell from these hypothetical rooms, the actual room size has little to do with the heat gain or loss of that room. Consider the factors of heat gain already noted: windows and doors, infiltration, exposed wall area, and ceilings. Then add the internal components of lighting, electrical devices, cooking appliances, and so on. And we haven’t even gotten into R values and U values—the individual building components and the rate at which they lose or gain heat. Even without those, the 150-square foot rec room could have a heat gain three to five times more than the bedroom! That’s going to call for very different cooling needs. The same goes for heat loss when sizing for a furnace or space heater. Windows, doors, and infiltration are the biggest factors and will determine what the actual heating needs are.

The bottom line is this: any method that doesn’t account for individual building components and heat gain and loss factors will simply not be accurate. The best advice we can give is to consult your HVAC professionals. They’ll be able to provide an accurate analysis and calculation for both heating and cooling.

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Topics: HVAC