How to Determine Heating Requirements per SF


A heater or furnace that is sized too large or too small for your home cannot perform well for you. A heater that is too large will turn on and off frequently, decreasing the heater's efficiency and wasting energy and money. A heater that is too small will not be able to heat your home adequately on those cold winter nights. Heaters and furnaces come with a BTU, or British thermal unit, rating that specifies how powerful the unit is. By calculating your home's heating requirements per square foot and finding the BTU output you need, you will determine the proper size heater your home needs.

Things You'll Need

  • Tape measure
  • Calculate the total square feet of your home. If you do not have the square foot measurement from a home inspection, you will need to measure the width and length of each room to calculate the square footage of each room in your home. Once you have the square foot measurements of each room, add them to find your home's total square footage.

  • Measure the height of your ceiling. Multiply this measurement by your home's total square footage to obtain the number of cubic feet your home contains.

  • Subtract the lowest outside temperature in your area from your ideal inside temperature. For example, if you want to keep your home at 70 degrees Fahrenheit and the lowest outside temperature is 10 degrees, then you would subtract 10 from 70 for a difference of 60 degrees.

  • Multiply your temperature difference by the cubic feet of your home. Divide this number by 55. Divide again by the total square footage of your home. This gives you the heating requirement per square foot of your home, in BTUs.

Tips & Warnings

  • A British thermal unit is the heat needed to raise the temperature of 1 pound of water by one degree Fahrenheit.
  • BTU calculators take into consideration sources of heat loss -- such as windows and doors -- and may give you a more accurate BTU rating.
  • The number of windows you have and the type of insulation your home has will alter the BTU rating you need. Windows and doors increase the BTU rating you need as they add to your home's heat loss.

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