How to Size a Furnance for Your House

Selecting the correct-size furnace for your home is an important task. Getting it right ensures the efficiency of the furnace and wards against problems with the unit. A furnace that is too small for your home will run for longer periods of time, which increases heating costs and often does not allow for the proper heating of your home. A unit that is too large will turn on and off frequently, decreasing the efficiency of the furnace and potentially causing damage due to moisture that builds up inside the heat exchanger because the furnace doesn't run long enough to evaporate the condensation that forms in it.

Tape measure
Calculator

Instructions

- 1
  Calculate the required BTUs (British thermal units) needed to heat your home efficiently. Measure the length, width and height of each room with a tape measure.
- 2
  Multiply the length times the width times the height to get the volume of each room. Multiply this number by five for rooms with one to two windows. Multiply this number by six if your room has more than two windows. Multiply the number by seven for rooms that face the north. North-facing rooms tend to be cooler and harder to heat.
- 3
  Add together the calculations for each room. The result represents the minimum BTU rating your furnace will need to properly heat your home.
- 4
  Look at the BTU rating on your existing furnace if you are replacing the unit. If the rating is drastically far from the rating you determined in Step 3, you must ask yourself how well the existing furnace performed, and consult a professional.

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