Furnace Sizing BTU Calculation
Sizing a furnace for your home can be difficult. All furnaces have a BTU rating that measures their power, but you first need to know how many BTUs it will take to heat your home.
All About BTUs
The BTU, or British Themal Unit, is a means of measuring how much heat a furnace generates. Your home has fairly specific heating needs, and purchasing a furnace that closely meets them is necessary for both comfortable temperatures and energy efficiency. You can easily calculate the BTUs your house needs using simple mathematical formulas.
The size of a home is the most influential factor in determining its BTU needs. The larger the space to heat, the more warmth the furnace must give off. Multiply the width, length and ceiling height of every room in your house to find its dimensions in cubic feet. Don't total the numbers yet, but keep the volume of each room separate.
Heat is generated by sources other than your furnace, as well -- two of the most common are kitchen appliances and human bodies. Simply living in your home will heat it to a certain extent, reducing the amount of work your furnace has to do. Account for this by multiplying the area of your kitchen by 3, your bedrooms by 4, and all other areas by 5.
The external environment also can impact your house's temperature. Windows are particularly susceptible to lost heat unless they feature multiple panes for insulation. North winds are also a common source of cold. Add 15 percent to the number for each room with a north wall and 20 percent for all rooms featuring large windows. Double or triple-glazed windows subtract 10 percent from a room's number, however. Add the numbers of all your rooms, together with a final 10 percent to the total to compensate for general heat loss, to calculate your home's BTU requirements.