What Size Oil Furnace Is Required for a 1200 Square Foot House?

Measures

• Two units of measure are important in sizing an oil furnace. The first is the British Thermal Unit or BTU, which represents the energy needed to raise the temperature of one pound of water by one degree Fahrenheit at normal atmospheric pressure. The input capacity of an oil furnace is often stated using this unit of measure. The second number is the fuel efficiency rating, which is expressed as a percentage. This represents the amount of energy that gets converted to heat, with higher numbers being better. To determine the energy output in BTUs per hour, multiply the input BTU by the efficiency rating. For example, a furnace with 100,000 BTU of input and an efficiency rating of 89 produces 89,000 BTUs of output. One with an efficiency rating of 80 and the same input amount, produces only 80,000 BTUs of output.

Guidelines

• AC4Life, a supplier of home heating and air conditioning products, offers a basic sizing method that depends on the climate zone of a home. It states that a home in a mild climate zone, such as Texas or Florida, uses about 30 to 35 BTUs per square foot. (The lower number applies to well-insulated structures, while the higher is for poor or no insulation.) A house in the middle zone of the country, such as in New York or Missouri, needs 40 to 45 BTUs per square foot. One in the harsh zone, such as Maine or Montana, needs 50 to 60 BTUs. Using this guideline, a 1,200-square-foot home in a mild zone requires a furnace with 36,000 to 42,000 BTUs. One in the middle zone needs 48,000 to 54,000 BTUs. And one in the harsh climate sound requires 60,000 to 72,000 BTUs.

Other Methods

• Alpine Home Air Products offers two additional methods for sizing an oil furnace. The first is to look at the nameplate on the furnace you currently have. This typically lists the input BTU and the fuel efficiency rating, which you can use to calculate output BTU. You can buy a smaller furnace to produce the same output BTU if it has a higher efficiency rating. The second is to ask a neighbor who has a house with 1,200-square-feet in a similar design. If she has a correctly-sized oil furnace, that size will also work for you.

Contractors

• Consumer Reports states that the most accurate way to size a 1,200-square-foot home for an oil furnace is to contact a heating contractor, who can then use calculations listed in Manual J from the Air Conditioning Contractors of America. This document takes into account the climate; location and orientation of the walls to the sun; insulation levels; number of windows, doors, skylights and other openings; and the number of occupants and their living habits. Contractors can also use Manual D from the same source to determine whether a home’s heating ducts are adequately sized for the furnace.