On a cold winter day, a cup of coffee can warm one person, and a blanket can keep a couple snuggled together. But to keep an entire family warm in 1,600 square feet requires a properly sized furnace. One that is too small will not work adequately, while one that is too large will cycle on and off too frequently.
The British Thermal Unit is a unit of measurement that frequently defines the heating capacity of a furnace. The Energy Information Administration defines BTU as the amount of energy needed to raise the temperature of 1 pound of water by one degree Fahrenheit under normal atmospheric pressure. Electricity provides about 3,412 BTUs per kilowatt hour, while natural gas has 1,027 BTUs per cubic foot. You can calculate the output BTU of a furnace by multiplying its input BTU by its efficiency rating. For example, a unit with 100,000 BTUs at input and an efficiency of 90 percent produces 90,000 BTUs of heat.
Michaels Engineering, an environmental consulting company, suggests a quick way to estimate BTUs required for residential furnaces in cold climates. It recommends multiplying the living space by 40 to 45 BTUs for the hourly requirement. In this case, a 1,600-square-foot home would need a unit that produces from 64,000 to 72,000 BTUs per hour. Another method is to ask neighbors who have similar 1,600-square-foot homes what their furnace capacities are. If each uses a different type of furnace, such as gas or oil, ask the amount they pay for energy during the winter months and during the year.
Alpine Home Air Products offers an online sizing estimator that shows how location (via Zip code) can affect the furnace size of a 1,600-square-foot home. For example, in 90210, which is sunny Beverly Hills, California, a 36,000-BTU furnace is adequate. In the Seattle, Washington, Zip of 98101, where temperatures are cooler but the winter is mild, furnaces must produce 56,000 BTUs. In the harsh weather of Chicago, Illinois (60601), BTU requirements climb to 59,000. And in perpetually cold Barrow, Alaska, at 99723, furnace needs jump to 85,000 BTUs. See Reference 1 and click "Try our Online Sizing Estimator."
The most accurate way to size a home furnace, according to Consumer Reports, is to contact a contractor. He can use industry standard calculations from Manual J, "Residential Load Calculation" of the Air Conditioning Contractors of America. Among other factors, this document accounts for the climate; the design of the home; the number of windows, doors and other spaces where air can escape; the insulation in the walls, floor and ceiling; the orientation of the walls toward sun and shade; the number of occupants, and the location and leakage of air ducts.