The size of a furnace is critical to efficiency. A system that's too large wastes energy warming up and cooling down as it continuously cycles off and on, and a furnace that's too small will fail to warm the house on really cold days. A properly sized system is designed as closely as possible to the needs of the house; on the coldest days of the year, it should run almost continuously.

The size of a heating system is measured by its "Btu" per hour input or output capacity, depending upon the manufacturer. A Btu (British thermal unit) is equal to the amount of heat required to raise one pound of water one degree Fahrenheit in temperature.

Input capacity refers to the amount of gas burned without taking into consideration the percentage of heat lost through the flue. Output (or "bonnet") capacity refers to the total usable heat generated.

Most manufacturers make several sizes of each model. For example, the Lennox Pulse furnace is available in 40,000, 50,000, 60,000, 80,000, and 100,000 Btu input capacities . These furnaces come in "upflow," "downflow," and "horizontal" models designed to accommodate basement, attic, or limited-space installation.

Choosing the right size and type is a job for a qualified central air conditioning contractor or dealer. Proper size should be based on energy-loss calculations figured from your house size, window areas, insulation levels, and related features.

You can't just replace a furnace with a high-efficiency one that's the same size unless you match output capacities; high-efficiency furnaces often have smaller input capacities than their inefficient predecessors because they generate more usable heat.