Gas-fired fireplaces, like other gas appliances, are measured by their "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 1 degree Fahrenheit.

Input capacity refers to the amount of gas burned without taking into consideration the percentage of heat lost through the flue. Output capacity refers to the total usable heat generated. Most manufacturers use the input capacity figure because it's always larger than the output.

If you're concerned about how much heat a fireplace will generate, the output figure is the one that's important. If you want to know how efficiently it will use its fuel, figure the Steady State Efficiency rating by dividing the output rate by the input rate.

Input ratings vary, and some units have a range, depending upon the flame's setting. If supplemental heat is important, choose a model with plenty of output. The Mendota DXV fireplace, for example, has a thermostatically controlled, variable two-level burner that can deliver from 25,000 to 40,000 Btus per hour with 75 percent efficiency.