Manufacturers size their gas appliances by the heat INPUT rating, measured in British Thermal Units (BTU's). This is a measurement of the heat
value of the gas the appliance burns per hour. If the rate of burn is adjustable, the input rating is given at both the lowest and highest burn
settings. One THERM of natural gas or one GALLON of LP gas each contain about 100,000 BTU's of heat value, so a gas appliance with an
input rating of 20,000 - 30,000 BTU/hr will consume approximately one therm of natural gas or one gallon of LP every five hours when adjusted
to its lowest setting, and every 3-1/3 hours when adjusted to its highest setting.
The steady state efficiency rating tells us how effectively a given gas appliance turns fuel into heat. Many factors affect this rating, including
altitude, composition of fuel gas, length of intake/exhaust pipe, outdoor temperatures, atmospheric conditions, etc. Some companies publish only
the lowest tested efficiencies, some just the highest, some publish both the lowest and highest, and some use an average. For this chart, we've
used the AVERAGE steady state efficiency rating, interpolating the unpublished data where necessary.
To determine the minimum & maximum heat OUTPUT, we multiply the minimum & maximum INPUT ratings by the EFFICIENCY rating. For
example, a gas stove or fireplace with an efficiency rating of 75% and an adjustable input rating of 20,000 - 30,000 BTU/hr will deliver 15,000
BTU's of heat per hour at its lowest setting (20,000 x 75%), and 22,500 BTU's of heat per hour at its highest setting (30,000 x 75%).