Heat Production & Fuel Efficiency
Highly efficient supplemental heating
Gas fireplaces are designed to provide maximum heat production and fuel efficiency. Just like air-conditioners and furnaces, gas fireplaces have a BTU (British thermal unit) rating. BTU ratings measure the amount of heat produced by the fireplace, usually at an hourly rate. Direct-vent gas fireplaces and top-vent gas fireplaces produce 20,000 - 60,000 BTU/hour, while ventless gas fireplaces generate a maximum of 40,000 BTU/hour.
Gas fireplaces also have an efficiency rating which measures the fireplace's fuel effectiveness. Efficiency ratings are measured as a percentage, and higher ratings represent a more efficient fireplace. Efficiency ratings measure the amount of fuel consumed by the fireplace to generate usable heat and reach its ideal BTU rating. Typically, direct-vent and top-vent gas fireplaces have an efficiency rating of 40 - 80 percent. This means that 80 percent of the heat produced by the fireplace stays within the home, while 20 percent is vented outside. As ventless gas fireplaces do not vent outside, their efficiency rate is higher, usually 90 percent.
A high efficiency gas fireplace burns less gas to supply the same amount of heat relative to a low efficiency gas fireplace. Essentially, if your fireplace is 80% efficient, for every dollar you spend on fuel you get 80 cents' worth of heat. A 30,000 BTU/hour fireplace operating at 80% efficiency rate will actually consume 37,500 BTU's of natural gas per hour, with the other 7,500 BTU's floating away out of the chimney. Thus, a 40% efficient fireplace would give you only 40 cents worth of heat per dollar that you spend on fuel – if you're buying a fireplace as a heater, that's a problem. But if you're buying a fireplace solely for ambience, efficiency is not such a major concern.
Heating capacity (in BTU's) and energy efficiency ratings also apply to wood fireplaces and stoves, electric fireplaces and propane fireplaces . However, the heating capacities of different types of fireplaces vary greatly, since different fuels burn at different levels and rates. Although the numbers may seem daunting at first, a heat capacity rating expressed in BTU's is a handy way to compare the heating ability of different types of fireplaces and woodstoves, even when they use completely different kinds of fuel.