Fireplaces offer a cozy view and a warm, radiant heat to enjoy in your home. But, if your primary source of heat is from a central heating system, a fireplace can waste the fuel it consumes along with the fuel used by the central system. It will also make the farthest places in your home colder.
Fireplaces can be fueled by wood or natural gas. One pound of fuel requires 15 pounds of air for combustion. Unless your fireplace has an outside air source, this air comes from your already heated home air. In addition to combustion air, the heated flue pulls in another 15-20 pounds of air from the house to the outside.
Glass Fronts can reduce the amount of air drawn from the home.
that contain a heat exchanger capture the heat that otherwise would go up the chimney with the smoke. They can provide heat to the home along with a central heating system.
If your primary heating system is a central gas, electric or heat pump system, the system should be turned off during operation of an open or glass-fronted fireplace. Wood Heaters and Fireplace inserts, on the other hand, can be operated together with a central system. In this part of the country they can produce much more heat than necessary to heat a home and are operated in a throttled condition. A variable speed fan on your central system can be used to distribute this heat to the rest of the home without causing a drafty feeling.
Prefabricated fireplaces now outnumber masonry fireplaces. They are much less expensive to build. I see very few of these with a heat exchanger like you would find on a fireplace insert, but that type does exist. The majority of these use gas logs with a triple-wall flue to the outside and have the efficiency drawbacks of masonry fireplaces as described above. There is a way to use these for home heating in an efficient manner: Vent-Free Gas Logs.
Vent-Free Gas Logs come with an Oxygen Depletion Detection System and do not produce dangerous carbon monoxide. They can be operated with a closed flue or without a flue. This makes them nearly 100% efficient in operation. They do present drawbacks compared to a central system; all products of combustion are dumped into the living space.
The products of combustion are:
- trace amounts of hydrogen sulfide
- large amounts of water vapor
- Carbon Dioxide
Hydrogen sulfide is not noticeable to humans in trace quantities but it is a corrosive gas. Large levels of water vapor raise the relative humidity in the home and can cause sweating of glass and window sashes. Carbon Dioxide is non-toxic to humans but elevated concentrations can make you drowsy. This is controlled for with the Oxygen Depletion Sensor that is part of a vent-free gas logs device.
I’ll include a note here about Carbon Monoxide. Carbon Monoxide is toxic and accumulates in the blood. It is created through the incomplete combustion of hydrocarbon fuels. High accumulation in the blood can be fatal. The symptoms are similar to influenza. If you have propane or natural gas supplied to your home you should own one or more carbon monoxide detectors. They are incredibly cheap and very reliable.
Gas logs are a much cheaper way to achieve high efficiency gas heating. A comparable 96% AFUE efficient Gas Furnace can cost as much as $5,000 or more. But of course there is a trade off – Gas logs degrade the quality of air inside the home.
I remember going to an elderly aunt’s home in Mobile in the 1960s who heated her home with free-standing radiant gas heaters. The indoor environment was oppressive because of the elevated humidity and odor of combustion products. I didn’t, of course, know these causes at the time but the indoor air was unpleasant nonetheless.
New furnaces draw combustion air from outside the home, recover almost all heat from the fuel and then dump the products of combustion outside. All this without adding or removing anything from the air inside the home. Gas logs are used primarily to heat the room with the logs and to also enhance the ambiance. As nice and quiet as our furnaces are, they’re not much to curl up in front of.