Rustic Precision Blog

Author’s note: the following article on home heating is the fifth in an eight-part series.

Open fireplaces have a reputation for polluting air. Actually, a fireplace, when burned hot and fast, creates very little pollution. The trouble is, a hot fire in a fireplace sometimes yields less heat than a smoldering fire. Where does the heat go?

The optimal amount of combustion air contains just enough oxygen to burn all combustible gases liberated by the heat. Any additional air grabs heat and sends it up the chimney. Under some circumstances, fireplaces can so far exceed this air-to-fuel ratio that they suck more heat out of a house than they radiate back into it. The fire actually makes the house colder!

The usable heat produced from the fireplace is primarily radiation, the same heat you feel on your face when you look at the flames. While fireplaces often contain lots of thermal mass (masonry), the unrestricted flow of cool air across this mass prevents it from capturing much heat. Nevertheless, if the damper is closed as soon as the fire burns out, a significant amount of heat will radiate back into the room instead of going up the chimney. Unfortunately, when the fire burns out, many fireplace users give up and go to bed without taking this critical step.

Here’s where pollution enters the picture: instead of burning a quick, hot fire and closing their damper, most people elect to burn their wood slowly to meter out heat.

Slow combustion means that the wood is burning at a lower temperature. At a lower temperature, a smaller portion of the combustible gases actually burn. More gases leave the chimney as smoke and soot (pollution).

With fireplaces, you really can’t win: if you burn a hot fire, you lose most of the heat up the chimney. If you burn a slow fire, you get very little heat, and lots of pollution.

The next article in the series talks about woodstoves.

Previous Articles in this Series:

• Heating Your Home: Thermal Mass
• Heating Your Home: Forced Air
• Heating Your Home: Heat 101
• Heating Your Home: Radiant Heat, Wood Heat

Photo credit: Wikipedia Commons