How Fireplaces Work

There are two main strategies for improving fireplace efficiency. The first is to use convection as well as radiation to capture some of the heat from the fire. Some fireplaces include a built-in heat exchanger -- channels where room air can circulate around the hot parts of the structure -- either through natural air flow or forced by a fan. The air absorbs the heat and returns it to the room.

The second approach is to block part of the front of the firebox in order to limit the amount of air that flows unnecessarily up the chimney. Usually, this is done with doors made of tempered, heat-resistant glass. Adjustable inlets allow enough air to reach the fire to keep it burning.

Here are some specific ways these two strategies are used:

A tubular grate is a series of open pipes that curve behind the fire and extend out the top of the firebox. The idea is to draw in cool air at the bottom, heat it and let it flow into the room. The problem is that much of the heated air is drawn back into the fire. Used with glass doors that block this air return, a tubular grate can help squeeze more warmth from a fireplace [source: Ace Hardware].

Glass doors reduce the loss of room air up the chimney and still allow you to view the fire. The drawback is that the glass can also reduce the heat that reaches the room by half (even a mesh screen reduces radiant heat by 30 percent) [source: Bortz]. The result is a small gain in efficiency.

Fireplace inserts are metal boxes -- usually equipped with glass doors -- that fit inside the firebox. They use a heat exchange chamber with channels to allow room air to pass through and absorb heat. Fireplace inserts usually require a full stainless steel flue liner, rather than simply connecting to an existing flue. An insert can put out up to five times as much heat as an open fireplace [source: Carlsen].

Some homeowners prefer to take advantage of the efficiency of a wood stove by placing the stove on the fireplace hearth and running the stovepipe into the fireplace chimney. By doing so, they lose the pleasures of an open fire but gain energy efficiency.

Next, we'll take a look at what may be the most efficient fireplace of all.