The Mechanics of the Traditional Fireplace
Lighting a fire inside your living room presents two obvious challenges. First, you have to avoid setting your house on fire. Second, you need to keep the smoke from spilling into the room. A fireplace solves both difficulties. It is made from materials that don't burn (traditionally, stone and brick, but also metal and tile), and it takes care of smoke by sending it up the chimney.
The most important mechanical function of a fireplace is to generate a draft. If you think of a hot air balloon, you know that a mass of heated air rises. A fireplace creates a column of heated gas inside the chimney. As that air rises, more heated air from the fire is pulled after it. The result is a draft -- a steady flow of smoke and hot gases -- up the chimney.
The draft serves another purpose, too. Any fire needs a steady flow of oxygen to keep burning. As the hot gas rises, it pulls fresh air into the pile of burning fuel.
You might remember from physics class that there are three methods by which heat moves:
- Conduction -- a hot object touches a cooler one
- Convection -- a movable substance, such as hot air or liquid, circulates into cooler areas
- Radiation -- warm electromagnetic waves, such as rays from the sun or a heat lamp, carry heat to cooler objects and warm them by making their molecules move faster
A traditional fireplace heats by radiation -- the flame and hot coals send out rays that strike objects or people in the room and speed up their molecules, thereby warming them up.
But the principle of convection is also at work in a fireplace, and this is one reason why they can be so inefficient. The major portion of the heat that a fire creates is in the form of hot gas. Convection sends this gas up the chimney, where it is wasted. What's more, the draft can draw more warm air from inside the room than the fire needs to burn and pull that air up the chimney as well, leaving the room colder than before. Some experts say that traditional fireplaces can draw four to ten times as much air from the room than is needed to burn the fire [source: Carlsen].
Sometimes, more heat is lost through convection than is added through radiation, resulting in a fireplace's negative energy efficiency. The colder it is outside, the colder the air that the fireplace sucks in and the lower the efficiency.
Next, we'll look at some tips and techniques about how best to operate a fireplace.