How Fireplaces Work

Fireplaces are often desired more for their aesthetic value and ambiance in homes than they are for their heating abilities.
Fireplaces are often desired more for their aesthetic value and ambiance in homes than they are for their heating abilities.
Maura McEvoy/Getty Images

Do you love sitting by a fireplace? Does a crackling fire define a home to you? If so, you're not alone. In a survey conducted by the National Association of Home Builders, 77 percent of home buyers said they wanted a fireplace in the family room [source:].

The indoor fireplace is a technology that dates from the Middle Ages, when people in medieval castles and homes used them for warmth. However, traditional fireplaces today are desired more for their aesthetics than to be used as effective heating appliances.

Even as far back the 18th century, Benjamin Franklin noted that "the strongest heat from the fire, which is upwards, goes directly up the chimney and is lost" [source: Carlsen]. And Franklin was right. The standard fireplace is among the most inefficient heating devices you can operate. In fact, it can be so inefficient that in some cases it actually makes your house colder.

Inefficiency is not their only drawback. In addition to the risk of setting the house on fire, the smoke that ends up inside your home can contain harmful chemicals, which is a problem in tightly sealed modern homes. And although many modern fireplaces use renewable fuel, they're not considered completely "green," because they can add to air pollution.

But it's not all bad news for fireplaces. Improvements have given fireplace fans a range of more attractive alternatives. Better designs provide more heat, less waste and safer operation. If you're willing to give up the smell and glow of burning wood, a direct-vent, gas-powered fireplace can be an efficient heat source that may even save you money on fuel.

In this article, we'll look at how a fireplace works and how to operate it safely. We'll also consider some innovations that allow you to have a fireplace without a chimney, on your patio or on the coffee table in your living room.

The Parts of a Traditional Fireplace

To understand how a traditional fireplace works, you'll need to know about its various components:

  • The hearth is built out of a fireproof material, such as bricks, and extends out beyond the fireplace.
  • The surround protects the walls around the fireplace and is often topped by a decorative mantel, perfect for hanging Christmas stockings or holding family pictures.
  • The firebox, the interior of the fireplace, contains the fire and collects the smoke.
  • The flue is the passageway at the top through which the smoke and gases travel for exit. Flues are often made of baked clay, but can also be stainless steel.
  • The chimney surrounds the flue, keeping its heat from contacting any flammable building materials that may have been used on the home.
  • The smoke chamber connects the fireplace and the flue. At the bottom of the smoke chamber is the smoke shelf, which deflects downdrafts and prevents any rain or soot from dropping directly into the fireplace.
  • Beneath the smoke shelf is the damper, a movable covering that separates the firebox from the space above. It prevents cold air from moving down into the house when no fire is burning. Some chimneys may also have a chimney damper, which is operated by a cable and closes the chimney at the top to eliminate downdrafts.
  • The spark arrester is a metal mesh that fits over the top of the flue and prevents the exiting gases from carrying burning materials onto the roof. A chimney cap prevents moisture and animals from entering the flue. It may rotate to block wind gusts.
  • Some fireplaces are equipped with an ash dump, an opening with a trap door where you can push the accumulated ashes into a pit below for later cleanout.
  • Fireplace doors can be made of glass or metal. They shut off the air flow when the fire has died down or the fireplace is not in use.

In the next section, we'll see how these parts work together to allow the fireplace to do its job.

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.

How to Operate a Traditional Fireplace

Operating a traditional wood-burning fireplace is not difficult if you follow a few simple guidelines. First, you should begin by choosing the right fuel. Be sure to burn hard woods, such as hickory, ash, oak and hard maple. Soft woods such as pine and spruce generally don't burn as well or provide as much heat. Also, be sure your wood is seasoned, or dry. Wood needs at least six months -- many experts suggest at least a year -- of drying to reach the 20 percent moisture level that is recommended for a good fire [source: Taylor]. One way to be sure your wood is seasoned is to knock two logs together and listen for a hollow sound, not a dull thud. Seasoned wood is also darker and has cracks in the end grain. Avoid using wet or rotten wood, and never burn trash or cardboard in your fireplace. Pressure-treated wood and chipboard are also inappropriate.

To start the fire, you need kindling -- smaller pieces of wood that will take flame easily. Stack a few split logs on your grate and place kindling around and below them. Make sure the damper is open before you light the kindling with newspaper. Don't use too much paper, as flaming scraps can be carried up the flue and onto your roof. Never use gasoline, lighter fluid or a butane torch to start a fire.

Once the fire is burning, you may still encounter problems with puffs of smoke entering the room. One cause of a smoking chimney is a house that's too tight. If there aren't enough openings to make up for the air drawn up the chimney, it can cause negative pressure in the room, creating a partial vacuum. Air pressure forces air down the chimney to compensate, resulting in a smoky house. The solution is to crack a window near the fireplace to let air in [source: HGTV].

Here are some other points to keep in mind:

  • Leave a few inches of ash in the firebox to help reflect heat and provide a bed for coals, which radiate heat.
  • Some experts recommend using andirons instead of a grate, so that logs to drop onto the bed of coals where they burn more efficiently [source: Carlsen].
  • If your damper is adjustable, gradually close it as the fire dies down to maintain a draft and limit cold air from coming down. But don't close it completely until the fire is out.
  • If your fireplace is equipped with glass or metal doors, make sure they are closed before you go to bed.

No matter how carefully you operate your traditional fireplace, much of its heat is lost up the chimney. Next we'll look at a few ways to increase its efficiency.

Improving the Efficiency of a Traditional Fireplace

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.

The Gas Fireplace

If you're looking for a fireplace that gives you both efficiency and the pleasure of watching the flames dance, you might consider a gas model. You can't roast marshmallows in them, but you'll have the advantage of a clean and convenient source of heat.

A modern gas fireplace emits no smoke and vents its waste gases to the outside through a tube in the wall rather than up a chimney. It incorporates air-movement channels that maximize the warmth supplied to the house. The fireplace consists of incombustible "logs" covering gas vents, and the fire itself burns behind glass doors. It gives off both radiant and convected heat and provides an experience similar to an open fire [source: Berendsohn].

One benefit of a gas fireplace is that it may help lower heating bills. It lets you heat the room you are spending time in while you keep your thermostat low and the rest of the house cooler. You stay toasty and your furnace gets a rest. Most gas fireplaces take advantage of sealed combustion. Their doors have gaskets that block all air. The fire draws air outside air through a pipe to support combustion, so no warm room air is drawn out of the house.

If you want to add a fireplace to an existing home, a gas fireplace makes sense. Because it doesn't require a massive masonry hearth and chimney, it can easily be included in a new family room. You sacrifice much less floor space and still have a fire to look at. You can also buy a gas fireplace insert that fits into a traditional fireplace hearth and boost its energy efficiency. As with its wood-burning counterpart, the gas insert has its own flue that snakes up the masonry chimney.

Because gas burns very cleanly, there are even vent-free fireplaces on the market. In this case, the combustion products -- carbon dioxide and water vapor -- simply enter the room, along with all the heat produced. Although highly efficient, they are also subject to debate. In a tight home, a vent-free fireplace can deplete oxygen or create an excess of moisture. The American Lung Association warns buyers to be careful of the emissions given off by vent-free appliances [source: American Lung Association].

Gas fireplaces are easy to use and require little maintenance. Some come with a remote control, so that you can adjust them from across the room. You will occasionally have to remove dust, soot and carbon buildup from the logs and make sure the door gaskets are intact. As with any gas appliance, if you smell gas, turn off the supply and call an expert.

Because they involve open flames, all fireplaces -- even gas ones -- create some danger. The next section will talk about how to use a fireplace safely.

Fireplace Maintenance and Safety

According to the U.S. Consumer Product Safety Commission, fireplaces and chimneys cause more than 25,000 house fires every year, resulting in at least 10 deaths annually [source: CPSC]. Some of the dangers of fireplace operation include the following:

  • Sparks popping into the room and setting fire to rugs or furniture
  • Combustible materials placed too close to the fire
  • Chimney fires
  • Carbon monoxide seeping into the house
  • Harmful particles in smoke

Careful operation and routine maintenance can minimize these dangers and allow you to use your fireplace in safer conditions.

For example, an annual inspection is a must. As a homeowner, you can perform a basic inspection yourself. Is the chimney in good shape? Are there obvious leaks or stains? Does the flue have a cap? Does the damper seal off the flue completely?

A professional chimney sweep will complete an internal inspection of the fireplace and flue and look for any internal or structural problems. He will also remove creosote buildup before it becomes dangerous. Creosote is the residue that results from fires and sometimes condenses on the inside of the flue. When it builds up, it can catch fire, resulting in chimney damage and potential spread of fire to the house.

Keep in mind, though, that an open fireplace will usually result in some smoke entering the room. The particles in this smoke could aggravate the problems of those who already have respiratory conditions such as chronic bronchitis or asthma. Breathing particles over the long term can contribute to lung disease. Older adults and children are especially vulnerable.

In order to lower the risk when using your fireplace, you should:

  • Keep all combustibles a safe distance away from the hearth.
  • Use a fire screen to prevent sparks.
  • Install a spark arrester at the top of the flue to guard against roof fires.
  • Keep a fire extinguisher handy.
  • Install smoke detectors on every level of your home. You should also have a carbon monoxide detector.

Read on to find lots more information about fireplaces, chimneys and efficient heating.

Related HowStuffWorks Articles

More Great Links


  • Ace Hardware. "Heat Saving Items." (Accessed Feb. 5, 2010)
  • American Lung Association. "American Lung Association Cautions Against Wood-burning and Urges Cleaner Alternatives for Winter Heat."
  • Behrendsohn, Roy. "Hearth and Home: A Guide to Gas Fireplaces." Dec. 1996. (Accessed Feb. 5, 2010)
  • Bortz, Paul. "Getting More Heat from Your Fireplace." Garden Way Inc. 1982.
  • Carlsen, Spike. "Warming up Fireplaces." The Family Handyman. Sep 1996. Vol. 46, Iss. 8; pg. 121.
  • CBS News. "No-Fuss, Modern Fireplaces." Jan. 12, 2010. (Accessed Feb. 5, 2010)
  • Chimney Safety Institute of America. "CSIA's Chemical Cleaners Position"
  • "Greening Your Fireplace; Sunset. Jan 2008. Vol. 220, Iss. 1; pg. 22.
  • Gulland, John. "Choose a Fireplace for Beauty and Warmth." Mother Earth News. Oct/Nov 2003. Iss. 200, pg. 90.
  • "Heat Up Your Home's Value with a Fireplace."
  • HGTV. "Fireplace Maintenance." (Accessed Feb. 4, 2010)
  • U.S. Consumer Product Safety Commission. "2004-2006 Loss Estimates."
  • Taylor, Adam. "Wood Products Information." University of Tennessee Extension. 2007. (Accessed Feb. 4, 2010)
  • Woodheat Organization "Can a fireplace behave itself in a tight house?"