Although wood-burning fireplaces have aesthetic appeal, they may actually remove more heat from a house than they produce. A typical, vertical-back fireplace with an open front is at best ten percent efficient in converting wood to energy and delivering it to a room. The rest of your wood dollars escape up the chimney. They also pull cold air into the house from small gaps around windows and doors. At the beginning and end of the burn, these convective heat losses are larger than the radiant heat provided by the fire. Also, most fireplaces are inappropriately situated on exterior walls. The large mass of masonry that makes up most fireplaces are poor thermal insulators and readily conduct room heat to the outdoors in cold weather.

There are several types of accessories that modestly improve the heating efficiency of a traditional wood-burning fireplace. A fireplace insert, however, is the only add-on device that makes a significant difference.

Dampers
All wood-burning fireplaces should have a damper, or flue, located in the throat of the chimney, above the fireplace. A damper has a cast iron frame with a hinged lid that opens or closes to vary the chimney’s throat opening. It is used to manually regulate the amount of air, or draft, going up the chimney, which controls how much air the fire receives, and the rate that it burns. The size of the damper opening should correspond to the size of the fire: a narrow opening for a small fire, wide for a big one. It is important to close the damper when a fire is not burning. This helps to minimize the loss of warm room air up the chimney. If you use your fireplace often, check the damper annually to be sure it closes as tight as possible.

Flue Sealers
Even brand-new dampers may not close tightly. Dampers can become warped after the first hot fire. They may even be installed incorrectly to begin with. There are products commercially available to temporarily seal off the flue and stop air from escaping up the chimney. One product is an inflatable stopper that you insert into the flue. Most models are made of heavy plastic with an attached tube for inflating the device. The tube hangs down into the fireplace to remind you that the stopper is there. The flue must be totally cool before installing the stopper. Most manufacturers recommend cleaning the flue before inserting the stopper to ensure a tight and effective seal. This also minimizes the amount of soot that will fall out of the flue when you remove the stopper. Stoppers cost $30 to $50. Depending on how often you use them and your climate, they may pay for themselves within a few years. A less expensive solution is to use an old foam rubber seat cushion or pillow: place it in a heavy plastic bag, and insert this into the flue. Attach a long red tail to it to remind you that it is there.

Glass Doors and Fireplace Covers Glass doors and fireplace covers are also ways to reduce heat loss from a fireplace. Both products are designed to reduce airflow, and thus heat loss, up the chimney. Glass doors have four major components: a frame with adjustable air intake slots or vents, tempered glass doors, an interior screen, and special brackets or anchor bolts to secure the frame to the fireplace. How much energy a glass door saves you depends on how it is used. Burning with the doors open is the same as not having a door at all. Closing the door during the burn allows for better control of the fire, but reduces the amount of radiant heat transfer into the room from the fire. But even so, the loss of some radiation is preferred over the uncontrolled air movement up the chimney. Always be certain to shut the doors just after the most intense part of the burn, when the fire is dying, and when the fireplace is not in use. Be certain to close the air vents and chimney flue damper too.

Most fireplace doors are made of tempered soda-lime glass. Tempered glass, often called safety glass, adds mechanical strength, making it more difficult to break than ordinary glass. If the glass does break, it shatters into many tiny pieces instead of large, dangerous shards. Fireplace doors should be kept clean. Glass that is darkened by soot and creosote reduces the radiant heat transfer to the room and may even weaken the glass during the extreme temperature changes of a fire.

When shopping for fireplace doors, look for models with a rigid frame and a fiberglass seal that goes against the outside rim of the fireplace opening. The frame should fit securely, the doors should seal tightly, and the inlet vents should be adjustable. If you have a factory-built fireplace, find out whether the unit is approved for use with glass doors.

Fireplace covers are less expensive than glass doors and also deter room air from escaping up the chimney. When installed correctly they seal off the fireplace opening once the fire has died out. To be effective, they need to fit tightly around the fireplace opening. Often a fiberglass gasket is used to aid in the seal that keeps room air in the building.

Heat Exchangers
There are a variety of products intended to recover heat from the fire that would otherwise be lost up the chimney. These are often a series of large diameter pipes or tubes either in the chimney or around the firebox. Their purpose is to circulate room air through the hot tubes to pick up some additional heat. Some models have a fan to force air through the tubes, while others rely on natural convection to keep the air circulating. In either case these devices increase heating performance by ten to five percent. They also require frequent cleaning to remove soot from the tubes, which will reduce their performance.

Inserts
Fireplace inserts are wood-, pellet-, or gas-fueled appliances that fit into a conventional open fireplace. Some models extend onto the hearth, while others stay flush with the front of the fireplace. Many inserts include aesthetic features such as decorative tile, cast iron, or enameled fronts, glass doors, and brass trim and handles.

Most inserts have tight-fitting glass or metal doors and built-in air circulation features. The most efficient models supply combustion air from outside the house. Virtually all of the inserts available are controlled-draft units, featuring manual or thermostatically controlled air inlets and gasketed doors. Some units are available with catalytic converters, which improve combustion efficiency. Combustion efficiencies for wood-fired inserts range from 63 % for non-catalytic inserts to 72 % for catalytic units. Pellet fueled appliances are about 80%. Gas units vary in heating efficiency, depending on whether they are vented or unvented.

Fireplace inserts may have a single or a double wall. In single-wall units, room air is pulled in under the firebox and rises between the back of the unit and the masonry walls of the fireplace. This heated air then moves back into the room at the top of the insert. Double-wall units have an air space between the exterior and interior walls of the unit. Air circulates and heats in this space. This type is much more common, safer, and more efficient than single-wall inserts. Both types have separate combustion air inlets and either vent into the chimney or a chimney liner. Some inserts have fans to circulate air around the unit and blow heated air into the room. Fans may improve heat distribution (not combustion efficiency) by 50% to 75%. A fan may not be necessary if the unit is in a small room.

In the past, an insert was often placed in the fireplace without any direct connection between it and the chimney. This method of installation led to an increase in creosote build-up, and a subsequent fire hazard, because the smoke lingered and cooled in the back of the fireplace instead of being immediately vented up the chimney. To prevent this, most building mechanical codes and the National Fire Protection Association (NFPA), require inserts to have a connector between the appliance outlet and the first section of the flue liner.

Studies have shown that proper installation of fireplace inserts is very important. Have a professional installer examine the fireplace and chimney to determine if they are suitable for an insert. Inserts should be as airtight as possible. The more airtight it is, the easier it is to control the fire and the heat output. The installer should use only approved fireplace insulating materials to fill any gaps between the fireplace mouth and insert shield.

Moving an insert to clean the chimney or liner can be difficult, and is a job best left to a professional chimney sweep. In some situations, a clean-out door can be installed above the insert connection so the insert does not have to be moved as often. Some models have wheels to simplify installation, cleaning, repairs, and other adjustments.

Gas-Burning Fireplaces and Inserts
Natural gas-burning fireplaces and inserts are an increasingly popular alternative to wood burning fireplaces. A wide variety of models are available, ranging from gas-logs to stove-type inserts. They may be vented or unvented. Unvented units do not have a chimney or flue. Unvented gas heaters are gaining in popularity since they are relatively inexpensive and easy to install, and are efficient heaters. However, they can also be a health hazard since the combustion gases are sent into the room. Ventless heaters also deplete the oxygen level of the rooms and may produce some carbon monoxide (CO). They also produce water vapor, which can cause problems when they operated often and/or for long periods, especially in a "tight" home. Most manufacturers recommend that they be operated for no more than 3 hours per day. Always operate ventless heaters when there is an "adequate" air source, such as an open window, nearby. Also, always closely follow the manufacturers operating and installation instructions. To avoid any of these problems, a vented unit is better and a sealed combustion unit is best.

Source: U.S. DOE, Energy Efficiency and Renewable Energy Network, Consumer Energy Information: EREC Reference Brief. EREC is operated by NCI Information Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of Energy. The content of this brief is based on information known to EREC at the time of preparation. No recommendation or endorsement of any non-U.S. Government product or service is implied if mentioned by EREC.