Fireplaces

Wood-heating devices are the oldest method of heating after the sun. Campfires evolved into fireplaces and eventually into the wood stove, which offers fuel efficiency within an enclosed space. Wood is popular for heating homes in regions where energy costs are high and local regulations permit burning wood. Not all fireplaces are good heaters, however, and any fireplace or wood stove requires timely maintenance if it is to remain reliable and safe.

Fireplaces and stoves can add to both indoor and outdoor air pollution, emitting carbon monoxide, irritating particles, and sometimes nitrogen dioxide. Wood smoke causes nose and throat irritation, and can trigger asthma attacks. To keep chimneys clean and minimize pollution risks, burn small hot fires, not large smoky ones, use seasoned wood, and provide adequate ventilation.

A fireplace (Fig. 24-1) is really a framed opening in a chimney, designed to hold an open fire and sustain combustion of fuel. Modern fireplaces combine masonry and steel construction; some are almost entirely steel. Fireplaces should be designed to carry smoke and other combustion by-products safely outside, and to radiate the maximum amount of heat comfortably into the room. The design must keep the fireplace adequate distances from combustible materials. Multifaced fireplaces are sensitive to drafts in a room, so avoid placing their openings opposite an exterior door.

The firebox is the chamber where combustion takes place. In traditional fireplaces, the firebox is steel, lined with noncombustible firebrick. The firebox is typically 91 or 107 cm (36 or 42 in.) wide and about 64 cm (25 in.) high. The top of the firebox tapers to a throat, a narrow opening that connects the firebox and the smoke chamber. The throat is fitted with a damper that regulates the draft in the fireplace.

The smoke chamber connects the throat to the flue of the chimney. At the bottom of the smoke chamber, a

Noncombustible insulation

Flue with fireclay flue lining

Noncombustible insulation

Flue with fireclay flue lining

Smoke chamber Smoke Shelf

Ashpit and outside air intake

Figure 24-1 Section through a fireplace and chimney.

Smoke chamber Smoke Shelf

Ashpit and outside air intake

Figure 24-1 Section through a fireplace and chimney.

smoke shelf deflects downdrafts from the chimney, preventing cold air and smoke from entering the room. The flue creates a draft and carries smoke and gases of the fire safely outside. The hotter air that has been heated by the fire, being less dense than the fresh air in the chimney, rises up and out of the flue. The flue is usually a metal or tile liner inside a masonry chimney. Masonry chimneys must have a minimum of 5 cm (2 in.) clearance from combustible construction. Normally, fire stopping is provided between the chimney and wood framing.

To burn properly, a fire requires a steady flow of air. Traditional fireplaces draw the air for combustion from inside the house. A grate on the floor of the fireplace holds logs so air can stoke the fire from underneath. Other fireplaces pull in outside air through an air intake underneath the fireplace floor. A flue damper enables you to regulate the draft and can be completely closed to keep air from escaping up the flue when the fireplace is not in use.

The hearth extends the floor of the fireplace out into the room, with noncombustible material like brick, tile, or stone to resist flying sparks. Many fireplaces have an ash drop in the hearth, which can be opened to dump ashes into a pit underneath. Some also have a gas starter, which sets logs ablaze without kindling. On the wall of the room, the chimneybreast often projects a few inches into the room. A mantle may trim the top of the fireplace.

Ordinary fireplaces generate mostly radiant heat, which warms only the immediate vicinity. Heat-circulating fireplaces produce some radiant heat, but mainly warm the air that circulates around the firebox. One, and sometimes two, shells surround the firebox in a heat-circulating fireplace. A room air intake under the firebox lets cool air in. As the air absorbs heat, it rises to a warm air outlet at the top of the fireplace. Some fireplaces have a fan that increases the airflow to speed up the heat transfer. Convection currents in the room carry warm air away from the fireplace and cool air back for reheating.

Most heat-circulating fireplaces gain further efficiency by enclosing the firebox with glass doors that let you see the flames without wasting their energy in radiant heat. A damper at the bottom of the door lets you control the supply of air for combustion. Some highly efficient heat-circulating fireplaces have an outside air intake and do not require room air for combustion.

In some areas, including Washington, Colorado, and parts of California, strict environmental laws prohibit burning of wood on certain days to all but certified, clean-burning appliances, which usually means factory-built fireplaces or wood stoves. These environmental measures have cut into the number of traditional masonry fireplaces that are constructed. The relatively low cost of zero-clearance fireplaces, which don't need space between their enclosures and nearby combustible materials, also compete with traditional fireplaces, as do gas fireplaces.

In response, manufacturers have introduced clean-burning masonry fireplaces that are certified under state environmental regulations. The manufacturers supply the pieces critical to combustion efficiency, and the lo cal dealer supplies firebrick, backup material, flue liner, and stone, along with accessories. Some of these clean burners are variations of the Rumford design that was developed in England in the later 1700s. The Rumford is a tall, shallow fireplace, about as tall as it is wide, that burns hot and easily radiates heat into the room. To prevent smoking, the fireplace is built with a rounded throat that draws air up into the smoke chamber. Some current models can be used in new construction or retrofitted into an existing fireplace.

Another design uses a standard brick fireplace on the outside and a masonry heater on the inside. Burning gases are mixed with oxygen in five separate locations in the fireplace, and the burning continues as the gases travel down hidden channels between the firebox and the chimney. The heat from the burning, which greatly reduces pollutants, is stored in the masonry mass. After the fire goes out, heat is slowly released over the ensuing 12 to 16 hours.

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