Owen Osborne, Extension Energy Specialist b1!11~~~· Hi ugh J. Hansen, Extension Agricultural Engineer II I • i,. r ..... - ..... ..... ...- ... Oregon State University Fireplaces are aesthetically appealing to many peo­ ple. Because of nostalg'ia, or for reasons of intuitive common sense, homeowners have for years been demanding open fireplaces in their homes. Estimates indicate that over half the single family -homes in the U.S. have at least one fireplace and about one million new fireplaces are being installed annually. Home buyers express an overwhelming preference fo ~ built-in fireplaces and a brisk business ·is being done in pre­ fabricated units. Where wood fuel is plentiful and inex­ pensive, people are turning down central heating sys­ tem thermostats, stoking up fireplaces and learning Fireplace Heat Distribution and Efficiency through personal experience the pros and cons of Heat, by laws of physics, is transfenred by three heating with wood. methods-convection, radiation and conduction. Con­ A fireplace must generally be considered a luxury, vection is transferring of heat from one area to another since its primary value is enhancing the appearance by moving air. Radiation is movement of infrared and atmosphere of a room. Fireplaces are basically electromagnetic rays through air with virtually no low-efficiency home heating units (only about 10% warming of the air but warming of any objects when efficient) unless extensive modifications are made in the rays stri·ke them. Sunlight is an example of radi­ their design and/ or operation to reduce the amount of ant heat. Conduction is transfer of heat along a solid heat lost up the chimney. Heating efficiency is net object-plac·ing a warm hand on a cold block of ice amount of usable heat generated from total fuel burned. conducts heat from. the hand to the ice mass. Heat output is simply total amount of heat produced Fireplace heat from burning wood is about 20% from fuel consumed. Several facts should be evaluated radiant and 80% hot gases. In actual operation, most and some accessory heat-intensification devices con­ of the effective heat from an open fireplace is radiant sidered if energy economics is a factor in justifying a heat. About 90% of the heat output goes up the home fireplace. By so doing many families have been chimney and is discharged to the outdoors, accounting able to set thermostats 10° to 15° F lowe,r than normal for even the well-designed and constructed fireplace and are enjoying the combined benefits of reduced cen­ being only about 10% efficient as a home heating unit. tral system heating costs and open fireplaces with ac­ Under many conditions, a roaring fireplace can ceptable efficiencies. actually remove more heat from a home than it dis­ charges into the room. To burn wood efficiently, the fire must be main­ tained at a high enough. temperature to burn all the combustible materials of the wood. The amount of radiation from a fireplace varies depending on type of fuel used, intensity and size of fi.re and burning temperature of fire, which can range from as low as 500° F to well above 11 00° F. Masonry br'ick linings in fireplaces radiate heat back to the fire to help create the higher-temperature fires necessary for optimum combustion. 111 Stsge 2nd SfatJe 3'd StatJe 6 soo'F- 6 + '• ' t 1100'F over ,,' '~tt'' ,',·f ~100'F ._ ·. ;.·:·/: ··: .. ·. =: .. - ~· --.· -. c~arcoal Efficient fireplace operation requires a system which provides Wood combustion is a three-stage process. First, moisture is adequate chimney draft, outdoor makeup air supply tor fire , evaporated and driven off; second, volatile matter begins to maximum use of radiated heat and appropriate devices to cir­ vaporize into gasses at temperatures above 500° F; and third, culate heated air throughout living area. gases and charcoal are burned at temperatures above 1100° F. Washington State University Pullman Issued by Washington State University Cooperative Extension, J . 0. Young, Director, and the U.S. Department of Agriculture in fur­ therance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and employment are available to all without discrimination. Published June 1980. I . I Pa. , ... _ ~- c..a.. Chimneys tightly block the outlet at the chimney top with a wet The fireplace chimney must be designed and built blanket. Smoke escaping through masonry indicates so that it produces enough draft to supply adequate the location of leaks. Such defects may be hard to air to the fire and to expel smoke and gases emitted by repair but should be corrected before chimney is used. combustion. The flue is the passageway in the chimney Chimneys should be inspected at least every fall through which air, gases and smoke travel. Its cross­ for defects. Check for loose or fallen bricks, cracked sectional area, height, shape, tightness and smoothness or broken flue lining and excessive soot or creosote determine the chimney's effectiveness in producing accumulation. Lower an electric trouble light into flue adequate draft and expelling smoke and gases. Sound­ to make inspection easier. Soot deposits can be dis­ ness of the flue walls may determine the safety of lodged by pulling a weighted sack of straw or a the building should a chimney fire occur. Overheated bundled-up tire chain up and down the flue. Seal fire­ or defective flues are one of the chief causes of house place opening or fJue opening to wood stove when fires. Chimneys located on interior walls mainta:in cleaning to keep soot out of living area. Chemical soot higher flue temperatures, thus reducing the tendency removers are not very effective in removing soot and for creosote to condense on flue walls when smoke may cause soot to burn, thus creating fire hazards or temperatures drop below about 250 ° F. even explosions under certain conditions. Creosote Proper ratios between the area of fireplace front collected on chimney flues is more difficult to remove; opening, area of fireplace throat or damper, area of the surest and safest method is to chip it from masonry, flue and height of flue are essential for satisfactory being careful not to knock out mortar joints or damage fireplace operation. A lined flue 22 feet high should flue lining. have a cross-sectional area at least 1/12 of the fire­ Fireplace Damper place opening area. An unlined flue or a flue less than The greatest heat loss from a conventional open 22 feet high should have a cross-sectional area at fireplace is through the damper. The damper consists least 1/10 that of fireplace opening. Fireclay tile flue of a cast-iron frame with a hinged lid to open or close liners are definitely recommended' for brick chimneys the throat opening above the fireplace. It is important because mortar and bricks directly exposed to flue that the full damper opening area be equal to or greater gases dis'integrate with use, causing cracks in the ma­ than the flue area in size. Dampers are not always in­ sonry which reduce draft and increase fire hazard. The stalled in fireplaces but are definitely recommended, throat area immediately above the fireplace must be no especially in cold climates and consistently windy less than that of the flue-length must be equal to width areas. of fireplace opening. The width is dependent on width A wel1-designed, properly-installed damper will: of damper frame. • allow regulation of draft Chimneys may contain more than one flue. A sepa­ • permit adjustment of throat opening according to rate flue should be used for each fireplace, wood type of fire and draft to reduce heat loss up stove and gas or oil furnace or water heater. Connecting chimney more than one combustion unit to a flue may result in carbon monoxide gases or sparks from one unit being • close off flue to prevent heat loss from living -· drawn into living area by backdrafting through an open area when fireplace is not being used draft, fireplace or wood stove. • close off chimney in summer to prevent insects, A chimney should extend at least 3 feet above flat birds or bats from entering house through chim­ roofs and 2 feet above a roof ridge or any other part of ney. roof within 10 feet of chimney. A hood should be pro­ Close attention should be given to damper posi­ vided if a chimney cannot be built high enough above a tioning in order to minimize loss of heat up the chim­ ridge to prevent trouble from eddies caused by wind ney. Generally, it should be opened just enough to being deflected from the roof or nearby trees or terrain. prevent fireplace from smoking, and no more. The open ends of the hood should be parallel to the Outside Air Inlet ridge. Metal pipe extensions are sometimes used to Wood requires large quantities of "makeup" air for increase flue height. Such extensions must have the proper combustion. This means a properly-operating same cross-sectional area as the flue. Spark-arresting fireplace will draw from 200 to 600 or more cubic feet screens securely fastened over top of chimney are rec­ of air per minute up the chimney. Unless special in­ ommended to reduce hazards of escaping sparks. Rust­ lets provide outdoor air to satisfy this makeup require­ resistant screen with openings no larger than 5/8-inch ment, combustion air for a fireplace is drawn from warm nor smaller than 5/ 16-inch across should be used. Every chimney should be tested before being used air inside the house. Cold outdoor air must infiltrate and preferably before it has been furred, plastered into the house to replace warm air being used to sup­ or otherwise enclosed.