Three types of heat.
Heat, by laws of physics, is transferred by three methods–convection, radiation and conduction. Convection is the transfer of heat from one area to another by moving air. Radiation is the movement of infrared electromagnetic rays through air with virtually no warming of the air but warming of any objects when the rays strike them. (Sunlight is an example of radiant heat.) Conduction is the transfer of heat along a solid object. (Placing a warm hand on a cold block of ice conducts heat from the hand to the ice mass.)
Wood burning fireplace heat.
Fireplace heat from burning wood is about 20% radiant and 80% hot gases. In actual operation, most of the effective heat from an open fireplace is radiant heat. About 90% of the heat output from a wood burning fireplace goes up the chimney and is discharged to the outdoors. So, even the well-designed and constructed fireplaces are only about 10% as efficient as a home heating unit. Under many conditions, a roaring fireplace can actually remove more heat from a home than it discharges into the room.
Wood combustion is a three-stage process. First, moisture is evaporated and driven off. Second, volatile matter begins to vaporize into gases at temperatures above 500 degrees Fahrenheit. Third, gases and charcoal are burned at temperatures above 1100 degrees Fahrenheit.
To burn wood efficiently, the fire must be maintained 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 fire and burning temperature of fire, which can range from as low as 500 F to well above 1100 F. Masonry brick linings in fireplaces radiate heat back to the fire to help create the higher temperature fires necessary for optimum combustion.
Wood burning fireplace heat loss.
The greatest heat loss from a conventional open fireplace is through the damper. The damper consists of a cast-iron frame with a hinged lid to open or close the throat opening above the fireplace. It is important that the full damper opening area be equal to or greater than the flue area in size. Dampers are not always installed in fireplaces but are definitely recommended.
A well-designed, properly-installed damper will:
- allow regulation of the draft
- permit adjustment of the throat opening according to the type of fire and draft to reduce heat loss up the chimney
- close off flue to prevent heat loss from living area when the fireplace is not being used
- close off the chimney in summer to prevent insects, birds or bats from entering the house through the chimney
Close attention should be given to damper positioning in order to minimize the loss of heat up the chimney (and your family’s safety). Generally, it should be opened just enough to prevent fireplace from smoking, and no more.
Outside Air Inlet
Wood requires large quantities of “makeup” air for proper combustion. This means a properly-operating fireplace will draw from 200 to 600 or more cubic feet of air per minute up the chimney. Unless special inlets provide outdoor air to satisfy this makeup requirement, combustion air for a fireplace is drawn from warm air inside the house. Cold outdoor air must infiltrate into the house to replace warm air being used to support the fireplace fire. A normal fire requiring 400 cubic feet of air per minute will draw the equivalent of all the air in a 15- by 20-foot room every six minutes.
CAUTION–With today’s tightly-constructed houses incorporating weather stripped doors, caulked windows and self-closing exhaust vents, it is possible for a fireplace to set up a reverse draft and suck poisonous carbon monoxide fumes from combustion-type water heaters or furnaces and discharge them into the living area. Also, in tight homes, the furnace may consume enough oxygen from the air in the house to cause problems to occupants. To be safe, a positive source of outside air should be supplied to all fireplaces and wood- or coal-burning stoves. This can be provided by installing an outside air vent or opening a window when the fireplace is being used.
Bob Beisbier, owner of BK Home Inspections, is a Certified Master Home Inspector who has been providing professional and thorough home inspections in southeast Wisconsin for over 12 years. Bob is Infrared certified, DILHR Certified, and provides Home Energy Tune-ups, Environmental Data Reports, Pre-sale Home Inspections and Pre-offer Home Inspections.