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Open fireplaces are inefficiently designed so that most of the heat escapes into the flue, rather than the home. In fact, open fireplaces use heated room air for combustion, thereby lowering the temperature inside the house. Exception: Rumford style fireplaces.
According to the Midwest Chimney Safety Council, chimney sweeps are reporting record calls for chimney service, no doubt due to the unusually long and bitterly cold winter. What the chimney sweeps are finding out is that their customers have been using their fireplaces as a heating source, rather than for just the occasional friendly fire. That is when trouble can happen.
Open wood-or gas burning fireplaces are designed with clearances to combustibles set at certain distances with the idea that people will use their appliance for short periods of time – up to four hours in duration. Once that time has been exceeded, or before if improperly installed, the nearby combustible materials may overheat and ignite. Combustible headers and studs, which are hidden from view by finishing materials may not be properly installed with proper clearance, which makes matters even worse. Unfortunately, improper construction around fireplaces is a common issue and many house fires have been the result.
Nearby combustible wood may pryolizes over time due to exposure to heat through the masonry or metal fireplace. Pyrolization is the chemical alteration of wood, which lowers the ignition temperature significantly. Wood normally ignites at 500 degrees, but pyrolized wood can ignite at 180 degrees or lower.
After a Raytown, Missouri house fire in 2009 Fire Investigator Gene Padgitt was requested to find the cause and origin of the fire. The customer had used their masonry fireplace as the sole source of heat for three days while they had no gas after moving in their new home on a Friday. Combustible wood framing, which was improperly installed above the fireplace without proper clearances, ignited due to exposure to heat for the prolonged period. Padgitt said that this scenario is all too common and that unfortunately, he sees it often.
In yet another fire investigation in Topeka, Kansas in 2012 Gene found that the homeowner used a manufactured gas fireplace for seven days straight when wood framing, which was properly installed, ignited. The fire caused a total loss. The homeowner thought that he could heat his house with the fireplace.
This type of burning is called "Over-firing" in the chimney and hearth industry. The fact is that masonry fireplaces do not come with a homeowner's manual, and rarely do new home buyers find a manual for their manufactured fireplace. So it is up to the homeowner to get operation information from his chimney sweep or from websites.
Certain wood and gas appliances are designed for heating, while others are designed for ambient fires. Since there are no standards in the industry that have been adopted for masonry fireplaces, Padgitt suggests that homeowners use common sense when operating a fireplace or stove. He suggests the following:
Appliances designed for ambient fires:
- Wood-burning manufactured fireplaces (read the manufacturer instructions)
- Wood-burning masonry fireplaces
- Gas-burning manufactured fireplaces (read the manufacturer instructions)
- Gas-burning masonry fireplaces
- Wood-burning freestanding stoves with proper clearances
- Wood-burning fireplace inserts in masonry fireplaces
- Wood-burning masonry heaters
- Direct-Vent Gas fireplaces (per manufacturer instructions)
- Non-Venting gas logs in masonry fireplaces (read the manufacturer instructions which likely state that they are not to be used for longer than four hours at a time)
Note: Non-Venting gas logs and wood-burning fireplace inserts should never be installed in manufactured fireplaces. To do so voids the warranty on the fireplace, and alters the fireplace which is a code violation. Any alteration can be a fire hazard.
Masonry Heater Association of North America
Chimney Safety Institute of America
Midwest Chimney Safety Council
National Fire Protection Association