"Understanding Your Home" by Building Inspector Mark Visser
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FRESH AIR SUPPLY
Air supply kits, which include a hooded intake, a length of duct, and an adjustable air inlet are available for existing masonry fireplaces. The one pictured is for new installations.


Never burn treated lumber in a fireplace or wood stove 'Wolmanized' lumber contains arsenic and chromium which are highly toxic. Do not use cedar shingles for kindling, they are also treated with chemicals.

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Fireplace Overview
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Wood burning fireplaces and Zero-clearance gas fireplaces. Fresh air supply kits. Pyrolysis.

Many people believe that a fireplace will provide a significant amount of heat to the house but fireplaces are in fact extremely inefficient. Only a very small amount of the heat available from burning wood is actually delivered to the house. The rest is lost up the chimney. A roaring fire can exhaust as much as 24,000 cubic feet of air every hour. This warm house air is then replaced by cold air leaking into the house through leaks and cracks.
In tighter, more energy efficient houses, adequate combustion air may not be available. In extreme cases, a roaring fire may compete with the gas furnace for combustion air, and may create a negative pressure strong enough to draw air down the chimney (even if the furnace is operating) causing a dangerous situation. To prevent this a fireplace should be provided with direct access to outdoor air, see illustration to the left. When this is not possible, you should open a window at or below the level of the fireplace whenever the fireplace is used.
Masonry fireplace. The traditional masonry fireplace is heavy and requires a foundation similar to the house foundation. The firebox is, in most cases, a 2" (50 mm) thick firebrick that is laid with high temperature cement.
Zero-clearance gas fireplace. This type of fireplace is factory built and has many advantages over the traditional fireplace. There is no need for an expensive foundation.
A zero-clearance gas fireplace weighs as little as 75 lbs. (35 kg) and a wood burning unit 200-400 lbs. (87-174 kg). The unit can be placed just about anywhere. Specially designed double-walled high temperature chimneys can pass through closets while direct vented gas fireplaces don't need a chimney at all. The efficiency of a zero-clearance unit is about the same as a heatilator type metal insert that is placed inside a masonry fireplace to heat and circulate room air.

PYROLYSIS WARNING. Wood stoves located too close to combustible materials can cause a phenomeon called pyrolysis. Over time, the heat from the stove causes a chemical change in the wood, lowering the temperature required to make it burn. For instance, a normal piece of 2"x4" will spontaneous ignite at 600 degrees F. If the woodstove or flue pipe is too close it will draw the moisture from the 2"x4". When this happens repeatedly, the piece of wood can spontaneously ignite as low as 200 degrees F. Generally this happens in 3-5 years. The wood stove can be used for years without a problem, then suddenly there is a fire.
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