Chimney Has a High Flash Point

QUESTION: My home has a heating system with a metal chimney that runs up through the attic and terminates above the roof. Since the temperature of the chimney in the attic is not high enough to ignite wood, why is it necessary to have a 2-inch clearance between the chimney and the wood framing in the attic?

ANSWER: The ignition temperature of wood is defined as the temperature at which it begins to burn. Wood and other combustibles undergo a physical change when continually exposed to elevated temperatures. This reduces their ignition temperatures.

Wood normally begins to burn at about 400 degrees to 600 degrees F. However, when it’s continually exposed to temperatures between 150 degrees and 250 degrees F., its ignition temperature can become as low as 200 degrees F.

The lowering of the ignition temperature of wood and other combustible materials can take years to occur. When it does, should the wood’s temperature coincide with its lowered ignition temperature, it will ignite and burn spontaneously. This is the basis for the clearance requirements in building and fire codes.

Hosing Down May Cure Chalky Bricks

Q: I’m concerned about the condition of our brick home. We bricked it two years ago. Every now and then a white chalky substance appears on it. Is there something we can do about this?

A: The white substance is deposits of water-soluble salts called efflorescence. These salts may be contained in the bricks, the mortar or possibly the masonry behind the bricks. When efflorescence appears on a brick wall of relatively new construction, it’s often the result of water absorbed by the bricks. The soluble salts dissolve in water (from rain, snow or ice) and are deposited on the brick surfaces when the water evaporates. However, it’s also possible that water is entering the wall through open joints or poor construction methods, and has found a path to the surface.

When there is a sufficient amount of water to dissolve the salts, there will be an efflorescence buildup on the wall surface. In this case, you should examine the wall and construction details for open joints and possible sources of moisture penetration. All open joints must be sealed and poor construction methods corrected.

Efflorescing salts will generally disappear of their own accord with normal weathering--especially when they are the result of absorbed water. However, you can remove the salts by dry-scrubbing with a stiff brush. Heavy accumulations of stubborn deposits may be removed by scrubbing with a solution of one part muriatic acid to 12 parts water.

It is imperative that the wall be saturated with water before and after the solution is applied.

Before you go to the acid, however, try several sessions of hosing down the whitened area of bricks to wash away the deposits, and see if that doesn’t do the trick.