Heating with Wood – Your Fireplace

By: Yale Wolk, Erinson Home Inspections

Many people use fireplaces, fireplaces inserts or wood stoves as heating devices. One of the critical components of this heating approach is the fireplace chimney and flue.

Escape Route For Exhaust Gases

A masonry chimney consists of an outer shell and an interior flue liner made of clay tile (terra cotta), poured concret, or metal. The flue directs exhaust gases up the chimney, and protects the chimney wall from heat damage and corrosion. A flue is sized according to the type of equipment it serves. You may have more than one flue, each designed for a specific task.

Masonry chimneys that are well built will last for generations, but they can experience problems. Deposits can build up inside, blocking the flow of exhaust gases, and causing a deterioration of the flue liner.

There are a number of materials which can cause deposits in your flue and chimney. The primary concerns are soot, water vaper damage and creosote.

Soot From Oil-Fired Equipment

Soot is a natural byproduct of oil combustion. The amount of soot produced is dependent upon how well the burners are turned, that is, adjusted for proper fuel/air mixing, and whether there is sufficient air for proper combustion. Poorly tuned untis produce excessive soot. The soot can accumulate on the walls of the flue liner, and can lead to a deterioration of the liner itself.

Water Condensation From Burning Gas

When natural gas burns, water vapor is a byproduct released into the exhaust. When the exhaust temperature is high, the water vapor simply evaporates. But lower exhaust temperatures can cause condensation to occur inside the flue liner. The water leads to deterioration of the terra cotta flue tiles in the chimney, and this deterioration can lead to blockage of the flue itself. When gas logs are installed in a fireplace, there are special requirements for venting the exhaust gases up the flue. Make sure that the installer had indeed followed those instructions so that you will not damage the flue tiles.

Creosote From Fireplaces, Stoves and Stove Inserts

When you burn wood in your fireplace, combustion produces a byproduct called creosote. Creosote is a brown or black flammable residue that deposits in the smoke chamber of the chimney and the flue liner. In severe cases, creosote can glaze and become hard as ceramic. Creosote can ignite and burn with intensity, reaching almost 2.000 degrees F. Creosote needs to be swept out periodically by a professional chimnet sweep every 1 or 2 years, depending upon the amount of wood you burn.

The typical fireplace has a large firebox and a correspondingly large flue vent to accommodate it. However, if you have a wood stove, there will have to be some modifications. A wood stove has a small firebox, and requires a much smaller flue. They can produce water vapor, or condensation. This is typically where ther more dangerous form of creosote, glazed creosote, originates. To overcome this problem, at a minimum there should be a metal extension pipe added to the back of the stove, which then extends inside the fireplace to join the flue liner. Even better, the new stove vent should extend all the way to the top of the chimney. The vent may be made of stainless steel, or poured-in-place (or pumped) concrete, or a new ceramic tile liner can be inserted. (If the stove is removed, the new flue liner will have to be removed.) And if you have a wood stove, it is very important to have a chimney sweep clean your flue annually.

To learn more about home maintainance and heating your home economically, go to www.janleasure.com and www.jan-leasure.com