The 'other' chimney - the one serving your furnace

Owners of old homes are often very proud of their working fireplace. They understand the function of its chimney and can see the smoke dischargingat the highest structure above the house. I often see a puzzled expression,however, when I begin to discuss the otherchimney, or second flue of a single chimney.

Many homeowners never consider the fact that the central heating system alsovents into a chimney. When I point this out and start talking about health and safety,the expression turns to one of concern.

How heating changes affect chimneys

The original central heating system in most older homes was fueled by coal oroil. Many homeowners replace these old furnaces with more efficient, modernsystems, or convert to cleaner burning gas heat. Unfortunately, the chimney isoften neglected in this switch, and this can cause a two-step problem:

• Large chimney flues required for coal and oil are often oversized for modern, moreefficient heating equipment. Much of the combustion heat from older boilers and furnaceswas lost up the chimney. This escaping heat warmed the chimney and created anadequate updraft. As a result, very little of the exhaust of the heatercondensed before reaching the atmosphere. Today's heating systemsextract more of the heat for distribution into the home. Flue gases nowenter the chimney at a much lower temperature, creating less draft and morecondensation.
• Condensation from gas fired equipment reacts with the deposits alreadyinside the chimney flue from the previous type of fuel. This condensation water combines with fuel emissions to create sulfuricacid, which can eat away at terra-cotta liners, exposed bricks or mortar.

Evaluating the flue

Beforecalling in a certified chimney expert for a thorough evaluation of your chimneyand flue you should get a general, unbiased opinion of your chimneyinterior.

Your chimney can be inspected in one of two ways:

• from the top of the chimney looking down, or
• from inside, looking up.

If you do not have theequipment or experience to observe the flue from the top of the chimney, it canbe viewed with a small mirror from the clean-out. If there is no clean-outaccess, have your heating contractor remove the heater's flue connector. With a powerfullight, you should be able to see enough to determine some of the condition orthe flue, and whether it is lined.

After your own preliminary inspection, you'll be better able to understandthe evaluation by a certified chimney specialist. When you do choose a chimney professional,consider one who useselectronic imaging to inspect the entire length of the flue, especially if yourchimney is very tall or has offsets that make a simple visual inspectiondifficult.

Relining the chimney

Your chimney needs to be relined if:

• The interior of the chimney, or the previous liner, has deteriorated to the point that the emissions from the heating equipment could escape into the home or cause further damage to the structure of the chimney.
• The flue is oversized for your current heating system. The relining will reduce the interior dimensions of the chimney. This creates a higher temperature inside the chimney, increasing draft and reducing the possibility of the gases condensing before reaching the top. Your local buildinginspector or heating contractor may be able to calculate the dimensions of the flue needed for your heating equipment.

There are three types of chimney liners:

• Cast-in-place concrete lining systems. This is the most expensive option, and the best option for a chimney that needs structural reinforcement. You can often avoid rebuilding the entire chimney by installing a cast lining system. These systems can be used for any type of fuel, but if your chimney is structurally sound, you may be able to use a less expensive metal liner.
• Stainless steel. These lining sleeves are recommended for coal and oil systems because they resist corrosive acid emissions these fuels generate. They can be used in chimneys that are structurally sound.
• Galvanized or aluminum sleeves. Similar in construction and function to stainless, this least expensive option can only be used if both the furnace and the water heater are natural gas. Never use galvanized or aluminum with a coal or oil system, because the metal will rapidly corrode.

Before you decide on a liner, check your local building codes. There may bespecific requirements forrelining systems.

What about rebuilding?

I rarely find a chimney that needs to be completely rebuilt. Occasionally, Iwill see a chimney that needs the section that extends above the roofreconstructed, where condensation and weather have eroded the mortar on theinterior and exterior. Some chimneys may be difficult to reline due to one ormore offsets, and it is sometimes necessary to create openings at one or morelocations to work the liner down through the length of the chimney.

The final word

Even if the chimney liner has been updated, it still should be inspectedannually. And don't forget that regardless of your heating system, allchimneys need to be kept clean and clear of debris.