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Photo: Wm Kibbel |
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Owners of old homes are often very proud of their working fireplace. They understand the function of its chimney and can see the smoke discharging at the highest structure above the house. I often see a puzzled expression, however, when I begin to discuss the other chimney, or second flue of a single chimney.
Many homeowners never consider the fact that the central heating system also vents 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 or oil. Many homeowners replace these old furnaces with more efficient, modern systems, or convert to cleaner burning gas heat. Unfortunately, the chimney is often 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, more efficient heating equipment. Much of the combustion heat from older boilers and furnaces was lost up the chimney. This escaping heat warmed the chimney and created an adequate updraft. As a result, very little of the exhaust of the heater condensed before reaching the atmosphere. Today's heating systems extract more of the heat for distribution into the home. Flue gases now enter the chimney at a much lower temperature, creating less draft and more condensation.
- Condensation from gas fired equipment reacts with the deposits already inside the chimney flue from the previous type of fuel. This condensation water combines with fuel emissions to create sulfuric acid, which can eat away at terra-cotta liners, exposed bricks or mortar.
Evaluating the flue
Before calling in a certified chimney expert for a thorough evaluation of your chimney and flue you should get a general, unbiased opinion of your chimney interior.
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 the equipment or experience to observe the flue from the top of the chimney, it can be viewed with a small mirror from the clean-out. If there is no clean-out access, have your heating contractor remove the heater's flue connector. With a powerful light, you should be able to see enough to determine some of the condition or the flue, and whether it is lined.
After your own preliminary inspection, you'll be better able to understand the evaluation by a certified chimney specialist. When you do choose a chimney professional, consider one who uses electronic imaging to inspect the entire length of the flue, especially if your chimney is very tall or has offsets that make a simple visual inspection difficult.
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 building inspector 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 be specific requirements for relining systems.
What about rebuilding?
I rarely find a chimney that needs to be completely rebuilt. Occasionally, I will see a chimney that needs the section that extends above the roof reconstructed, where condensation and weather have eroded the mortar on the interior and exterior. Some chimneys may be difficult to reline due to one or more offsets, and it is sometimes necessary to create openings at one or more locations 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 inspected annually. And don't forget that regardless of your heating system, all chimneys need to be kept clean and clear of debris.
