The second and most obvious problem from cool chimney temperatures is the condensation of water vapor inside your chimney. As odd as it may seem, the combustion of any hydrocarbon-and gas is a hydrocarbon-results mainly in carbon dioxide and water vapor. In fact the average furnace puts about 1 1/2 gallons of water into your chimney every hour! The high stack temperatures of the older inefficient furnaces kept this moisture from condensing inside the chimney, and it was often visible as steam escaping from the chimney top. Because the newer high efficiency furnaces now steal this extra heat from the chimney, all this water now often condenses inside the cooler flue.
The problem becomes even more complicated however because this water is also usually highly acidic and corrosive. The air used for combustion is generally contaminated with not only normal air pollution, but often with household cleaning products, especially chlorine from bleach. If the chimney was previously used to vent coal or oil there are most likely also sulfur deposits left inside too. So now not only do you have a gallon or more of water an hour in your chimney, you now have a gallon or more of dilute hydrochloric or sulfuric acid eating away at the mortar and brick of your chimney from the inside!
The situation is aggravated by cold exterior chimneys and long runs of connector pipe between the furnace and the chimney. Although your chimney may be suffering from an improper heating/venting match without producing any visible symptoms, sometimes the excess moisture produced causes visible results. Any of the following symptoms could point to a venting system problem.
- Damp patches on interior walls or exterior walls
- Peeling wallpaper
- Blistered paint
- Stains on the ceiling around the chimney
- White stains(efflorescence) on the outside of the masonry chimney
- Eroded mortar joints
- Crumbling bricks
Eventually corrosion caused by this acidic water condensing inside the flue may cause the liner, mortar, and brickwork to flake and crumble. Chimney sweeps often find this debris creating blockages in the flue, potentially exposing the occupants of the home to carbon monoxide and other dangerous combustion byproducts, a situation not to be taken lightly.
The first step is to have the chimney/venting system evaluated by a competent Certified Chimney Sweep, someone who understands the relationships between furnace and chimney type. A qualified sweep can offer advice and recommend measures to make your entire system function safely and efficiently.
If a problem is found, the solutions often involve installing a correctly sized, insulated liner, and/or reworking the connector pipe between furnace and chimney. These upgrades are designed to resize the flue for better draft, minimize the condensation, and contain the acidic byproducts within the liner to protect the surrounding masonry. (See the area on liners for more information)
A few dollars spent on corrective measures could save thousands in expensive chimney repair down the road, and will help protect your home and family.