A chimney that appears fine on the outside can often reveal a different story on the inside. The flue gases from your heating system that vent through your chimney system are highly acidic and oftentimes toxic. These corrosive gases can deteriorate the original clay liners found inside your chimney. Over time, these compromised clay liners can crack, spall, and even collapse on themselves. In homes built prior to 1940, chimneys may not have a flue lining at all. These old unlined flues are especially concerning as the unlined masonry is unsafe to vent the products of combustion.
What Exactly Is A Flue?
Essentially, a flue is the passageway for the exhaust of a fireplace, wood stove, or heating system. Flues are typically made of clay tiles or metal. Safe, serviceable flues must remain free of perforations, cracks or damage of any kind that could allow carbon monoxide to pass into the living spaces of the home. A flue must also protect nearby combustible materials such as framing, walls, ceilings, insulation, or floors from the heat and smoke.
Years ago, low efficiency gas heating appliances sent almost as much heat up the chimney as they put into your home. This put your utility bills sky-high and wasted resources.
Today's high-efficiency gas appliances extract more heat during the burning process and send much less of it up the chimney flue. But for all the benefits these systems offer, there's one important side effect that must be dealt with -- excessive moisture inside the flue system.
Water is a by-product of burning natural gas. When you burn one cubic foot of natural gas, you're also creating two cubic feet of water vapor. Those old, inefficient heating units sent so much heat up the flue that any water created in the combustion process stayed in the form of steam. The hotter the air, the quicker it rises. These hot temperatures within the flue allowed the water vapor to be carried up and out of the chimney without premature condensation inside the chimney flue.
New high-efficiency heating systems don't put out as much exhaust heat into the flue. When these new systems are installed into old chimneys, the water vapor that's created doesn't have enough draft power to make it up and out of the chimney flue. The flue gases cool, thus losing their ability to rise until they stop entirely and condensate inside the chimney flue.
The old chimneys of yesterday weren't designed for the higher efficient units of today. Internal condensation issues can make it seem like you're chimney is leaking