Gas Furnace and Water Heater Chimneys

Un-lined masonry chimney

This photo shows some of the debris that is falling down to the bottom of an old and un-lined masonry chimney. The debris can be seen at the back of the metal flue pipe. Most of this debris consists out of pieces of brick and mortar. Not visible in the photo are some of the damaged bricks and missing mortar. At the time this photo was taken, the debris had not blocked the entire chimney.

This chimney was originally designed for and used with an oil burner. It was later converted for use with a gas furnace and water heater and most of the damage occurred during the gas period. This kind of damage occurs when water vapor (the major by combustion by-product of gas) in combined with some other combustion by-products comes into contact with the bricks and mortar in an unlined chimney. The damage is slow at first but the rate increases as the brick and mortar become more porous and more of this moist air is absorbed into the chimney material. The damage is compounded by the expansion of the moisture during freezing temperatures.

There is not way to know when this chimney will plug up and allow carbon-monoxide (CO) and other awful stuff to vent into the house. Cleaning the chimney is not a sufficient solution, it might even hasten the deterioration of the brick and mortar. The only reliable and safe answers are:

  • a metal chimney liner,
  • a new and separate metal chimney, or
  • a new high efficiency furnace that vents combustion gases through PVC pipes to the outside.

Un-lined masonry chimneys should not be used with gas appliances. The carbon monoxide and other combustion by-products from furnaces might not kill you, but they may cause some other very nasty health problems.

And don't depend on carbon-monoxide detectors to overcome the dangers of un-lined masonry chimney's. Such detectors may not be accurate enough to prevent poisoning from low level CO exposure.