Trained Eye


Water From Chimney


I live in an older home with an outside chimney. Two years ago I replaced the old furnace with a new mid-efficiency gas furnace. When the cold weather was upon us, condensation formed in the chimney with water running down the wall and onto the basement floor.

Is there anything that can be done to prevent this from occurring again?

I will be looking forward to your reply with great interest.


It is not unusual for ice to form at the top of a chimney that has been converted from an old, inefficient natural draft furnace to an induced draft mid-efficient unit. The exhaust from the new furnace is much cooler and may condense as it reaches the top of the chimney and form ice. I have experienced this with my own chimney after conversion a few years ago. If the metal chimney liner within the brick chimney is properly installed the ice should cause little concern, as long as it does not block the flue at the top.

To minimize this problem, new furnaces often have a double-walled B-vent installed to replace the older single-walled chimney liner. This will keep the exhaust somewhat hotter as it exits the home up the flue. There may still be some condensation and ice, but it should be on the very top of the chimney and little should run back down. In your case there is likely a problem with the chimney liner or the chimney itself.

Allin Leray, licensed HVAC contractor and owner of Heat Plus suggests that the chimney flashing or the liner itself may be the culprit. He says that outside chimneys may be subject to condensation, but it is unusual for this to be enough to run down and wet the basement floor. If the flashing at the roof or the top of the chimney is damaged or has openings, snow and ice may run down the outside of the metal liner or the brick structure itself and may seep into the home through the old cleanout. If the chimney liner is properly installed, it will have a metal cleanout attached to the bottom of the piping, which will catch excess water running down the inside. I would expect this to be leaking if the problem was strictly condensation, not active water running down the foundation wall.

Leray states that a smaller diameter pipe may be inserted inside the older chimney liner to accommodate the newer furnace. The newer furnace will normally have a lower heat output and require a smaller diameter vent. If this is not done correctly or sized properly, the exhaust fan in the new furnace may not be able to properly expel the combustion gases. In this situation, condensation is likely, but problems will normally show up in the furnace fan area, or the bottom of the new chimney liner or cleanout. If the old chimney liner has not been upgraded with the furnace, then problems are much more likely.

Galvanized metal chimney liners will corrode, over time, and need to be replaced. Often the first sign of this is deterioration to the bricks and mortar of the masonry chimney. When holes rust through the liner, heat and exhaust gases can leak out into the brick chimney and cause deterioration. The excess moisture seen in your basement may be the result of too large a liner, and this moisture will run back down to the bottom of this pipe. In older installations, the chimney cleanout may be embedded in the old brick chimney with the access cover near the bottom of the chimney, in the basement. If this cleanout is rusted through at the bottom, condensation will leak into the brick chimney and may seep through into the basement.

Because the water is seen in your home on the coldest days, condensation is much more likely than leakage through the outside of the chimney. Older brick chimneys may leak below grade, but this is normally seen in the spring, when the ground thaws and the snow melts.

Whichever of these possibilities explored is the cause of the condensation; damaged chimney liner, loose flashing, or improper liner, repairs are warranted. Water should never be leaking out of a chimney on to a basement floor. Some ice outside the chimney or minor condensation in the flue or furnace may be normal, but should never be excessive. Consultation with a certified chimney contractor or licensed heating contractor should be able to identify the specific defect causing the moisture intrusion.