No quick fix for build-up of ice on older chimney

Hey there, time traveller!
This article was published 24/03/2002 (8669 days ago), so information in it may no longer be current.

QUESTION — Ten years ago, I purchased an 80 to 90 year-old home with an original boiler. About eight years ago, I had a local company replace the old furnace with a new hot water boiler. Every year since the new boiler was installed, I get a build-up of ice on the flat part of my chimney. The ice is the result of condensation dripping from the chimney cover, which forms large icicles. These icicles get very large and eventually fall. If one ever fell on someone they could be seriously injured. I have had the original installer out on several occasions. They have explored every imaginable source of the problem and are unable to stop it from occurring. I have had Centra Gas out and they say that it is the installer who should fix the problem. The size of the chimney liner meets the required specs. I have also added a fresh air intake and that has had no effect whatsoever. Any suggestions?

–Adrian Challis, e-mail

Answer — The ice problems experienced in the Challis home are all too common in recent years. Updating older boilers and furnaces with more efficient units will create a change in the quality of the exhaust products from these heating units, and the method in which they exit the home. These changes can create problems previously not seen. Ice build-up near the top of the older chimney is frequently encountered. Some possible solutions will be explored, but it may not be possible to prevent the ice completely.

Older natural draft, gas-fired furnaces and boilers incorporated a draft hood on the units to help with the flow of combustion products up the chimney. This draft hood would draw dilution air from the area around the heating unit to mix with the hot combustion products, and increase the draft up the chimney. This was necessary to allow proper removal of these waste products of combustion from the home. This warm dilution air contained moisture from the house and the natural draft would reduce the humidity in the home. Unfortunately, the dilution air and the hot exhaust products carried a significant amount of heat with it, reducing the efficiency of the heating unit, increasing the cost of heating the home.

Newer, more efficient furnaces and boilers do not have a draft hood and force the waste products of combustion up the chimney with the aid of an exhaust fan, integral to the units. This fan is also necessary to help the lower temperature exhaust from the new appliance rise to the top of the chimney. Many newer furnaces and boilers are significantly smaller in size and heat production than their predecessors, and require much smaller chimney flue sizes. The smaller flue or chimney liner size helps the furnace exhaust fan, increasing the flow up the chimney. This will minimize condensation as the exhaust cools, but may not eliminate it. This exhaust may carry a significant amount of water vapour, which may condense and turn to water and ice as it exits the chimney.

One possible solution is to install a B-vent or double-walled chimney liner, which provides an extra air layer between the warm exhaust and cold outside air. This may help, but according to HVAC contractor Allin Leray of Heat Plus, may not solve the problem. Leray says that heating contractors should install new furnaces or boilers with the minimum diameter liners allowable, but are limited by the specifications of local authorities. He notes that homes have become much more airtight in recent years, raising indoor relative humidity levels, which is compounded by the absence of draft hoods on the natural gas appliances. Leray also suggests that recent winters have had higher outside relative humidity, due to unusual weather patterns. High winds may hit the exhaust as it exits the chimney, causing substantial ice build-up.

Unfortunately for the homeowner in question, and others with similar problems, there is no quick fix. Changing the style of rain hood on the top of the liner or adding a better flashing to the liner or chimney may reduce the size of the icicles, but likely won’t prevent the ice build-up. I do recommend periodic removal of the icicles, if they become too large, to prevent injury to passersby, which is a real possibility.

This is reminiscent of a recent visit to my neighbour’s house, where he informed me that something had fallen on his roof from an airplane, passing overhead. He heard a loud bang on his roof, while renovating a second floor bedroom. On further investigation, I discovered a large piece of ice in the yard, which had fallen from the chimney. He had been experiencing problems with significant ice build-up and luckily no one was below this area when the ice fell off the roof.

Ari Marantz is owner/inspector of Trained Eye Home Inspection Ltd. and is the P. R. rep. for the Canadian Association of Home Inspectors — Manitoba (www.cahi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com