Installing Bosch tankless water heater flue

by Daryl
(Edmonton, Alberta Canada)

Here's a story from the great Canadian north. I live in Alberta where winter can be severe. About 10 years ago, I had a tankless Bosch water heater installed in the cabin and it has worked really well for us. It was located in the same area as our old Lennox furnace.

Both appliances were vented through the roof through one combined flue. The furnace drew combustion air from outside and there is also a combustion air duct directly into the furnace / water tank area. In December, I had a high efficiency furnace installed which has dedicated exhaust and combustion air venting. This resulted in orphaning the hot water heater flue. As a result, when the furnace runs, it draws cold air down through the water heater flue which has caused the heat exchanger to freeze and crack.

One question I have is, if my HE furnace draws dedicated combustion air from outside, why does it also pull air down through my water heater flue? I am assuming it must also draw combustion air from inside the house. I was told by the installer that the furnace was a "sealed unit". When I asked him about this problem, he talked a lot, but didn't really seem to understand either.

I am trying to understand what's really going on before I make a decision on replacement of the water heater.

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Feb 06, 2017
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starstarstarstarstar 		More Testing Required
by: Anonymous

Thanks very much for your comment Marshall. I have removed the heat exchanger from the heater which left the flue unattached. I taped a plastic bag over the flue to prevent the cold air from entering the mechanical closet. When the furnace runs, negative pressure in the house fills the bag with air from outside. When the furnace stops, the bag deflates. There are no fans running or fireplaces lit. I only have a wood burning fireplace and the flue in closed. The house is a 2x4 construction, 1600 sq. ft. house that was built in the mid 60's. I cover most of the windows with plastic in the winter and I make sure the doors seals are good. Still, the house is not very air tight.

I am going to bring in my HVAC guy and do some testing on the return air. It may be that I have insufficient return air to the furnace and so it is drawing air from the mechanical closet. I will update once we have done the testing.
Feb 05, 2017
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by: Marshall

Hi Daryl,

i have a couple suggestions... are there any devices in the structure that create negative pressure? bathroom fans, fireplaces, etc. could be drawing outside air when they operate.

second, the flue may reverse airflow when it cools since the column of cold, dense air would travel downwards.

sorry to hear about the cracked exchanger!

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