Installing Bosch tankless water heater flue
(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.