ERV exhaust up a chimney
Can I run the main exhaust pipe of my ERV into the abandoned flue of my chimney?
I am remodeling an old house and installing a Zehnder ventilation system with an ERV. The house has a masonry chimney with a 7×7″ flue out which an old (1933) oil-fired boiler used to exhaust. Since I was planning on running the 7″ round (interior) ERV exhaust pipe right past the flue opening, I thought I could terminate it into that opening rather than seal it shut. I will inspect the flue to make sure it is sound, but I am assuming for now that it is since the boiler was installed and functioning right up until I started the remodel. I am also assuming the total run length of the pipe, including the 20 ft or so of flue, is acceptable as well, but I will verify with Zehnder.
Is there any downside to using a chimney flue to exhaust an ERV? Unlike, say, gas-fueled appliances, the ERV exhaust won’t contain any corrosive elements, it’s just air. Presumably the ERV exhaust will not have a significantly different moisture content or temperature than the outside air, so condensation should not be an issue. And I am not relying on a “chimney effect” to expel the air, I am relying the the ERV’s fan. While masonry may not be as smooth as galvanized pipe, the 7×7″ square flue has 25% more surface area than a 7″ round pipe, so that should reduce the overall resistance to airflow.