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ERV exhaust up a chimney

noone95 | Posted in Mechanicals on

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.

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  1. charlie_sullivan | | #1

    Is the chimney interior or exterior? If it's interior, in the winter, the air exiting the ERV will be cold. That will cool the chimney and you'll have some heat loss to the chimney--you'll be using the heat from the house to conduct through the chimney walls and re-heat the air that you just recovered the heat from.

    If it's exterior, I don't see any real problem, but if you are already at an exterior wall, you could run the vent through the wall without going up. That would shorten the run and might decrease your fan energy use a bit.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Is there any downside to using a chimney flue to exhaust an ERV?"

    A. At a minimum, masonry chimneys have two openings: the flue termination at the top of the chimney, and the cleanout door at the bottom. Cleanout doors are not airtight.

    If you connect the exhaust duct of your ERV to an old masonry chimney, some of the exhaust air will exit the chimney at the top, and some of the exhaust air will enter your house through the cleanout door. The air entering your house through the cleanout door will carry a sooty smell that you probably won't like.

    Even if you manage to seal the leaks in the cleanout door, old masonry chimneys can have poorly sealed thimbles that are hidden behind drywall chases. I think you will discover that it's hard to seal all the leaks in this chimney, so I advise against using it as an exhaust duct.

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