[jtbayly]

I’m interested in this, but I think I just realized that what I want it to do is impossible.

We have a fireplace, which is not efficient at all, in part because it sucks cold air in from the outside. I was thinking it would be great it we could use an ERV to condition the air that gets brought in.

However, as far as I can tell, the moment you exhaust air from your house in any way except through the ERV itself, the ERV cannot help you with the replacement air that comes in.

Is that correct?

[open_erv2]

That is half correct. The TW4 units operate in synchronized pairs. When one is exhausting (egressing), the other is ingressing (bringing air in). This always happens. They even have quite nice pressure sensors that precisely regulate the pressure the fans exert, controlling flow precisely and preventing the house from becoming pressurized at any time, which as you say would imply heat loss.

[tempestn]

It would be cool if someone could build a chimney ERV: extract heat from the dirty air produced by fire, and inject it into fresh air pulled into the living space. So I guess basically it would function like a regular ERV, but with a fire in the exhaust path. (Probably not feasible with a wood fire, but maybe with gas?)

[tempestn]

Maybe this thing works that way? https://www.modlar.com/brands/nu-air-ventilation-systems-inc...

[turtlebits]

If you can use the exhaust heat to temper the incoming outside air, that's all an ERV does. Or, get a direct vent fireplace (draws in outdoor air)