I made a post a while back:

HRV via bathroom exhaust from hvacadvice

Basically, trying to figure out how to do my hrv, the gist of it is I have only the fresh air intake and nothing else, though fresh air out-take (whatever you wanna call it – air going to furnace) would be really easy.

What I DO have however, is a bathroom fan on the main floor and the vent goes right through the furnace room, right near where the hrv would go. So in my original post and in this diagram (option B):

https://i.imgur.com/tWGzJsO.jpeg

this is what I could do. (if the picture isn’t obvious, just have the hrv go ‘midstream’ in the bath fan)

However, the biggest issue I have is the bathroom fan is all 4” duct which from my understanding with hrv’s, 5” is kind of the minimum. It is possible for me to replace the bath fan exhaust with a 5 or 6” –  through some difficult limited access and complicated finished area (it would involve removing ducts, reaching my hands in tight spaces, yuck).  But before I do all that, I guess my biggest question is:

Can I run my HRV exhaust on 4” ducts?

I know the answer may come with some ‘yes but restrict speed and airflow’ so my follow up is;

should I just buy the weakest hrv in that case? Or one that has variable speeds?

Another big question, Knowing that my stale air comes from the bathroom.

Should I keep the bath fan and wire it so it turns on every time the hrv is on, the bath fan turns on, and vice versa? Or rip bath fan out and have it just hole in ceiling (which feeds hrv)?

I know this may be a no, since it probably isn’t best for an hrv to be fed by forced air. obviously, the alternative is to rip out the bath fan and it basically just be a hole in the ceiling and whenever someone hits the bath fan switch it turns on hrv. Side note: it’s a half bath so no shower.

I will also mention, that my plan also involves for the intake (stale) is to have it come from two sources

a. the bathroom

b. the ceiling of the basement bathroom or anywhere else in base, which will be painfully easy.

so if its allowed my intake stale air would be multiple 4” ducts merged into a 6” or so, so it would only be the exhaust that would be a bottle neck (again, I may be able to jerry rig a 6” pipe to replace bathroom exhaust but for now I have to plan for the worst). So last question

For stale air intake on an HRV is it ok to have two or more sources merge into one and then into HRV?

On another note, it is possible to run new exhaust duct, and new intake (stale air) ducts as oppose to jerry rigging the bath fan. The new options involve going through uncharted walls etc so the bath fan is easier and to a lesser extend satisfies the ideology of less holes in my house. But I could one day make these changes and the return the bath fan to its original state. I only mention this as maybe buying a larger hrv still makes sense if one day I do have the higher airflow bandwidth.

Final question:

What is everyone’s consensus on air purifier or cleaner? I wanted to also add something like the lifebreath tfp but my research has run me short. HRV solves most of my stale air and moisture problems but I am happy to invest in slightly cleaner air