ERV/HRV in existing home
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When setting up ductwork for an ERV/HRV, can you achieve a “exhaust ducted” system by attaching stale air return towards the end (closest to the vent) of an existing HVAC duct that is in the kitchen? So you’re essentially just using the very end of the existing HVAC duct to take stale air back.
If you did this, can you draw in all the stale are from one location (the kitchen in this example)?
Also, can you draw in air from floor vents rather than wall? All the diagrams show air being drawn from wall vents.
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Replies
Why would you want to draw exhaust air from your supply duct? It is typically recommended to use exhaust from restrooms for ERV/HRV.
There should be no reason you can't exhaust from a floor register. I would recommend you exhaust from a ceiling or wall though because if it is on the floor it will cause more dust and debris to enter the exhaust duct.
I didn't mention drawing stale air from supply ducts.
In your plan, where would the fresh air be piped back in?
I would think, as long as you return the same cfm downstream as you remove upstream, and respect the separation distances in the installation manual, it should work fine.
My plan was to return fresh air directly into the central return air of the HVAC
Seems ok to me but I do wonder what your duct layout looks like and whether you'd be missing out on an opportunity to exhaust stale return air from an important part of your house, like bedrooms, if the junction for the ERV is upstream of that.
When adding ERV supply air to a ducted H&AC System the fresh air will be distributed to all the spaces the conditioned air goes to in the same proportion as the conditioned air.
But the fresh air proportion distributed to bedrooms is typically proportionally greater. A typical scenario wants 25% of the fresh air to go to a primary bedroom, but only 17% would be getting there via H&AC ducts.
Stale air to the ERV is withdrawn from bathrooms, Powder rooms, laundry , mud rooms, and kitchens (optional if an outside exhausted hood is available). After the stale air passes through the ERV, it is exhausted to the outside.
ERVs often have a recirculation option. This allows intentional distribution of bathroom humidity to living areas which can become too dry (in heating dominated climates and dry regions) .
Withdrawing the stale air from an H&AC return duct does not address need to remove high moisture levels from bathrooms. Separate ventilator fans would be necessary.
Each bathroom already has an exhaust fan that vents directly to the outdoors. Would there be a need to withdraw stale air from these rooms in this situation?
In a word, no.
You need to assess why you’re thinking about an ERV/HRV and whether the added benefits of ventilating the bathrooms is of value. Much depends on where you are located, current efficacy of your H and AC system, air tightness of the house and anticipated system replacement / upgrades / electrification.
There’s a lot of material on balanced, exhaust driven and supply driven solutions for residential ventilation.
And whatever approach you take, be mindful of return air pathway adequacy.