I’m trying to figure out how to deliver ERV supply air to three 2nd-floor bedrooms without making more holes in the ceiling. I installed the ERV last year in a “hybrid” configuration, where the supply goes into the AHU in the attic and the exhausts are in the two 2nd-floor bathrooms. The blower fan is interlocked to the ERV.

Overall, this has worked reasonably well. It will soon be hot and humid in the D.C. area, and I’d like to experiment with dedicated ERV supply air to each bedroom. It would be quite challenging to get ERV supply air ducted to the 1st floor, so I still need some AHU blower operation for distribution (by way of an “X min/hr” setting on the thermostat).

So, I’m wondering if I could cut a hole in the ceiling diffuser boot and add a metal flex duct for ERV air that would terminate at the face of the diffuser? The bedrooms have 8″ round diffusers supplied by 5″ rigid ducts. By area, it seems like it wouldn’t cause a flow problem if I upsize the boots to 8″. The max AHU fan speed is 875 cfm distributed among nine 5″ ducts, or roughly 100 cfm per diffuser. (Side note: seems like the supply ductwork is undersized, and the old A/C had a bigger blower!)