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What kind of air handler is with each furnace? If it has a variable speed ECM motor the energy use running it 24/7 may not be so bad. If it's not and uses several hundred watts per hour that may not work so well. Even if variable speed ECM I would just tie in with one air handler, preferably the one that serves the bedrooms unless the other is much more efficient.
Are you set on having the ERV ducted to every room, or do you have a central location it could be ducted to? I have been happy pulling stale air from two upstairs bathrooms and bringing fresh air to a single location in the upstairs hallway since it's between the three bedrooms and within a few feet of the upstairs return for the heat pump when it runs.
Most ERVs I've seen need to be installed in an area that is at least in the 40s if not 50s Fahrenheit. You could maybe sacrifice some part of your conditioned space such as a closet to store the ERV, or make a small conditioned area within the attic or crawl space for it if not conditioning the whole space, and then be sure to insulate all ductwork either going through unconditioned areas or carrying unconditioned air.
I would say none of this would be cheap, particularly if hiring someone else to do it. I don't know if I could have hired someone to do it where I am since even the energy auditor I had come by was unfamiliar with ERVs. I'm not sure what counts as breaking the bank for you. My total material cost was around $3k but could have been less if getting a smaller ERV and sticking with the washable filters that come with the Broan vs adding in-line filter boxes. Sticking with the control panel on the ERV itself vs wiring the advanced touchscreen controller would have saved a little money but it's much nicer being able to control it from my bedroom vs the attic and the control provides features that weren't accessible without it.
One thing that made installing an ERV feel approachable was seeing someone on Healthy Home Guide install a Broan ERV in a rental using a window for the inlet/outlet and having everything in one room. Seeing their success made me feel I could do something similar on a somewhat larger scale. I'd had experience adding ductwork for a kitchen range hood and a bath fan, and the Broan AI ERV measures and adjusts CFM by itself for commissioning.
https://www.youtube.com/watch?v=CnLwJxCbxfM&t=473s
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I had a challenging situation as well in a 100 plus year old house, retrofitted but with limited mechanical room. The HRV is installed into a basement stairwell with a dropped ceiling. Return is in that stairwell and supply is on the main floor, central. It works very well, although at night I do run the ECM furnace fan to keep 2nd floor bedrooms CO2 in check. The automation system is typically running that system at 60-75 CFM (family of 4) to keep CO2 levels (measured on each floor and averaged) under 750ppm. Zone7A.
The system responds to CO2, PPm2.5, VOC and Radon as well as running in an asymmetric mode to do some supply air for the kitchen exhaust.