What happens if you retrofit an ERV/HRV to an existing supply/return ductwork, and heat/cool with mini splits instead?
So my questions are largely retrofit-based, since we are not expecting to tear down and rebuild our house from scratch. As we tighten up our forced-air house, I’m not very excited by the idea that we will have to put more careful thought into ventilation, which might mean yet more ductwork if we do an ERV or HRV installed the “right” way. But I was curious if we could actually just use the existing supply/return setup and add an outdoor fresh air supply/return for ERV, hooking up just an ERV to that (no furnace), and then adding a few minisplits to heat/cool instead. Would the minisplit(s) actually have some temperature controlled air circulated throughout the house if the ERV is pulling through the old returns? Would the balance of the ERV be off if there’s both a supply AND a return in each room? I’m assuming we would still keep direct outward ventilation through the bathrooms and kitchen range hood, unless that would mess with the pressure too much? This seems like such a simple solution to me for a retrofit that I’m wondering why it isn’t just done, unless the ERV is just too weak to help circulate the air if it’s being both pushed and pulled into a given room (that has both air supply and air returns).
House is 1400 square feet slab on grade with a planned addition in 5-10 years that would add another 150-200 square feet; climate zone 5. Obviously if we’re reusing some/all of the existing ducts for an ERV, we’d want to air seal and insulate them. The eventual plan would be to get to pretty-good house standards, so R-20 against slab, R-40 against exterior walls, and R-60 ceiling in the vented attic (and covering all the attic ducts, of course!).
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The cfm of a right sized ERV is an order of magnitude lower than a typical furnace. For a house your size you're looking at less than 100 cfm for ASHRAE 62.2 levels of ventilation, less than 50cfm using Building Science Corp's recommended ventilation rates. A typical small-ish furnace is pushing 800-1200cfm. The air velocities at that level of duct oversizing for ventilation would be very low, and the mixing & delivered room cfm wouldn't be something that could be counted on. (There's a reason why ERV ducts for houses your size are typically 3"-4" rounds.)
The ducts MIGHT be usable for a ducted mini-split though (TBD). The design heat load of a tightened up 1400' slab on grade house in zone 5 could be brought within range of a single 1.5 ton Fujitsu 18RLFCD, even before major insulation upgrades, and at Pretty Good House levels would probably be within range of the 1-ton version in that series. If you have a heating history on the place, run a fuel-use based load calculation to ballpark the "as-is" heat load, duct losses and all.
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
With not-so-tight not-so-insulated ducts the fuel use based load estimate would be an absolute upper bound.
If the ducts are in an unvented attic above the insulation forget about using them as-is with a mini-split They need to be completely inside the pressure and insulation boundary of the house. It's also important to look at their sizing relative to the output cfm of a mini-duct cassette. The Fujitsu cassettes are good for roughly 500 cfm, and the target duct velocity is about 450 feet per minute give or take to stay within it's static pressure limits, so a cross section of about 150-170 square inches for the main plenum/trunk before any branches would work, say a 10" x 16" or 12" x 14", or a 12" round. Ideally it would taper off after branch points to keep the velocity in range. Measure up what's there and report back.
Onnie,
As Dana has explained, the type of duct system that is appropriate for a 1,000 cfm furnace looks a lot different from the type of duct system needed for an 80-cfm ventilation system.
The duct system for a ventilation system needs to be scrupulously airtight -- a description that excludes the vast majority of forced-air duct systems in the U.S. Moreover, the system needs to be commissioned -- meaning that the air flow at each register and grille needs to be measured.
Finally, the duct system for an ERV or an HRV is really two duct systems -- a fresh air delivery system, and an exhaust air extraction system. These two duct systems usually connect to different rooms than forced-air heating system ducts.
If you hook up an ERV to an old forced-air duct system, you'll have no idea where the fresh air will be delivered, or from which rooms the exhaust air will be pulled.