Our finished basement will be one big 20' x 24' room. We will have 1 window which is big enough for a secondary egress, but it doesn't have quite enough net ventilation area to meet the 4% of finished floor area requirement. Therefore, we'll need an ERV to pass the building inspection.
1st 20' wall is mostly below grade (front of house).
2nd 20' wall is only 1/3 to 1/2 below grade (back of house).
1st 24' wall separates the finished basement from the garage.
2nd 24' wall separates the finished basement from an unfinished storage/mechanical room.
The ceiling will be about 7' high.
I was thinking we would use the Panasonic WhisperComfort 60 ERV (FV-06VE1), which should provide at least the minimum required 0.35 air changes per hour when set to 20 CFM, but I'm having trouble figuring out the ducting. We can't use the wall cap accessory (FV-WC10VE1) because it looks like it needs to be at least 90.5" from the ground. Also, we couldn't go out of the rim joist on the rear side of the house anyway because our exterior deck is basically right on the rim joist. The rim joist on the front of the house is only 2' off the ground, which is much less than 90.5". If using 2 separate ducts, other than keeping the intake 6' away from the exhaust of a dryer or fuel-burning appliance, the installation manual says to refer to "local regulations" for the distance between the intake and exhaust hoods. The local building inspector says he is not aware of any specific requirements other than the distance from fuel-burning exhaust. I asked Panasonic for clarification via support email and was quoted from section 502.2.1 from the IMC 2021 which says "Environmental air duct exhaust shall terminate not less than... 10 feet (3048 mm) from a forced air inlet". This seems crazy because the intake only needs to be 6' from fuel-burning exhaust according to the installation manual! Whatever the required distance, we could send both intake and exhaust ducts toward the rim joist on the back of the house, then turn them both down 90 degrees to get under the deck before another 90 sends them out the wall. I just don't know if the 10' intake/exhaust separation is a real requirement or a misinterpretation of the IMC.
Regarding a mini split AC, the contractor has proposed putting in a 9K BTU LG Red heat pump, which is about right-sized for the winter heating load (assuming 10 degrees outside air, and 50 degrees ground temp), but WAY oversized for cooling. According to my own cooling load calculation and it's nearly break-even at 70 degrees (assuming 90 outside air, and 60 ground temp) due to the cooling effect coming from the concrete floor, EVEN with 2000BTU put out by a dehumidifer running 24/7 (which it won't be). People and other electronics will definitely add some heat though, particularly when running on the treadmill or riding the exercise bike. For heat, I'll also have a hot water baseboard zone from the boiler, so I don't necessarily need the heat pump for that. The 1st and 2nd floor have single-zone central AC, but the air handler is in the attic and we can't bring another duct to the basement.
Do I even need a mini split for AC? My heat loss calculation through the floor during summer time is: 400sqft x (10 degrees delta) / (1.2 R-value of floor) = 3333BTU of cooling from floor. Does that make sense or are my assumptions wrong?
I'm in Massachusetts FWIW.
You asked about HRV exhaust, 6' apart is the recommendation but is not a hard rule. You can go down to 3' apart and it will still work. It's not a big deal if there is some recirculation, it's just air. The intake needs to be 10 feet away from a combustion exhaust (like a furnace) however.
The hoods should be 12" above the anticipated snow level. So yes they can be close to the ground. I've done some really close to the ground and it still works, you are just more likely to suck dust and snow into the HRV so keep an eye on it and clear the snow away if necessary.
For a basement you could install some baseboard heaters, either electric or hot water if you already have that. I don't think you really need A/C in the basement if the upper levels have it, the basement usually stays cool. Maybe you could add one later if it's bad.
Regarding AC, one thing I left out is that one of the motivations for adding it is because we are gut remodeling an already existing finished basement, which in its current incarnation, gets a bit uncomfortably warm in the summer when the dehumidifier has been running, particularly when working out. However, that's with barely any insulation. After the remodel, the walls and rim joist will be sprayed with R21 closed cell foam. Not sure if the insulation will make enough of a difference to skip out on the AC. The GC thinks I should just put it in now while it's "new construction" so that we don't have to poke holes in brand new walls after the fact.
OK, but maybe skip the heat pump and just get an A/C minisplit plus baseboards. Price it out different ways.
Usually when finishing the basement you just add some ductwork to the existing furnace/central air for the upstairs, but I don't know what your other HVAC equipment is.
There's no good way to get a duct from the attic (above 2nd floor) to the basement.