I’m in the design phase for a large residential build and I’m having difficulty picking out a general HVAC topology.  I’m going to track down some high performance house HVAC folks to consult with a little later in the process.  But right now, I want to narrow down to a few options that I can look at now with my architect so the house layout has enough room for ducting and I don’t have to deal with a bunch of expensive redesign down the road.

Here’s the basic details about the house:
– Build will be in metro Detroit
– Roughly 8400 SF of floor space, roughly 6600SF and  64K cubic feet of air volume inside the HVAC envelope.  Remainder of house is 3 work rooms for metalworking, woodworking and electronics work that have large airflow venting to the outside, significant ACH to the outside and will have some temperature regulation to keep temp swings modest.
– 4 story (basement, 3 above grade) ICF build.  Not explicitly going for passive house compliance but hoping to get into that general range of R-value and air tightness for the envelope.  Aiming for an ACH number well below 1, hopefully in the 0.5 range.
– House will be a personal residence and a rental property with 8 rental rooms.  Expecting roughly 10 adults in residence at any given time.

While I’m open to considering radiant floor or coil boxes and other modernized radiators, standard forced air just seems to be the simplest and cheapest approach for this house.  But even with forced air, I’ve got a lot of questions since most HVAC resources are aimed at a more traditional house in terms of building tech and general design. 

The low ACH of the house and large volume will require a lot of external makeup air.  I’m having trouble finding good data for the proper amount of fresh air required.  I’ve seen 0.33 ACH of fresh air being tossed around for passive house builds but that seems like kind of a low number if rooms like bedrooms and bathrooms are going to see enough ventilation.  I’m personally very sensitive to stale air and want to err on the side of overventilating in the HVAC design rather than the other way around. 

I’ve made a very rough draft HVAC design.  It pushes 24/7 external fresh, conditioned air at roughly 1 ACH to all bedroom spaces, 0.4-0.5 ACH to the large common room areas and 5 ACH of flow through the bathrooms.All of the non-master bedrooms have their own supply and return vents to maximize soundproofing for privacy.  The master bedroom has its own supply and its return is roughly 50/50 through a standalone return and a returns in the master bathroom.  The common areas have several supply registers and the air return is done through several bathrooms.  With those ACH numbers, the total incoming fresh air rate is just shy of 600 CFM.

With that setup, all of the return air is passing through a bedroom or bathroom and I’d like to avoid recirculating any of that air back into the house.  To keep the HVAC as simple as possible, I’d love to avoid the typical topology where there’s an intermittent, high flow supply that recycles internal air and adds in a smaller amount of fresh air.  I would rather just have external air come in at a constant low rate through the ERV and heat pump coils, go to the supply registers, through the returns and go back through the ERV to the outside – 100% of the HVAC airflow being external fresh air.  There’s some commercial grade ERV/HRV units that can provide 600-700 CFM of airflow with relatively high recovery efficiency, so I can hopefully mitigate the heating/cooling and de/humidification loads of bringing in that much fresh air.

The main concern I have is that the register flow rates don’t give anywhere enough throw to properly circulate the air in most of the bedrooms.  The rental bedrooms range from 12×13 to 12×20 and at 1 ACH of flow, the supply registers are seeing 20-35 CFM of continuous flow rate.  I was looking at the Hart/Cooley catalog and at those sorts of flow rates, I can get *maybe* 6 feet of throw which seems completely inadequate to mix the air in those rooms.  In the master bedroom, it’s even worse.  It’s two connected rooms and a bathroom that are about 20×40 and the best setup I can think of is a ceiling mounted 2-way in the midpoint of the room that reaches about 7 feet from the centerline of the space and there’s 13′ of additional room on either side.

Do I just have to put in a bunch of ceiling fans to properly mix the room air?  (I don’t like ceiling fans much, so this would be a last resort)  Do I rethink the whole topology and increase the register flow rates by recirculating internal air through the system?  That would require completely rethinking how it works since I don’t want to recirc bedroom or bathroom air at all.  Are there special low-flow registers that can get more throw at lower CFM rates?  Am I worrying too much and 24/7 ventilation will mix the bedroom air sufficiently? 

Any feedback on whether this HVAC scheme is workable or needs to be rethought from the ground up would be greatly appreciated.