How many fully ducted systems are really needed?
Designing phase. 1340 sq ft ranch in 5a. Heating load will be around 18k. Will a centrally located minisplit air handler do the job if supported by a fully ducted HRV and a partly ducted heat pump water heater mixing the air all around? This seems obviously true to me, but I know how appearances are.
I would like it to be true since I can do it pretty easily without plenum trusses or a basement. I also want it to be true that simple building practices can reduce HVAC costs in general, but tight well insulated houses seem to need much larger investments here. Now we need ventilation. Now we need dehuminification. Now we need pricey makeup air. Now the heat pump or furnace or whatever still costs the same even though much smaller. What do I get for figuring out the furring strips? Where are the drawings of how actually to use a hair dryer to heat the home?
Bonus question: Where are all the mini ducts? I can work with 4″ stuff, but sheesh. Come on Menards!
Bonus bonus: Is it bad to have fresh air dischargd into a walk-in closet with louvered door to buffer the noise a bit?
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What is the airflow of the HRV? The answer is likely, no. To move heat with a forced air system you generally need to move air and the airflow of the HRV is likely not enough to move that much heat around. Let's see though, if you undercut the doors of each restroom and exhaust air from those rooms that will condition the two restrooms so that would just leave the rest of the house. For 18kBTU you'll need in the neighborhood of 675CFM moving around to distribute all the heating and cooling (~0.5CFM/sf).
How are you ducting the HRV? Why not just get a ducted minisplit and run the ducts from the minisplit to the different rooms similar to how you would the HRV?
What about multiple indoor units with one outdoor unit and run refrigerant lines instead of ducts?
The airflow of the heat pump water heater is somewhat irrelevant since it will only operate intermittently.
Thanks for your reply. My plan was ducted heat pump but it bugs me. I want to try out Jake Bruton's detail of drywalling the whole ceiling at once before interior walls and I don't think I like the plenum trusses approach. You may be right about the 675 cfm, but I found my current house to be overkill on all that ductwork. I have done 2 or 3 projects with ducts and am comfortable with ducts and furnaces, but this new house will have a very low heating load. I also hate the sound of furnaces. I likely will but baseboard radiant in each bedroom as backup, though I'm certain I'll never use it in my own bedroom. Good point about the water heater. I think I'll put a fresh air register over there after all. I wish I knew more about how loud that thing will be and for how long.
There are fans you could put in the walls to each room above the doors or something that could push air and it could filter back out if you undercut the doors. The fans would need to be connected to electrical but you could always have a switch for them to turn them off or on depending on conditioning preference or CO2 levels.
Having a backup heat source for heat pumps is a good call even if it's just space heaters. When it gets really cold out they struggle to keep up.
Using the HRV is not a bad idea I just don't think it will quite distribute all the air to the bedrooms but if you think it's worth the risk, which is fairly minor, you could just go for it and see how it works out for you. You can always make modifications later. What about a ducted minisplit with the indoor unit in your mechanical closet with the water heater and HRV, a return grill in the wall to the common area and duct from the indoor misplit to all the bedrooms and kitchen area instead of ducting the HRV to those areas? Maybe if you install a ducted unit but just have it discharge to the common area you could install duct later if you determine it's not distributing the air effectively?
Where's the door(s) to access your mechanical room?
I think ducted will make you happier. Dehumidification can be handled by the heat pump.
Another vote for ducted. Connect the fresh air from the ERV to the mini split so you only need to bump up the size of the ducts a bit, won't require much more ducting.
No need for a plenum truss. You can install a dropped ceiling either in the closet or the bathroom and install the unit there. The attached is a 1.5 ton unit with similar dropped ceiling install.
The ducting can run in small bulkheads above the doors so it won't take up much space.
This gets you a proper air filter, much better air distribution and cooling and heat in all the rooms. Worth the extra bit of work.
You could run the duct using all metal ductwork, leave it exposed, and go for the industrial chic look if you're trying to avoid the extra work/materials to conceal it. Personally I think exposed ductwork looks pretty cool if it's a quality install. Might have to dust it every now and then though.
I have done exactly that on a couple of occasions, in CZ6 successfully. Both were Passive Houses; getting at least close to PH helped greatly. In addition to that, I recommend adding electric resistance backup heat to the bedrooms and bathroom. You would also need to be ok with opening and closing interior doors as part of the heating and cooling system. For many people the last one is the dealbreaker, and others don't mind at all.
I would be more concerned about adequate cooling in bedrooms than backup heat in Zone 5.
I’d consider ductless if the heat load were at passive house levels—say 6kbtu or so.. At that level the mech ventilation should be enough to mostly even out temp differences esp if the occupants are tolerant of a couple degrees across different spaces.
Your ERV might be moving 60-80 cfm while an 18kbtu ductless head is around 500 max. Also the ERV will be supplying into bedrooms and common spaces, so that fresh air will be a couple degrees cooler due to efficiency losses in the heat ex. So you can see how an erv will move some btus around, but you could still end up with comfort issues when it’s cold out.
18kbtu is high enough to be an issue I think. For example in a bedroom with two exterior walls or at the opposite end of the house if the ductless head is not centrally located. As you say you could deal with that by installing a couple of feet of electric baseboard in strategic places.
The ducted option buys you more certain comfort but at higher cost of course. If it were a client I’d be hesitant to propose the ductless option. If it were my own house I’d probably do it.