Revisiting “Just two minisplits heat and cool the whole house”
After much research on this site (specifically in the article/comments about Carter Scott using ductless mini splits for whole house heating) I believe air source heat pumps are the way to go for a new house I’m building. I’m working with a fairly square, open floor plan and electricity is cheap in my area. My question is whether or not insulating interior walls has a material impact on performance of ductless mini splits when using them for heat/cooling of all rooms. Sound deadening aside, I’m curious whether or not omitting interior wall insulation helps “even out” temps from room-to-room when there are no ducts running into rooms. Any advice or reference to articles would be much appreciated.
Thanks,
Brandon
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Brandon,
Uninsulated walls are around R3, even without insulation, you'll still get a fair bit of temperature difference. Assuming 1500btu loss for the room, you are looking at around 3F delta. This is usually fine for bedrooms in the winter, insulating it would definitely increase this at which point you would need supplemental heat.
I think the bigger problem would be AC in the summer, bedrooms would be uncomfortable unless you keep the doors open.
In my area (zone 5), for mini splits used for cooling, I find what works best is a single head in the living area and ducted unit for the bedrooms. This lets you insulate to your hearts content and still have comfortable rooms.
P.S. Insulation in the walls doesn't do as much as you would think for sound. If you want quiet, your best bet is 5/8 drywall over resilient channel.
Thanks Akos. My heating and cooling loads came back pretty low at 18kbtu heating and 14kbtu cooling for the entire house. Sounds like I either need to be OK with leaving doors open to even out temps or do a zone that drops some ducts into individual rooms. I'm in zone 5 also and have no humidity and our elevation affords us much cooler nights than daytime highs - I'm wondering if doors open during daytime plus pretty cool nights would be enough to avoid running ducts?
You can probably get away with it but I don't think it makes too much sense.
If you are building new, it is simple enough to include service chases for a slim ducted unit. Provided you have a reasonably compact layout, you need minimum ducting. You can also share the ducting with the HRV/ERV so that it doesn't add all that much cost.
I've attached a picture of a unit for a retrofit install to feed 2 bedrooms, 1 bath plus kitchen. The whole thing fits into the ceiling in the hallway (takes up about 12"). Not the most efficient ducting but good enough to keep it bellow 0.15" pressure drop the head required.
The bigger problem is usually finding somebody to the install, you'll have to do a bit of searching. What I find works is installing the duct work and just having the HVAC tech install the unit. Helpful to have the HVAC company drop off the ducted unit ahead of time, makes running the ducting much easier.
>"...loads came back pretty low at 18kbtu heating and 14kbtu cooling for the entire house..."
A 3/4 ton ducted unit upstairs (assuming the bedrooms are up there?) and another half-ton or 3/4 ton wall or floor coil serving an open floor plan first floor is probably about right, though you could make both ducted.
What are your 99% and 1% outside design temps, and your elevation? (Elevation affects capacity- it matters.)
Here's a floorplan screenshot. The plan is an open great/dining/kitchen and then rooms next to that. Above the bedrooms and bathrooms is an open, unfinished loft. My design temps are 94/4 and I'm at about 4,800ft elevation. I've got cheap electricity and am building with the intent of a PV array in the near future - I'm not at all opposed to supplemental heating in rooms, but not sure whether or not the cooling of individual rooms is worth the extra complexity. Other note: my grandpa is my HVAC contractor who is retiring as soon as my house is complete. He's admittedly the old school rule of thumb guy often read about on this site, however he does trust the numbers in design loads and will build whatever design I decide on. My preference is to not overcomplicate the entire system for a marginal gain in comfort if a very simple system will suffice.
With such a simple floor plan and low load, I would be temped to go with a single ducted unit for the whole place.
You can mount it in the unfinished loft area and easily feed all the rooms from it. Run the ducts in such a way that you can bury them in interior walls if you ever decide to finish the loft. The ceiling of the mechanical room or the pantry is also an option but would mean more bulkheads in the hallways.
A bit of electric floor heat around the perimeter of the living space as well as the corner bathroom would also help a lot with comfort.
Standard practice in a single family residential home is to leave interior walls uninsulated. If any insulation is installed, it's either for sound (which does also thermally insulated, but that's a side effect rather than goal), or it's for a specialty room like a cold cellar or wine cellar.
You should calculate, not guess about an open door being sufficient. It depends on the bedroom load and how much drop/rise in temperature you can tolerate.
With a relatively open floor plan and low heating and cooling loads I doubt you'll have enough of a temp difference across interior walls for the difference to matter. The bigger issue is exterior wall area. Even in a super insulated house, a corner bedroom might drop a couple of degrees overnight with the door closed.
But mechanical ventilation will tend to even things out, if you are going down that road.
I'd put electric resistance heat in the bedrooms and master bath as back-up. Set it a few degrees lower than the main area.