Planning a “Pretty Good House” HVAC system
Good day GBA!
I’ve just started the design phase of my future residence. Im located in zone 7 (just north of Thunder Bay, Ontario). I’m working with one of the only Passive House certified architects in the area. We will be shooting for slightly better than PGH but not quite Passive House. The build location is on 150 acres. Space is not an issue and it will have perfect south orientation.
The footprint will be approximately 1800sq ft (30×60’) with a walkout basement. I’m planning on a monolithic slab (6” foam under) with ICF basement walls (undecided on foam thickness). I’d like to keep the slab as the finished basement floor. I’d like to use a slightly modified version of Lstiburek’s ideal double stud wall at around r50. It will have a unconditioned attic with a gable roof. It will have a 3rd south facing gable that will use scissor trusses. Tightness will be <1ach50. An ERV/HRV will be used. An airtight woodstove will also be installed.
I’ve been researching and thinking about heating and cooling extensively. The common recommendation seems to be air to air heat pumps, especially since cold climate performance has been steadily improving. My climate routinely sees -20F temps for extended periods and occasionally -40 a couple times throughout winter.
I’m conflicted as to whether I want to install hydronic tubing in the slab. The common theme on GBA is that in floor heating in a high performance building is much less advantageous. I’ve also started considering a ground sourced heat pump (water to water) which could also serve the DWH needs. I have the benefit of lots of space and some heavy equipment to do the trenching for only fuel costs.
At the point I am looking at air sourced ductless mini split with as many heads as needed vs ground sourced heat pump (water to water)
Ductless mini split: simplest, least costly, possible eye sore, possible fan noise(both interior and exterior), low COP/performance during cold spells, temp swings in different rooms (I prefer bedrooms cooler)
Ground sourced heat pump to hydronic: good performance regardless of air temp, less eye sore, no complications for installing a deck around a mini split. Minimal noise. Equipment more expensive (several 1000ft of 3/4” tubing), minimal excavation costs, radiant floor cooling is questionable, can provide DWH, will only provide heating/cooling to slab not main floor (I don’t plan on installing hydronic tubing on main floor)
Can anyone provide a rough costing estimate for both of these two systems in the 2 ton range excluding dirt work? Is the price that dramatically different?
To add to this, I’d also like to get on the Net Metering program to feed PV into the grid to subsidize my electrical use.
Thoughts are welcome and appreciated!
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Replies
Ducted air to air is the most popular choice- no indoor units on the walls, can easiest incorporate backup heat, provides whole house filtration/heating/cooling, and isn’t that expensive. I wouldn’t choose a system based on DHW. The ground loops can often be less efficient than air source - 3/4” pex has high pressure drop and that pumping energy adds up
I forgot to add that I’d like to steer clear of ducted systems. I understand that an ERV/HRV requires ducting, however they’re much smaller diameter ducts and much simpler.
A PGH should also need small diameter ducts too! You might end up needing only 800 cfm total, that’s easy to conceal if designed from the start.
Or, you can go a different route and install one or two ductless indoor units - should be significantly <$10k USD, but cooling won’t be happening well and you’ll have distribution concerns.
The route you don’t want is one outdoor unit connected to multiple indoor, ductless units (a few ducted units might be okay) - you’ll be almost assuredly really oversized at the room and outdoor unit level.
I'm with Paul here.
A small fresh air feed is 3". You want to upsize that to 4" so it will be quiet when the ERV is on boost. To also heat/cool the room, that duct would have to be 5". So what are you saving really by not using the ducting you already need to run anyways?
Good point.