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Community and Q&A

OVER-thinking mechanicals for Zone 7 deep winter homestead

welivehonestly | Posted in Mechanicals on

Hi All!

We are making a modest 25×28′ addition onto the 1bd/1ba cabin we live in, to convert it into a family home.

The current structure is 1400sq-ft and its sole automatic heat is radiant slab in the basement. We use a Jotul wood stove as our primary heat. This means when we are traveling in the winter, the upstairs bedroom 2 floors above the radiant slab is cold. This makes me uncomfortable from a building health standpoint, but also convenience since we often return from travels at night and the kids are freezing until I get a fire blazing. There is a Venmar HRV running, of course.

With the addition, the structure will ultimately be ~2700sq-ft with 4bd/3ba.

I am looking at the CERV2 to replace the HRV.

The Jotul (which we love) is being replaced with a fireplace, RSF Opel 2C (with the catalyst). It should provide more than enough heat, although it will be on the West wall and the addition is attaching on the East, so I don’t expect bedrooms to get heated by the fireplace directly.

Ultimately, my idea is to expand the hydronic system to keep the temps consistent throughout by adding a radiant zone in the tuck-under 2-car garage which will be at the addition basement level, and low-temp Jaga radiators in the bedrooms/bathrooms. But considering ductless units can now operate at very cold temps, should I be looking at them instead?

A basic requirement I have for this house is to consider possible future guests and even future owners should we ever sell for some reason. We love using wood as our primary heat, but realize a future owner may not do that. So between that reality, and the fact that the fireplace is at the far end of the house from the bedrooms, I need to make sure that between the CERV2 (or whatever HRV we use), possible hydronic, and possible ductless, the house is nice and comfortable even without a fire burning. The idea is that the automatic systems will just run more efficiently or not at all when there happens to be a fire!

I also have not figured out domestic hot water. I have experience in our (detached) workshop with the use of a Navien combi mod-con which provides 3 zones of heat plus DHW, so I considered a similar setup for the house. But I am exploring getting away from propane dependency.

So – here are my ‘dumb’ questions:
1. HPWH – what do they do when it is colder than they can get heat? They use their backup element like a traditional water heater, right? Would that still be cost-effective in northern Minnesota?

2. Could an air-to-water heat pump run the hydronic system (like the which is made in MN), using an integrated boiler when it can’t get heat from the outside air? Or would that be totally impractical cost-wise for a theoretical future winter where for whatever reason nobody starts a fire?

3. I know this ultimately comes down to modeling, but with the CERV2 and assuming we’re building a pretty good envelope, can you see a need for cooling? I am thinking we’d get a single-head ductless unit just in case, but is that in conflict with adding the hydronic radiators? I feel like the radiators are better since the heat will go directly into the bedrooms, but I have zero experience with ductless.

4. Why is it so many folks are saying hydronic is just a thing of the past for passive/pretty good construction? just the efficiency and low cost of mini split units? Is it folly for me to be looking at expanding our system to another slab zone plus upstairs radiators?

Thanks all! I hope this discussion is helpful to other folks in my position.

P.S. FWIW regarding our energy costs:
1. We currently pay ~$2/gal for LP. So we have multiple reasons – cost, sustainability, and the simple fact that we have a sloped 1/4mi long driveway which is precarious in the winter, suggesting it would be wise to go all electric.
2. We pay ~0.12/kWh for electric from the local coop, and pay extra to source it from wind (I realize this is primarily for warm fuzzies…). Our homestead was originally 100% off grid solar, so we will be building that up again as a new grid-tied system over the next 1-5 years, so I feel like this further bolsters the all-electric idea, but I don’t want to shoehorn it if it will get us in a difficult financial position later in life, either.

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  1. welivehonestly | | #1

    Addendum: the existing structure is ICF foundation, 2x6 conventional with R-21 bats, R50 in the ceiling. So, not horrible. BUT, due to an apparent design flaw with the original windows, probably compounded with previous owner behavior (leaving off-grid property during winter and not ensuring HRV ran), the windows are all compromised.

    So... once we do this addition, and update the existing structure with triple pane fiberglass windows/doors and an extra R10 worth of GPS insulation on the exterior, I am sure it will hold onto warmth a lot better than it does now. That is also a consideration in all of this, and more evidence I am over-thinking.

  2. rockies63 | | #2

    So you have a propane fired boiler in the workshop which provides hot water for the basement radiant slab and DHW, a Jotul wood stove in the living room and an HRV?

    A lot of people don't like radiant slabs simply because they have a very slow response time. Have you thought of abandoning the radiant slab completely and just installing baseboard radiators in every room?

    Also, you might examine how the air circulates between rooms and floors. There may be no way for warm air to get up to the bedrooms (especially if the bedroom doors are closed).

    1. welivehonestly | | #3

      Hi Scott! Thanks for your reply.

      I should have clarified - the shop is a separate 30x40 building which has an open attic space which is my office, recording studio, etc. It is a self contained, livable structure with its own mechanicals.

      I have considered abandoning the slab, I am only thinking about it since it's already there, and since that seems like a fine way to keep the tuck-under garage temperate. The kitchen and master bed/bath are above the garage, and kids' bedrooms/bath above that. I'd figure on keeping the garage just at 45F. But yeah, I am not hung up on radiant slab, it is just a design consideration.

      If we do hydronic at all, yeah, I was thinking of using Jaga baseboard radiators in every bedroom and bathroom.

      My hypothesis with the CERV2 (with its integrated heat pump) is that it will do a "pretty OK" job of recovering whatever heat is available in the building, whether it be from the fire or whatever else, and distributing it.

      When the fireplace is burning, and assuming upstairs bedroom doors are open, heat should find its way up there because the fireplace is in a vaulted living room open to the upstairs hall. But that's not something I want to count on as primary heat.

  3. rockies63 | | #4

    With a heated tuck under garage below the kitchen and bedrooms I would be more concerned with carbon monoxide leaking into the house than heating it. Perhaps turn off the heat in the garage to save energy, increase the insulation in the interior garage walls and ceiling and make sure that the garage is completely air sealed from the rest of the house would be a better idea.

    1. welivehonestly | | #5

      yeah that is a good point. even if we run tubing for slab heat in the garage we could just keep it off. We definitely should insulate the garage ceiling anyway. The garage walls will be ICF.

  4. _Stephen_ | | #6

    The reason radiant is not used often is two fold. One is cost. Doing just my basement as radiant would have added $20k to the cost of my mechanicals, and that's just for the piping and pumps.

    Two, in really right, efficient houses, your slab wouldn't need to get warm enough to get the warm toes effect radiant heat is known for, due to the low loads.

    I think they might make a comeback of we ever get cost effective air to water heat pumps.

    1. welivehonestly | | #7

      Got it - yeah. I need to decide whether or not we will expand our existing radiant slab to the garage, especially since the garage will need to be insulated and sealed from the rest of the house anyway. We use a radiant slab in our shop and I keep it at 45 - that sure is nice in the winter...

      So I am torn between expanding the hydronic to have radiators in the bedrooms and bathrooms, and just betting hard on ductless which I have no experience with. I don't know if 1-3 heads in common spaces on the first and second floor would distribute heat well enough into bedrooms and bathrooms.

      As for air to water, I am really excited about the NorAire units The slab would not need tremendously high heat (as you alluded to) and the Jaga radiators we would use only need ~130F. I personally like hydronic, but I don't want to invest further in it if in fact the future is ductless or some other magic box.

    2. welivehonestly | | #8

      Steve, in your house, did you end up using air source ductless heat pumps? For us, I am happy continuing to depend on heat from the (super efficient, dedicated combustion intake, catalyst) wood fireplace. But as I wrote above, I need the mechanicals to support comfort without the wood fire, both for times we travel in the winter and for possible future scenarios. I just don't know what that system should be.

      1. _Stephen_ | | #9

        Sorry! I didn't see this.

        Yes we went with an air source heat pump. A slightly oversized Mitsubishi P- Series air handler, with hyper heat. This past winter we had cold down to -30C and she kept up just fine. It was really neat to see it work.

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