HVAC Design for Cold-Climate New Home
Hi everyone, and sorry in advance for the long read. We’ve just broken ground on a new project in the White Mountains of New Hampshire (Thornton), and I’m looking for some input the mechanical systems. First of all, a little background on the project:
slab on grade
1600 sqft, 1 story, 3 BR, 2 BA with an open loft space, +372 sq ft on one end. Has a +400 sq ft conditioned mech rm on the other.
12 pitch cathedral ceiling in the open great room, loft, and scissor truss cathedral in the MBR, and std ceiling in the other BRs.
2×6 construction, R21 (cellulose) + Zip R9 spec’d
2×12 roof with cellulose (R42) and 1.5″ polyisio on the inside (R14 per plans?)
Windows/doors – lots of them, mostly fixed panels but a mix of triple (U-0.18) and dbl paned (U-0.27) Euroline tilt turns, so pretty good.
We’ll be using the place here and there year round, as we need a wheelchair accessible vacation home to ski, hike, etc., but the rest of the time it will be a short-term (hopefully income generating) rental year round.
My architect and build team have engaged a local plumbing/heating contractor (one they’re comfortable with), to design the system. I have asked the builder to apply for the energy star homes certification (offered through NH Saves Program), in hopes that will drive the proper sizing of the systems, but I still haven’t heard back from them on system design parameters. That’s not to say that it’s not happening, I’m just not privy yet to any specifics. The team knows I’m interested in right sizing the systems, and I hope that the concerns I hear about oversizing won’t apply to us. I did suggest that a 3rd party do the load calcs/system design (it was more reasonable than I expected), and I think there was some offense taken at that suggestion (any suggestions welcome on how to make those conversations go smoother?).
Anyway, we are about to start the footings and stem walls, and the panelized walls should come in the next month, so with that said, I have a couple of questions (finally, right?)
1. Given the slab on grade design, it has been recommended by our deign build firm to have hydronic, in-floor heat run by a HE boiler with an indirect water heater. I’m not opposed to it, but, from a short-term rental perspective, I’m concerned it will have a hard time reacting to different guest’s comfort needs. Of course, this means separate cooling/ventilation as well. So what about in-floor heat, running low temp (like 65 or so), with mini split heat pumps/AC, so the heat pumps could boost the heat if needed? We also have plans for a propane stove, ’cause it’s a cabin, and that thing throws 7-25k btu if desired.
2. I went against my team’s advice and had the manual J calcs done by a reputable engineering firm, just because I was curious. (It was cheap insurance?) The outdoor heating design temp used was (-18F) but that is for Mt. Washington, NH, and although that’s what the energy star guide uses as a minimum design temp, I think that data is taken from the Mt Wash. weather station, which seems a bit extreme.
3. DHW – indirect off a boiler, if I go that route, how do we size it? 2 standard showers (no tubs), typical kitchen with dishwasher. (Keep in mind for us, it’s just 3 people, but rental guests don’t seem to care how much water they use…)
4. Would a little 80K (or so) mod-con boiler be sufficient for the DWH and lower heating loads of in-floor heat?
If you’ve made it this far, thanks! Appreciate any input. I’m comfortable enough with my design/build firm, but since they haven’t really specified any specific system info yet, I prefer to have as much info up front as possible.
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