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HVAC Design for Cold-Climate New Home

jimgove30 | Posted in Energy Efficiency and Durability on

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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  1. paul_wiedefeld | | #1

    If you want AC and responsiveness, then you should use ductwork. In that case the in-floor heating is very expensive and rather inefficient compared to a central heat pump, while adding much complexity and trivial redundancy. You didn’t mention the sub-slab insulation, but assuming it’s adequate or better, the floor temp will be rather low. Manual Js typically run oversized, so a 50kbtu could easily be 40kbtu, which would be about 78 degree floors when it’s -18 out. An indirect for only two showers is pretty inefficient and expensive as well.

    1. jimgove30 | | #2

      Thanks for the insight Paul. The slab is to be insulated 3" XPS under and 1" XPS at the edge. Could 2 smaller central ducted systems work? One on each end? The way the house is split by the great room doesn't leave much space to hide ducting. It would be an afterthought at this point, could work.

      1. paul_wiedefeld | | #3

        Great underslab insulation.
        Definitely! Two 24,000btu units would get you zoning and some redundancy. Adding backup heat strips, electric baseboard, or propane furnaces would help with the lowest temperatures. The propane stove would be good for power outages.

  2. brp_nh | | #4


    Lots to think about. We've been in our house in Jackson, NH for almost 8 years now. We're at 1400', so possibly a bit colder/snowier depending on your location. We have a small and simple house that we built with energy efficiency in mind, some posts on GBA about it, start here:

    Looks like your slab edge is actually 2" of xps, that's good. What is your ventilation plan? Hopefully an independently ducted HRV.

    Some thoughts:
    -We went through the Energy Star program. Well worth it for the advice, load calcs, and $4k rebate.
    -Not sure if the outdoor heating design temp is actually the Mt Washington Observatory weather station (which would be crazy) or the Mt Washington regional airport in Whitefield. Check out some sites in NH, ME, and VT on this document ( Something around -10f might be more realistic.
    -Your ACH is listed as 3, that is pretty leaky for an energy efficient home, should be pushing for 1.
    -Your projected heating load of 50k seems excessive for a 2400 sq ft home. We are heating our 1500 sq ft house with a single 12k minisplit.
    -Are you or can you plan for some solar PV?!
    -We just rough it during power outages and our house isn't in danger of freezing, even in winter.

    1. I would consider ditching the in floor heat, ditch the propane, and just go for air source heat pumps (mini splits). Either ducted or ductless or a combination. If designed/installed properly, they will handle all your heating and cooling needs. No need to add heat strips or electric baseboard, except maybe as a nice touch in bathrooms. And we do need cooling/dehumidification in NH summers.

    2. I would have this redone with more accurate input on the weather station and confirm your target ACH with builder.

    3. How about just a simple electric tank or heat pump HWH?

    4. Go with the air source heat pumps.

    Keep in mind I'm just a semi-knowledgeable homeowner, but we GC'd our house and learned a lot.


    1. jimgove30 | | #5

      Hi Brian,

      Not sure how I missed your reply, but thanks so much for sharing your experience. I'll check out your build. We're in the Thornton area, over by Waterville Valley.

      We're just about to place footings, and I have yet to see an HVAC proposal, so this has me very nervous about what we will end up with for a system. I'm having a bit of a tough time getting the builder on board with using just the heat pumps. I'm also concerned the budget will be blown with basically 2 systems in place. Trying to get as much info up front so when the conversation happens, I'll be prepared.

      Yes, the plan is for ERV, but again, no details yet. I've been trying to push these concerns at build team meetings, to ensure the architect considers space requirements, but haven't been very successful.

      I was very surprised to see the -18F design temp as well. It seems that around -7F would be more appropriate. The Energy star manual does list that -18F, but I can't seem to find out why. That's quite a bit north from where I'm at. Is there any guidance on what a designer can do to alter those #s? As I read it, that's the minimum number that should be used, so should they just change to out for -7F and rerun the calc?

      The leakage rates were just a guess, I guess, based on the plans. They only list "full air sealing" so who knows.

      I went out and used a company known to the GBA community to perform the load calcs, mostly out of curiosity, as I'd played with and a couple other online tools.

      I'd definitely like to be able to add PV down the road. That'd be nice to offset the energy costs. South facing steel roof lends itself well to that I think.

      We also have a Rais Viva 120L propane stove planned for the great room, more for ambiance, but it can throw some heat for sure (about 25K btu max) to cover any really cold spells.

      Keep in mind, this is a short term rental as well, so we don't want guests calling us saying they need to bundle up either.

      Also- I'm really nervous about being held hostage by a propane co... having never had to deal with a 1000 gal underground tank to rent.

      Appreciate all the input.


      1. brp_nh | | #6


        It can be tough to give advice on these forums because we all have different opinions and goals. We wanted a simple (but nice), small, affordable, and energy efficient house...and we made many significant choices with that in mind.

        I would recommend checking out what some of the more well known energy efficient building companies are doing for systems:

        Personally, I don't see the benefit of adding the expense and complication of propane to this build when electric systems (heat pumps, electric/induction stove, etc.) can meet the needs, but I understand that may be important to you.

        Planning for solar PV would be great if you have a nice south facing roof.


      2. paul_wiedefeld | | #7

        It's tough - once you make the decision to add AC, most forms of hydronics become an expensive luxury with subjective benefits. Most American choose AC and forced air over AC and a separate forced water system.

  3. jimgove30 | | #8

    Thanks guys. I offered to have to heating system designed by a 3rd party, but that idea was met with much disdain. One of the issues I see for ducting the systems (ERV included) is that the great room in the center has no real provision to supply and return from both ends. I suppose exposed ducting could work to solve it (although nor preferable from an aesthetic standpoint), or a dedicated wall mount unit could work, less desirable again. I'll pursue our options at the next meeting. Thanks for the support.

  4. Expert Member
    AKOS TOTH | | #9


    Something is off on your man J. Even assuming the -18F design temp is right, 51000BTU for 1600sqft structure with your level of insulation is way too high. If you can post the full Man J, we can take a stab and figure out where the issues are. I would expect your place to come out at about one third that load.

    Once the Man J is fixed, you can re-visit the equipment options again. A combination of ducted cold climate heat pumps or a combination of heat pump plus some resistance heat is all that will be needed. Anything hydronic is defiantly a giant waste of money if you also need cooling.

    Also in zone 6 you need much more than 1.5" of polyiso for a hybrid roof. You can read more about the proper ratios here:

    In your case, it might be worth while to reduce the amount of insulation in the rafters, simplest is to use thinner batts pushed up against the roof deck and held in place with insulation wires, otherwise you will need a very thick (and costly) layer of exterior rigid.

    1. jimgove30 | | #10

      Akos, I'm a bit bigger than 1600, and the Manual J was completed for 2400sqft, which includes about 1600sqft main floor and 360 of 2nd fl open loft and 450 of conditioned mechanical space on either side of the full height great room. I attached the full Man J, and I'd love to give the engineer a chance to fix it if you see something that looks off.
      Also, I was JUST notified that we need to consider adding a full basement (we're days away from footings), based on grade issues/costs to continue with the slab on grade approaching that of a full basement. Not what I wanted to hear, and now I need to go back and read all those sections that I skipped about full basement construction :( . It does however, make it a little more attractive for ventilation/ductwork etc, as well as opening up space for a heatpump water heater, maybe.)

  5. Expert Member
    AKOS TOTH | | #11

    Lot of your losses are glazing, infiltration and slab loss. As Brian mentioned earlier, there is no reason why you can't hit 1ACH, that will take a huge chunk off.

    Slab loss is moot now that you might need to go for a basement. With a well sealed and insulated basement, the loss should be still be less than the slab loss in the calculation.

    There are some big losses form the 0.27 windows. I would look at upgrading the larger ones to triple pane, you can save some cost by making some of the smaller operable ones double pane to compensate. Watch when mixing triple/double on the same wall as they can be much darker. I would look at some mix and match coating options to get them to a closer shade if needed.

    Nailing down your actual outdoor design temp will also help in getting your load right. -18F sound more like zone 7.

  6. Expert Member
    Dana Dorsett | | #12

    >"he 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. "

    A bit... Thornton is at ~600' of elevation, the Mt. Washington weather station is at ~6000'. Lebanon's design temp is -3F, Laconia is +1F, Concord is 0F. I suspect Thornton's 99% temperature bin is somewhere in that range- call it 0F. See:

    While the first page of the Man-J _SAYS_ they used the Lebanon airport for design condition criteria, they must have overridden the default 99% design temp for Lebanon (elevation ~600').

    Starting out with a design temp 15-20F colder than reality is a pretty fat thumb on the scale, adding 20-25% to the heat load numbers. That's a pretty inane thing to do even with a code-minimum type house, let alone a tighter & better than code house. I'm sure there are other thumbs on the scale buried in there somewhere, but that's a pretty fat one- so fat it hits you right between the eyes on the very first page. Is the rest crap? I don't know, but the rest is at best suspect, given the starting point.

    >"Also- I'm really nervous about being held hostage by a propane co... having never had to deal with a 1000 gal underground tank to rent."

    With 99% design temps warmer than -10F (which yours SURELY is, even atop the highest hill in Thornton) it's more reasonable to go with cold climate heat pumps (ducted or ductless) that with condensing propane equipment. Fossil-burners are rapidly becoming the past, and there is no good reason to go there (unless you own a gas well & local processing plant extracting the propane :-) ). The capital sunk into a 1000 gallon tank & buried installation is better applied to rooftop solar and an LFP battery (or a much smaller above ground tank + propane backup generator.)

    1. jimgove30 | | #13

      Hi Dana, thanks for weighing in. Here's the thing, the Energy Star Design Temperature Guide actually lists that -18 and Mt Washington as the design temp for Grafton County, where Thornton is located. I can't fault the designer if that's what's listed can I? I am trying to apply for the Energy Star certification, so that seems odd to me. Could I ask that it be rerun using Lebanon's -3F?

  7. jimgove30 | | #14

    So, met with the build team earlier today, and looks like we're going with the full basement. I'm told it includes insulation to code, but we'll see once we have a revised foundation detail. The window package is already on order, so my mix of triple/doubles will be what it is (We've already pushed harder than they'd like to get the Euro windows, so that will have to do).
    The builder is NOT a fan of the heat pumps, basically saying: "It won't work. We've tried it. You need propane backup at least up here." I've asked them to take it to their HVAC contractor anyway to see what he says, but I'm not optimistic. More awkward discussions to follow, I'm sure.
    If I can get some more clarification on my outdoor design temp issue, I'll go for a rerun of the load calcs and see where we're at.

    1. paul_wiedefeld | | #15

      If you have the ability to run ducting in the basement ceiling and use a single HVAC unit, adding a propane furnace as backup isn't the worst idea. Just because it's installed doesn't mean you'll need it! You could even disable it if you want. Avoiding the in-floor heating is the huge costs savings here, so a propane furnace, while probably unnecessary, is an easy concession.

    2. Expert Member
      AKOS TOTH | | #16

      Heat pumps were an issue in colder climate before hyper heat units become available. A number of current generation hyper heat units will deliver their nameplate capacity down to -5F and some can run down to -22F.

      In my area (-2F design temp) I'm heating a house that is a mix of uninsulated double brick and new construction with a pair of ducted heat pumps without issues. The heat pumps carry the whole place without any resistance backup.

      Up further north with -16F design temperature, I'm heating a 550sqft pretty basic 2x6 build cottage with a single one ton hyper heat unit.

      If you look at the current price of propane, around me heat pump is about 2.5x cheaper to run. Since the cost of furnace+AC+propane tank is more than a cold climate heat pump it makes no financial sense to go for a fuel burner.

      This is not some noble environmental goal but simple economics.

      Get your man J dialed in and select a right sized heat pump. You'll end up with a much more comfortable place that is cheaper to run. Fuel burner backup is not needed and a waste of money, if you want a piece of mind install a resistance strip heater in the air handler.

      There are threads on here for people using heat pumps in much colder place than you for example:

  8. jimgove30 | | #17

    I spoke with the manual j designer, and he said they use the energy star reference when no other design temp is provided. He's happy to change it to whatever suits me. I couldn't find any data to support an actual 99% design temp for my exact location, but I feel like we're definitely a little colder than Laconia and Concord. I've asked them to update the manual J for a -3F design temp, and I'll report back with the new numbers. Thanks for all the coaching so far!

  9. jimgove30 | | #18

    Attached is the updated summary, for a -3F design temp. It came in almost 10K BTU lower!

    So, at a load like this, thoughts on using a single ducted hyper-heating unit like:\M_SUBMITTAL_PVA-A42AA7_PUZ-HA42NKA1_en.pdf

    Or better to use 2 separate ducted units like a 24k and an 18K *(or another 24k?)?


  10. jimgove30 | | #19

    Also, attached is what the PUZ-HA42NKA1 looks like on the NEEP tool (which is pretty neat BTW) according to the new load calcs.
    Looks like it would handle most of my heating load (I'll have that euro-style propane stove as back-up, because it's pretty). It appears that it will have some low load cycling, and the modulation % looks lower than ideal. Am I reading that right? FYI - I plugged in the data for heating capacity at -13F from the data provided on the submittal sheet. Is that the way that's supposed to work? Should I keep looking?

    1. Expert Member
      AKOS TOTH | | #20

      Looks better. The ACH @50PA is still at 3. With minimal effort you should be able to half that and 1ACH is not really a stretch for a ZipR build with some attention to detail. That would knock about 3/4 ton of heat load off which would put you into the range of a SVZ-KP36NA SUZ-KA36NAHZ which has better modulation range and much cheaper than the P series. The data for this is not in the NEEP database, you'll have to pull the engineering manual for it from:

      With a lot of glazing, two zones might make sense if parts of the house get significantly more solar gain. This can be an issue as those areas tend to overheat whereas the rest the of the house will run colder. Unshaded large west facing windows are typically the worst for this. Your hvac designer can take a look and advise on how to split it up. This will add a bit of cost to your mechanical budget. In case of two units, make sure that each one is on its own outdoor unit, don't go for a multi split. Multi splits have much lower modulation range and tend to be less efficient for about the same BOM cost as two one-to-one units.

      It is also possible to zone a large SVZ/PVA. The simple setup (thermostat module + Honeywell zoning controls) is not the best as you loose some modulation, the expensive one (Airzone) might be a challenge for your HVAC installer to configure properly.

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