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Community and Q&A

Workshop heat pump

benjamin_wooten | Posted in General Questions on

Hi everybody! I have a quick heat pump question, and hope somebody might have some insight, experience, or opinions to share in response.

We live in Nova Scotia, Canada, a mid-temperate climate zone with average temps of -8C during our coldest months (January/February) up to 23C in July/August.

We have a detached outbuilding on our property – it was here when we purchased the home a few years ago – that is 25’x25′. It has 8′ ceilings. It is insulated, walls and ceiling, with fiberglass batts. The floating slab it is built upon is NOT insulated. The majority of the interior (entire ceiling and maybe 85% of the walls) are sheetrocked.

I am a woodworker, and am starting a small business in this shop, building furniture. I am researching the best option for heating the space. I have a small wood stove available, that I could install in the workshop, that would provide more than adequate heat in the winter… however consistent humidity control is a big consideration for me, so I am looking into heat pumps as an alternative.

My understanding of heat pumps is that the very edge of their function comes in at about -8C, which is the low end of our winter temperature average here. It certainly gets colder than that some days, but today, for example, it is 14C and sunny.

I am also gun-shy with heat pumps. Before we moved into our current home, my wife and I rented an apartment about 30 minutes from here, in downtown Halifax. We paid our own utilities, and thought the heating oil expense was painful enough… until the landlord upgraded to “an ultra-efficient heat pump,” which promptly doubled our heating costs. In addition to the apartment heat-pump fiasco, I also have a general sense of skepticism coming from what I will refer to as a “heat pump craze” currently going on here in Nova Scotia. I think the price of heating oil has gotten so exorbitant, that customers are ripe for the picking, and now the heat pump folks are sprouting like mushrooms after a thunderstorm. And nothing against them for taking the opportunity to provide an alternative where not too many alternatives exist… but I know every dealer and installer will tell me they work great in this climate. My understanding of how heat pumps actually work tells me that may not be the case, or at least not quite as efficiently and enthusiastically as all the radio-ads say.

My question is, basically, would a 12,000 BTU (the recommended size, from what I can tell, for a 625 sq. ft. space such as my workshop) ductless heat pump be a reasonable investment to provide heat and humidity control in such a space? I plan on laying down vapor barrier, rigid insulation sheets, and plywood on the cement floor to try and provide some slab insulation, as well.

Thanks for your thoughts!

Very best,


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  1. benjamin_wooten | | #1

    Also, the shop space is just one big open space. No interior dividing walls etc.

  2. GBA Editor
    Martin Holladay | | #2

    Mitsubishi and Fujitsu both sell heat pumps that work at outdoor temperatures of -25°C or even -28°C.

    The temperature you cite (-8°C) is equal to 17.6°F. Your Mitsubishi or Fujitsu heat pump will work at much colder temperatures than that, with high efficiency.

    When it comes to paying your electricity bill, we can't help you. I don't doubt that you can heat your shop more cheaply with firewood than electricity.

  3. benjamin_wooten | | #3

    Reason number nine-hundred-and-one to invest in the flexible PV panels sooner than later!

    I would love to do the wood stove, but I need humidity levels to be constant day-to-day, and I don't trust I could do that very easily with the wood stove. Also, the real tricky times here are early-Winter and early-Spring, when moisture levels can be very high, but its not cold enough to fire up the stove... having the pump running low to keep things steady would be a plus. I suppose I'd just have a big dehumidifier running in there otherwise.

    Thanks, as usual, for your help Martin!

  4. Expert Member
    Dana Dorsett | | #4

    When #2 heating oil was running USD$4/US-gallon with electricity running USD$0.20 even in locations as cool as northern NH or ME (colder than Halifax, for sure!) cold climate mini-splits saved enough money to break-even in less than 5 years.

    Now that #2 heating oil has moderated to USD$2.50-$3.00/US-gallon it's still cheaper to run mini-splits, but the cash break-even point is quite a bit longer, but still well within the anticipated lifecycle of the equipment.

    Heat load is not directly proportional to floor area- it's a function of temperature difference, the construction of the exterior surface assemblies of the building (including the floor), and the amount of air leakage. That said, in typical residential construction a TIGHT 2x4/R13 house with clear-glass double-panes(not low-E) at typical window/floor area ratios and R19 or more in the attic at -17C (the 99th percentile temperature bin for Halifax NS) with a 20C indoor temperature would come in around 10,000-13,000 BTU/hr, and if it has substantially less than 15% window/floor area it will be lower.

    Most cold-climate 1-tonners (12,000 BTU/hr nominal cooling rating) can deliver 15,000 BTU/hr or more at -17C, and at least some 3/4 tonners (9000 BTU/hr nominal cooling rating) can deliver more than 10,000 BTU/hr @ -17C, eg:

    If the building is 2x6/R19 with less than a 15% glass/floor ratio the likely heat load is even lower, and 1-tonners would probably be sub-optimally oversized for the actual load.

    Getting to the right solution starts with an aggressive heat load calculation, (Manual-J or other), taking every possible feature that can lower the heat load into account. In a shop building that is occupied you'll likely have at least 500 watts (1700 BTU/hr) of lights on, and at least one conscious adult human (400-600 BTU/hr), heat sources that need to be subtracted from the raw load, to only include the load that has to be supported by the heat pump.

    A guy who used to post here regularly (under the handle "Jin Kazama') heats his whole house in Quebec solely with four Fujitsu 3/4 tonners (first-generation cold climate -9RLS2H units) , and they keep up with the load just fine at -30C outdoors, +20C indoors, even though they don't have a specified output capacity at that low temperature.

  5. benjamin_wooten | | #5

    Thanks Dana! That is a lot of good information, thank you for posting it.

    One frustration I have with this building is that the slab is such an enigma. The walls are 6" studs, there are only two small (20x30) windows (single-pane), the walls and ceiling are all insulated... and then nothing in the floor at all. I have worked in there on Winter days where I'd be comfortable in just a long-sleeved cotton shirt (and this is with no current heat source whatsoever) except for my feet turning into blocks of ice through my insulated boots. Since the concrete pad seems to be such a heat sink, I am not sure how that might affect the calculations?

    I have nice(r) double pane windows we pulled out of the house during the recent renovation, which I plan on using to replace the tiny single-pane ones that are in the shop now. I'll be adding some glass, but not much.

    I presume getting a unit that does too much BTU for a space's needs would be inefficient?

  6. GBA Editor
    Martin Holladay | | #6

    You might want to invest in perimeter insulation for your slab. Get 2-inch thick rigid foam, and cut it into 2-foot wide strips. Dig a trench around your building, and install the rigid foam vertically, up against the slab, the the top edge of each piece of foam even with the top of the slab. Install Z-flashing at the horizontal seam between the rigid foam and the lowest course of siding, slipping one leg of the Z-flashing under the siding. Protect the above-grade portion of the foam with metal flashing or pressure-treated plywood, and backfill the trench.

  7. benjamin_wooten | | #7

    That's an excellent idea, yes. We did that with the main house when we moved in... discovered it had no insulation on the edges down to the footings, just insulation under the slab itself. So we trenched down 4' deep to the footings and put rigid foam on... it's helped a lot.

    You're suggesting, though, in the case of this shop building the foam only needs to go to a depth of 2'?

  8. Expert Member
    Dana Dorsett | | #8

    US building codes in climates like yours (DOE zone 6A) would require R10 to a depth of 4 feet for slab edge insulation on new construction, but even 2 feet is WAY better than 0 feet.

  9. calum_wilde | | #9


    I'm in Middle Sackville, I'm guessing not far from you. I have two Fitijsu minisplits, a 15k and a 12k, heating my 2400 sqft home. I have yet to even turn on the breakers for the baseboard electric as the hestpumps have had no issues with keeping up. Even the late December early January cold snap wasn't cold enough to make it an issue.

    I had Ground Hog in Lower Sackville install both units, one early last year, the other early the year before, and would happily recommend them.

    Depending on the insulation and air sealing of the garage I'd say a 12k unit would be too much for that size of a space. My 12k in the basement can heat the whole house down to roughly 3°C outside temperature. Below that I separate the basement from the main floor and the 12k unit is fine down to -18°C so far.

    As for cost. My power bill is down from an average of 109kWh per day the last time we had a December-January with similar amount of heat degree days, to 43kWh per day. The vast majority of that difference in the heat pumps. My summer time, no AC, base load is about 20kWh per day. I'll let you extrapolate the costs, you know what we pay for oil better than I do and NSPowers rates are available as well.

  10. GBA Editor
    Martin Holladay | | #10

    Of course 4-ft.-wide rigid foam is better than 2-ft.-wide rigid foam. You just need to invest 2x the money, and you need a shovel with a longer handle. But you end up with a better-insulated building.

  11. benjamin_wooten | | #11

    Ok, understood, Martin. I thought maybe the 2' vs. full 4' had something to do with a floating slab vs. a foundation with footings etc. I am attaching a couple of my favorite pics from the trenching process, sometime in mid-November 2015. I still laugh at the one of my spouse finishing off the last few feet, and I could barely see her behind the piles of dirt and stones. Some of the boulders we dug up were unbelievable! We hate digging here :(

    Calum, thanks a lot for weighing in. Yes, we are just down the road from you, in the Head of St. Margaret's Bay! I'm happy to hear you have been pleased with your pumps thus far. Thanks for those figures on your energy consumption. I'd never go back to oil, for a host of reasons - I honestly haven't even looked into the infrastructure costs of having a tank put in etc. Our goal is to use solar extensively, as we face South and get quite a bit of sun here, so hopefully we can feed ourselves a good bit or at least feed NSPower via a net-meter at some point in the nearish future. That, and we've been utterly spoiled by our wood stove, super-insulation, and the solar gain we get through the windows on sunny days. We use maybe one, one-and-a-half cords of wood to heat the house (roughly 1200 sq.ft.) all season, usually starting to burn in mid/late November.

    The shop isn't super tight, but it will get better as I improve the space (replacing the single pane windows, changing out the floppy swinging barn doors for something that seals up tighter, and re-insulating the walls). The envelope of the building leaves something to be desired... when we inherited the building, it was OSB sheathing with two layers of Tyvek home wrap on the outside, and that was all. I think the previous owner would just reskin the place as the outside layer decomposed in the UV light? I put up 1/2" plywood, caulked all the joints, and we painted it with exterior paint. One day we might be able to take it all down and do it properly from the studs out... or would it be ok to keep going right over the OSB, Tyvek, plywood and paint? My plan was to mimic what we did on the house... Frogskin over the plywood, then Roxul exterior insulation, furring strips and siding over the top.

    Anyway, now I'm getting into future questions about the shop envelope it seems :) I love this forum. Thanks so much for the helpful advice, everybody!

  12. calum_wilde | | #12

    I can't believe you dug that by hand. I had to dig down to my septic tank one time and that was atrocious. I don't dig anymore, now I call an excavator.

  13. benjamin_wooten | | #13

    Yep, "never again" is our new motto.

  14. lance_p | | #14

    Ben, you've stressed that humidity control is very important for your woodworking business.

    Humidity control in the winter months will be more about dealing with a lack of humidity. Neither a wood stove or a heat pump can humidify air, and it can get very dry in winter time. Keeping a constant amount of moisture in the air 24/7 would probably require heating the shop 24/7. I'm not sure you would want to do that unless it is very well insulated and tightly built?

    The more efficient mini-splits are not very good at pulling moisture from the air, a sensible/latent design ratio that favors highly efficient cooling. Depending on your desired indoor RH in the summer you may need supplemental dehumidification.

    Food for thought.

  15. Expert Member
    Dana Dorsett | | #15

    Most mini-splits have a "DRY" or "DEHUMIDIFY" cooling mode to manage high latent loads, or or latent load when there is only modest sensible load. It takes a significant hit in as-used SEER operating in that mode compared to it's normal cooling mode, but given the modest sensible cooling loads in NS that doesn't matter a whole lot. There's nothing that's going to be dramatically MORE efficient at dehumidifying (unless you have a gia-normous capital budget for this project.)

  16. Jon_R | | #16

    Would be interesting to see an accurate cost comparison of reaching a given level of %RH with typical dehumidifiers vs a mini-split in dehumidify mode. The former supplies some heating and the latter supplies some cooling.

  17. benjamin_wooten | | #17

    Thanks everybody for the continued input and thoughts.

    I had read about the "dry/dehumidify" modes on these units, and am curious whether that can be used in Winter alongside the heat? Like defrosting the windshield of a car, where the vent runs full-tilt on high heat, but also automatically turns on the AC unit.

    My experience in this area has been that Winter is frequently quite moist, above freezing but wet outside. I find that on those days (like today, for example, having just returning from a week-long out of town trip) there are moist spots on the concrete in the shop, and some of my tool tables are perspiring.

    I'd certainly be open to humidifying the space a little in the Winter if we had a real cold/dry spell. I am hoping the need for dehumidification in the Spring/Summer won't be very much at all. I imagine that the best iteration of this is to replace the windows, fix that big swinging door, reinsulate and seal the building as best as I can, and insulate the floor and foundation... then see how much it takes to keep it at a nice median RH% and working temperature.

  18. calum_wilde | | #18


    My house remains a comfortable 44ish %RH in the winter. That's with all the typical activities, showers, cooking, humans breathing, and my wife drying a few pieces of laundry inside. But I didn't find the heat pump made much difference there, it's just the HRV that seems to have any effect on humidity in the winter.

    During the summer the HRV is useless for humidity (though still required for a safe living environment), but the ductless heat pumps do a great job of reducing humidity.

    If you're going to be drying/maintaining wood I'd suspect that you'd need both systems to manage all year. I'd start with finishing the building, then adding a heat pump and HRV and monitor your RH levels. In the end you may need more dehumidification than that, but with out knowing the moisture content of the wood, how much of it you'll be storing, or the air tightness of the building, I doubt anyone could even hazard a guess.

  19. benjamin_wooten | | #19

    Yes, I think you're right. I'll only be storing and using properly air or kiln dried wood to begin with... and if I end up processing green stock (which is what I'd like to do eventually) then I will build a separate solar kiln on the property.

    The HRV addition is a good suggestion for out there... I'll just have to be at the filters all the time to make sure they don't get loaded with particulate :)

  20. Expert Member
    Dana Dorsett | | #20

    It's not possible for a mini-split to condense moisture out of the air with a mini-split in heating mode, since the coils are well above the room temperature, which is above the dew point temperature.

    Indoor humidity in winter can be controlled by the ventilation rate, since the outdoor dew point temperatures tend to be very low.

  21. benjamin_wooten | | #21

    Thank you Dana, makes sense. Another upvote for an HRV unit! We just had the first installer come out... recommended a 15K Fujitsu, which seems like a big unit for that space.

    I'll see what installer #2 has to say tomorrow...

  22. lance_p | | #22

    Ben, your equipment was sweating inside the shop because the temperature dropped below the dewpoint of the air. Keeping the shop heated to above the dewpoint temperature will eliminate this.

    I would suggest that if humidity control is a priority, air sealing should be your number one goal for the shop's envelope performance. This will help you keep moisture inside when it's cold outside, and keep moisture out when it's warm outside.

    I would start by determining how much moisture you need in the air. For example, if preservation of wood requires a certain amount of moisture in the air you can relate that to a certain Temperature and Relative Humidity. If 18C @ 40% RH is a good place to be, you now have a dewpoint temperature to work with, which is 5C. This means any object or surface within that space that is 5C or colder will be at risk of moisture damage since condensation will form on it.

    Check out this dewpoint calculator from the Image Permeance Institute. There are some preservation indicators relating to artwork:

    Playing with the sliders we can see that a dewpoint of 5C would let you go as low as 9C without exceeding 80% RH. In reality, you would probably need to keep it a little warmer to ensure the interior shop surfaces stayed above 5C.

    Going the other way in the summer, we can see that to maintain that level of moisture in the air (dewpoint of 5C) at 26C requires a much lower 26% RH, so keeping the shop at a cooler temperature would be beneficial.

    I'm not sure how precise you need your humidity control, but getting a good idea of how to deal with it ahead of time can't hurt. Either way, keeping air infiltration to a minimum will help with those goals. Also, using an ERV instead of an HRV can help with maintaining your moisture levels as is transfers moisture between the incoming and outgoing airstreams. An HRV will bring in dry air in the winter months and humid air in the summer months, where an ERV will affect your interior moisture levels much less. Now, if you bring large quantities of wood into the shop that are actively drying and releasing moisture you may need to counter that somehow.

    Absolutely in a shop you'll need some above average level of filtration maintenance. A large furnace filter installed in the inlet ductwork would most certainly make sense as they are abundant and cheap compared to the smaller specialized filters found in many recovery ventilators.

    Food for thought.

  23. benjamin_wooten | | #23

    Excellent information Lance, thanks a lot!

  24. MKCF | | #24

    I'm wondering how the heat pump project turned out? I'm specifically wondering how it performs in a dusty environment (just started a new thread on this topic). I'm a furniture maker here in CT but I work in concrete (these days, anyway- started out in wood).
    My FIL has a furniture/cabinet studio up in VT, and he heats his shop with wood only. He's a big hippie and wouldn't do it any other way. He claims he's never had any issues with humidity. If he leaves home for a while I guess he just brings his glue somewhere warm?
    Also, at least where I am, hardwood lumber is usually kept in an unheated warehouse. It takes a pretty long time for lumber to acclimate to a dry workshop, and once you deliver your piece to the client, it's going to swell up again in all likelihood. Variation in humidity is something that wooden furniture has to accommodate. Few people condition their houses perfectly. So design accordingly.
    Why not have both a stove and a mini-split (provided they can handle the dust levels)? When it gets real cold switch to wood. Use the electric system during the shoulder seasons.
    Did you insulate your slab yet? I'm not sure but I think heavy machinery might crush plywood and rigid foam. My old shop had a slab like that and I actually ran radiant heat pex over rigid insulation and between sleepers on 12" centers, then plywood over that. It worked! The downside was lower ceiling height, which drives me nuts in my current shop. At this point I'd rather spend a little extra on heat and keep my ceiling as high as possible.

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