GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Heating ideas for a shop with living space above it

AHTTAR | Posted in General Questions on

Hey guys.  Hoping to get some opinions here.  I’m building a small detached shop with about 600sf of living space above it and I’m trying to decide on heating methods.  I guess, really I’m trying to decide on a heating method for when we are out of town so things don’t freeze up.  Our main heat source is a catalytic wood stove but I’m torn on location.  The downstairs is mostly shop but a wall seperates about 150sf of living space, bathroom, laundry room and stairs. We’re in Montana so every now and then we get extended periods below zero.  I can’t say its a super insulated envelope but it is tighter than most.  Slab is poured on R10 XPS.  Walls are flash and batt with 2″ closed cell and 4″ batts so around R30.  Roof is single pitch unvented with 6″ of exterior foam under rubber membrane.  Standard 12″ batts in the ceiling, no recessed lights, sealed electric boxes, etc.  We do have a fair amount of glass, lots south facing but the windows/doors are nothing special, somewhere around U.28.  Pella Fiberglass dual pane.

Basic drawing attached.

So the questions:
– Where would you place the wood stove?  Ideally I would just put it in the shop and vent it upstairs and into lower living space but I don’t want to bring shop air into the house.
– If the stove was upstairs, how would you heat the bath/laundry rooms downstairs?
– How would you heat the shop?


Thanks guys

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Peter Engle | | #1

    I don't know about code enforcement in your area, but there could be code issues that help drive your decisions.

    Most code enforcement people would consider your shop to be a "garage" if it has rollup doors. That said, the IRC requires that "garages" be separated from living spaces with a specified fire barrier, generally 2 layers of 5/8" gypsum. There can be no air exchange between the garage and the living space and doors between the two must be fire-rated. Therefore, your idea of heating both the garage and living space with the same wood stove would not be allowable.

    Electric strip heaters are not very efficient, but they are very inexpensive. They are also automatically zoned based on where and how many thermostats are installed. i would heat the shop and lower level living space that way. The bathroom and foyer are so small that they will require little heat anyhow, and if the shop is not used/heated all the time, the payback with anything more sophisticated would be rather long. If the strip heaters get in the way of the shop layout, an electric unitary heater hanging from the ceiling would also work.

    Upstairs, I would use a minisplit heat pump so that you get heating, cooling, and dehumidification if necessary.

    How often is the living space used, and in what seasons? That might also make a difference in the recommendations.

    1. AHTTAR | | #2

      Thanks for the reply. We're not driven by any code enforcement but obviously based on my insulation levels I don't use code as a baseline. I'm not worried about the fire wall though I do wonder about mixing air from the shop. I wear a mask when I'm wood working and welding, why would I want to bring that inside is my mindset.

      That said, the main reason for the heat other than a wood stove is so the place doesn't freeze if I go out of town. Minisplits would be great but I'm afraid they would run non-stop if at all during a real cold snap. Two winters ago we were below -10 for over 45 days straight.

      Wood is our main heat source no matter what so I guess placement of the stove should be my first question.

      Problem with electric heat in the shop is that I will have to run it all the time to heat it up in there. I'd like to be able to blast heat quickly and get to work.

      We are building the shop/guest house to live in right now for possibly a couple of years. We gave my parents our main house so they could travel around for a couple of years without thinking about their own house. So this new place will be fully used and needs to be as comfortable as my main house.

      Btw electric is cheap here, I believe .7 a kw or something like that. Propane is about 1.50 for winter fuel.

      1. Expert Member
        Dana Dorsett | | #5

        So what if a mini-split ran non-stop during cold weather? That's the way they SHOULD be run, not starting and stopping? Even at -10F a cold climate mini-split uses about half the kwh than baseboards to provide the heat. Both Fujitsu & Mitsubishi cold climate mini-splits can be set to 50F for freeze control when the space isn't being actively used. If you need to ramp the temperatures quickly you can oversize it by one size.

        For VERY intermittent use, using radiant ceiling heaters (either electric or propane) rather than baseboard is a standard approach, providing considerable comfort benefit well before the room reaches the target temperature.

        Like any other heating system specification problem, getting to the right solution starts with a heat load calculation.

        1. AHTTAR | | #8

          Makes sense but what about when it gets too cold and the mini split shuts down? Just have to hope the building holds heat long enough? Once the mini freezes up, how long will it take to un-freeze in those temps? Also, when it is very cold, is a mini really all that efficient? Don't get me wrong, I would love to use minis, I've had them in warmer climates and thought they worked great, I'm just not sold in a cold climate yet.

          1. Expert Member
            Dana Dorsett | | #10

            Most mini-splits don't shut down, but Mitsubishi units do at about -20F or a bit cooler (spec says it could be as warm as -18F, but field reports have them still running at -22F.) For that reason in your climate I'd pass on Mitsubishi cold climate mini splits.

            Fujitsu's cold climate mini-splits never shut down, and are reported to still be delivering real heat (albeit an unspecified amount) at -30F. A guy in Quebec who used to post here under the handle "Jin Kazama", heats his house with four 3/4 ton Fujitsu 9RLS2H units at temps that hit below -25F every winter.

            Efficiency at 0F outdoors, 70F indoors most cold climate mini-splits are still beating a COP of 2, but not 2.5. A -10F and in indoor temp of +70F it'll have a COP of about 2 or a hair less. At -20F outdoors, 70F indoors it's probably less than a COP of 1.5, but still better than one. In a freeze- control situation with an interior temp of 50F, at -20F it'll have the same temperature difference as 0F out, 70F in, and comparable capacity & efficiency.

            A Fujitsu 9RLS3H is fully rated with a specified output at -15F outdoors. Find it's row on the NEEP spreadsheet, and look way to the right in the double-letter columns to find it's -15F min/max capacity and COP efficiency:


          2. AHTTAR | | #11

            Great, thanks Dana. Thats what I'm looking for is real world use. I will check these out now.

          3. AHTTAR | | #12

            Ok so first, I appreciate you guys setting me straight on the minisplit option. Like most of us, it was my first choice but I obviously didn't do enough research on the very cold climate performance.

            I'm gonna try to power my way through a Manual J today to really figure out our sizing needs, but I'm curious to hear your thoughts on this layout. Two zone mini split system, one upstairs at the top of the stairs facing the living space and one in the shop. (not sure yet on how fresh air is used, I'm assuming each zone grabs its own fresh air?) I would also run a wire to the bathroom downstairs for a baseboard heater just in case and for some extra comfort. Lastly, I would install a small wood stove upstairs in the living area.

            Should I look at other brands besides Fujitsu or just go with it? I guess one possible issue is going to be to find a way to install it myself since it is so simple.

            Thanks again guys.

    2. gusfhb | | #29

      Am I behind on code, 2x 5/8 firecode is the commercial standard[staggered seams etc], i have only seen single 5/8 in residential, but haven't dealt with it in a while

      1. Expert Member
        MALCOLM TAYLOR | | #30

        I can't speak to your code, but under ours the fire-rating depends on what the two occupancies being separated are. Separating a house from an attached garage would be quite different from separating a living space from a shop - and much would depend on the classification of the shop.
        The required fire-separation also has an effect on whether to use one or two heat sources, as ducts may not be able to penetrate the walls or ceiling.

  2. AHTTAR | | #3

    Also, I'm not worried about cooling. Both the main house and guest house are placed correctly with a lot of roof insulation and at least 36" of overhang over all the windows and doors. Cool nights allow us to cool things down even on the triple digit summer days which aren't often. South facing glass allows enough sun in to allow us to open up several windows and run fans to circulate fresh air. Dry wood stove air also helps. Even on the coldest winter days, as long as the sun is out we keep humidity around 25%.

  3. Expert Member

    I be careful about dismissing possible code issues because you are located where they aren't enforced. Most areas where permits and inspections are not done, the codes still are in effect (Parts of the US and all of Canada). There are areas where codes haven't been adopted, but building-in code violations opens you up to both civil liability, and possible insurance claim exclusions.

    1. AHTTAR | | #7

      I have no intent on ignoring codes. I basically said everything I build far exceeds codes.

      1. Expert Member
        MALCOLM TAYLOR | | #17

        No, what you actually said in response to Peter Engle pointing out you need a fire separation was "I'm not worried about the fire wall".

  4. Expert Member
    Peter Engle | | #6

    The codes also establish MINIMUM standards for building safety and durability. There are good reasons that you don't want to violate the codes. Your "mindset" about mixing the air is based on good data: You don't want to mix the air. Period. So, you've got to have separate solutions for the shop and the living space.

    Again, the tiny living space on the shop level is just too small to worry too much about to cost of heating. Electric baseboard seems like the only way to go.

    For the living space, I still think minisplits are the right choice. As Dana pointed out, if they are running flat out, then they are providing the absolute best efficiency they can. If you really want to rely mostly on wood heat when you're home, that's OK. You can certainly fire up the wood stove and turn off the minisplit. If you want to save a little bit of money on the front end, size your minisplits to just barely maintain 50 degrees on your coldest day. That way you can safely leave the place and it won't freeze. The A/C mode is then a bonus. While you don't think you will need it now, it might come in handy, and it comes free with the minisplits.

    As Dana also mentioned above, any rational decision starts with a heat load analysis. You can't decide what to do or how much any of the possibilities costs or saves until you know how much heating and/or cooling you need

    Propane options seem like a waste, especially in the west where wind energy is coming on strong. With a 20-30 year lifespan, it seems you would be using more expensive energy and designing in obsolescence. Go with an electric option that meets your needs.

    1. AHTTAR | | #9

      Codes only establish anything if you're willing to question them and make your building better. Most codes are outdated and not even enforced due to lazy or overworked inspectors and planners. I've done commercial and residential projects in California building hell as well as small rural towns and the inspectors in all places are clueless to anything that hasn't been enforced for decades. For example, drive around montana and look how many unvented single pitch roofs there are that are only 6" thick. Long and short, use your brain not the codes. If you can't afford to build it right, don't.

      To be clear, this is a guest house next to my main house. I'm very familiar with what I need and don't need. A/C has zero influence on my decision. Its not free with a minisplit because you pay the cost of the minisplit compared to say a direct vent wall unit.

      The best heat load calculator I could find puts around 18k but I don't think its all that accurate. Local HVAC guys put us around 30k but hes way off I'm sure.

      I will never run the upstairs heat while I'm home. The wood stove will always be run. If the stove is upstairs, then the heat in the downstairs living area will probably get used if I can't get the stove heat down there in a simple manner. When I'm away, it doesn't really matter if the upstairs gets cold. The main water line comes into the shop and all the plumbing is pretty much right around the downstairs bath. So from a heating standpoint while away, the area in the shop around the bathroom is the most critical.

      Thanks for the replies.

  5. AHTTAR | | #13

    From the local HVAC guy in my climate. He says he installs tons of minis so not sure what his real thoughts are.

    I’m not sure where you are getting your information, but it doesn’t match up with the information I’m getting from my distributors, from Fujitsu’s spec sheets or the independent data from the AHRI database. For example, according to the AHRI ratings from the AOU24FLXFW extra low temp outdoor unit provides 66% of its heating capacity at 17 degrees above zero (AHRI certificate attached). Fujitsu’s specs sheets do not include published outputs for heating capacity below 47 degrees and simply list an operating temperature range so there is no way for me to extrapolate heat outputs at -15.

  6. Expert Member
    Dana Dorsett | | #14

    The AHRI numbers are only the modulated output levels that the manufacturer chose to test at. The NEEP spreadsheet is a better source of what the thing really does.

    A 2-ton head for just one floor is a bit much and the AOU24FLXFWH (remember it's the -H version) only modulates down to 7500 BTU/hr @ +47F- it'll cycle on/off like crazy. But according to row 260, column BE of the NEEP spreadsheet it can deliver 19,720 BTU/hr @ -15F outdoors, +70F indoors.

    The 2 ton AOU24RLXFZH may be what you want if you want separate heads upstairs and a head downstairs.

    According to row 148, column BE of the NEEP spreadsheet it delivers 13,500 BTU/hr @ -15F outside, 70F inside. At +50F inside that would be approximately it's capacity at -25F outdoors, but they never test or specify them that way.

    When married to multi-split compressors the heads don't really modulate with load. You'd be more efficient and more comfortable with a pair of 1-tonners (or 3/4 tonners). A 1-ton 12RLS3H (remember the "H" at the end it's important in your climate) delivers 11,500 BTU/hr @ -15F (see row 199, column BE) so a pair of them would be good for 23,0o0 BTU/hr @ -15F outside.

    The 3/4 ton 9RLS3H delivers 11,000 BTU/hr @ -15F so a pair would be good for 22K. (Row 142, column BE). Either model modulates down to 3100 BTU/hr @ +47F (each), but you can turn one off if the other is capable of carrying the whole load. I suspect this is closer to what you really want.

    1. AHTTAR | | #15

      Thanks for taking the time here Dana, I really appreciate it. My biggest problem right now is really figuring out heat loss calcs or manual J so I can size this thing and get it on order. I'm struggling to find an online tool that can handle even my insulation, I can only imagine some super insulated houses. Do you know of any that might get me in the ball park without doing a full blown manual J? Some of the online calculators without taking into account glass and extra insulation puts me in the range you were talking about. How did you come up with those numbers?

      None of the local HVAC guys will sell me the system but they will come pressure test and set levels for me. If anybody recommends a good place to buy the system online, please let me know. I've been dealing with AC outlet and they seem fine so far.

      Thanks, and let me know when I start asking too much of you.

    2. AHTTAR | | #16

      One thing to remind you here Dana, this system is mainly for freeze protection when we go out of town. Of course if I sold the house I would like to say the heat will keep you comfortable, but the wood stove is the main heat source in there. Doing two seperate units will add at least $1000. From an efficiency standpoint, that will take a long time to pay back since I will rarely have it on. If the multi-split will do the job I feel like I'd rather save the money. Also with two units it would be more labor to get the dealer to test/fill it for me.

  7. Expert Member
    Dana Dorsett | | #18

    It's hard to rationalize the cost of a mini-split if it is ONLY used for freeze protection while you're away.

    Two zone multi-splits are often more expensive than a pair of 3/4 tonners when sourced & installed completely turnkey by a pro. With mostly DIY installation that may not be the case.

    1. AHTTAR | | #19

      It's only for freeze protection but also for resale. If the next guy that owns my house wants to be lazy and not heat with wood, the mini needs to work well.

      You mentioned turning one off if a single unit could carry the entire load. Problem is one will be in the garage and the other is in the living space so they need to run separately.

      Looking at costs, its really not all that different from other options. Our power company has a $100o rebate in place and there are federal rebates as well. A 27k btu dual zone 15k/12k unit ends up being less than $2k with linesets. 24k BTU unit is about the same. Two single zone units is about $3k.

      So to be clear, you would go with 2 singles because they modulate lower resulting in less short cycles? Also because of that they can put out more heat at lower temps so you can ultimately get away with less tonage?

  8. Expert Member
    Dana Dorsett | | #20

    If they're in two separate places I don't know how you were intending to use the AOU24RLXFWH, which is a single zone compressor(?)

    The smaller single zone units are more efficient, have more cold-temperature capacity, and with the modulation range are more comfortable than multi-zone compressor solutions.

    Spending more money now for resale later to a buyer who may not even know or care about the difference doesn't necessarily make sense. If the 2 ton multi-zone AOU24RLXFZH (not WH) has the capacity and saves you a grand and it's only for freeze protection that's probably the "right" way to go. If you're using it for comfortable space heating/cooling while occupied a pair of 3/4 tonners is the ticket.

    I checked to be sure- "Minimum Heat" mode still works with Fujitsu multi-zone units (such as the AOU24RLXFZH) , the same way it does single zone units:

    "Sets room temperature to 50°F, keeping temperatures above freezing and reducing power consumption."

    1. AHTTAR | | #21

      I wasn't planning to use the single zone. Sorry for the confusion. That model was just cut and pasted from the local HVAC guys response to using a mini split. He essentially told me he didn't want the job my guess is because he thinks I'd be calling back all the time saying I'm cold.

      My intention was to use either a 24k or a 27k dual zone if I can ever get an accurate load calc. They're about the same price but of course I don't want it short cycling all the time.

      Its not just resale, its also a possible rental. Wouldn't want tenants calling me saying the heat doesn't work. Unfortunately I can't picture how the difference in comfort is going to be from a single 24k unit as opposed to separate single zone units. Makes more sense in your head than mine.

      Thanks for your time.

  9. Jon_R | | #22

    With a few fittings and an air compressor, you could make it easy to blow out pipes such that freezing wouldn't hurt the pipes. Much safer since it works even if the power goes out while gone. On the other hand, there may be other things that aren't freeze proof.

    A wood boiler would address issues of airflow between spaces.

  10. AHTTAR | | #23

    I plumb it so I can blow it out as well or even just draining the system from the lowest point is enough. The main water line still has to come into the garage though and don't want that to freeze. Also, probably not the best for all the other stuff in the house to be freezing. For example, the refrigerator would probably stop working. Not to mention, I don't know anybody that likes to get home to a house that is -15F!

  11. AHTTAR | | #24

    Funny how all the HVAC places have different info. People must get screwed all the time. Just had a guy from tell me the mitsubishi and LG don't shut down but the Fujitsu they have in their building shut down last winter. I'm guessing they just are more incentivized to sell certain brands. Anyway, LG actually does seem interesting. Does anybody have opinions/experience with LG?

  12. Expert Member
    Dana Dorsett | | #25

    I haven't seen an LG that has a rate capacity at temps lower than -12F dry bulb/-13F wet bulb.

    According to the extended temperature capacity charts he 1-ton LAN120HYV1/LAU120HYV1 can deliver only 9420 BTU/hr -12F outdoors, at a cool 60F indoors. (See table 17: )

    That indicates that it's probably not a vapor injection type compressor (or not as well tuned for cold tempertures as the Fujitsu) and it would likely have a COP less than 1 when it's -25F outdoors. The 1.5 ton LAN180HYV1/LAU180HYV1 might get you there, delivering ~14K @ -12F. (Table 18)

    1. AHTTAR | | #26

      You're too sharp for me Dana. Thanks. I'm gonna order something today most likely the Fujitsu. Daikin seems to get a lot of love so I'm getting a quote for good measure.

      1. Expert Member
        Dana Dorsett | | #27

        Daikin is a high quality manufacturer, but I've yet to see a cold climate mini-split spec from them. They may have a newer lineup that I don't yet know about, but most of their stuff a few years ago didn't have extended temperature capacity tables that went cooler than +5F.

        Their 1.25 ton RX15QMVJU/FFQ15Q2VJU is only good for 7,270 BTU/hr @ +5F outdoors, 70F indoors. A pair of those isn't going to cut it at -25F at your estimated loads.

        1. AHTTAR | | #28

          Fujitsu it is. So the first online company finally got back to me and it turns out they were trying to sell me the wrong model. Not the "H" model. I've been dealing now with PED who seem to be pretty good and actually fairly knowledgable. The difference between a dual zone 24K BTU and two single zone 12K and 15K units is only about $350 so that route is starting to make more sense.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |