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Community and Q&A

Space and water heating for a micro-load house

JustHousing | Posted in Mechanicals on

I’ve been re-reading Martin Holladay’s posts on water heating as I once again deal with a small-ish super-efficient house: 1900 ft2 with a heating load of less than 20 kBtu/hour. The homeowners have access to propane and electricity and here in MN, electricity is still generated by a lot of coal, although the homeowners are planning for a pv system.

At the moment I’ve narrowed heat/hot water system ideas down to three:
1. use a 6kW modulating electric boiler for space heat (hydronic radiant) and also to fire to an indirect tank for hot water. Also, use a drainwater heat recovery device.
2. Use a modulating propane-fired boiler that is oversized for the space heat and use a Marathon electric tank water heater for hot water. So, space heat and hot water stay completely separate.
3. Use the modulating propane-fired boiler for space heat and hot water via indirect-fired storage tank.

I’m pretty sure we can run the boiler output temp at 120 – 125F for both space heat and domestic hot water. The smallest gas-fired boilers I can find do have low range firing at under 20,000 Btu/hour, so maybe that will be okay?

Input/thoughts from GBA community?

Additional info:
Design heating load is 17,000 Btu/hr
There is no cooling load.
The house is in a rural area. It will have a well, and if the water is too hard, a water softener.
Family of 3; they are conservationists.
They will have a wood stove located on the main floor, which has a very open plan. All bedrooms (3) are upstairs.

finally, I’m trying to devise space and water heating systems that are reliable, fairly simple, and easily serviced by pretty much any plumber.

Rachel Wagner

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  1. Expert Member
    Dana Dorsett | | #1

    At those heat loads using a condensing Vertex or Polaris employed as a combi-heater works, and would be less expensive hardware than a mod-con boiler + indirect HW solution. The thermal mass of the tank keeps them from every short-cycling. At that design heat load even the smallest mod-con boilers would never modulate, so there's little advantage to that control sophistication.

    But ....

    "wood stove located on the main floor, which has a very open plan. All bedrooms (3) are upstairs."...

    ... and...

    "Design heating load is 17,000 Btu/hr"

    ...just SCREAMS "mini-split heat pump". With the wood stove as either primary or backup heat, and some amount of resistance heating as backup/balancing 2-3 well-placed & sized ductless heads would work fine right down to about -18F or so, and would be cheaper to run than a propane-fired system. Mini-splits are fully modulating and can be sized reasonably to track even those fairly low loads. A Mitsubishi MSZ-FE12NA can deliver ~8-9000BTU/hr @ -13F at a coefficient of performance of about 1.8, and is still chugging away until it self-protects by shutting down at about -18F (it automatically re-starts when it warms back up a few degrees.) I understand Fujitsu has released a similar cool temperature series as well. You may need a 1,5-ton (-FE18NA) to handle the first floor, depending on the actual room-by-room heat load numbers and your outside design temp. Fujitsu has released a similar series capable of -25C/-13F operation too.

    During the shoulders seasons the COP efficiency of these units soar into the 4s.

    For climate info purposes, got a ZIP code?

    See third party bench tests of the -FE12NA here:

    Alternatively, the Daikin Altherma air source heat pumps provide both domestic hot water and low-temp hydronic heating down to at least -8F or so. It's more expensive than a mini-split solution, but can be micro-zoned with a low temp panel radiator in each bedroom. The ERLQ030 can deliver the requisite amount of heat if you use very low temp radiation, such as a radiant slab on the first floor. See:

    While the MN grid still has has a heavy coal component, it's most rapidly rising fraction is wind + combined cycle gas (at nearly 2x the thermal efficiency of baseload coal, and less than 1/3 the carbon footprint per kwh) with no sign of abatement. About half is from coal, a fifth is from legacy nukes, and wind is now into double-digits, the rest being mostly a combination of high-efficiency combined cycle gas and conventional gas fired peakers, with a smidgen of solar as dressing on the electricity salad. Going forward it appears combined cycle gas & wind are poised to render legacy coal & nukes uneconomic. So while heating with heat pumps may initially have a slightly higher carbon footprint than condensing propane, within the lifecycle of the equipment my expectation is that it will have flipped the other direction. (And of course, the more they can offset power use with a high-efficiency woodstove, the lower the overall global warming hit will be.)

  2. JustHousing | | #2

    THANK YOU Dana!

    Some responses to your question and comments:
    First, the zip code is 55803
    This is Duluth, MN with approx. 9800 HDD

    I'm considering using mini-splits. I also very much like the idea of the Polaris or Vertex - I'd forgotten about these, and they make much more sense than a modcon boiler (unless we go with an electro-boiler, but I'm losing interest in that route).

    I've scheduled a meeting in a week with the mech contractor and general contractor - and wanted to have some GBA input before that meeting, so your comments are helpful in that regard as well.

    I need to continue the discussion with the owners about their commitment to using the wood stove. Initially they wanted it for "back up" but given the design we've accomplished, they are now open to considering it their "primary" heat source, and that really changes things.

    Again, thank you. Once I've talked with the mech contractor, I'll post our proposed approach.

  3. Expert Member
    Dana Dorsett | | #3

    The 99% outside design temp for Duluth is -16F, which is banging the low temp limits of the colder-temp mini-splits, which means you WOULD need 100% backup for those occasional cold snaps where it drops below the unit's shut-down temperature. But 17,000BTU/hr is still only 5,000 watts of panel radiator or cove-heater. The room-by-room heat loads will tell you what to to install in each, and may be useful for temperature balancing when it's really cold out anyway should you only go with 1-2 ductless heads.

    With a design heat load of 17KBTU/hr it's important not to over-size the wood stove or it'll turn into a sauna. Having some amount of thermal mass to the wood-burner itself (ceramic or soapstone stoves work) let's you oversize it a bit, but even then you'll want to keep it under 40KBTU/hr to be able to burn in the low-pollution/high efficiency zone. A house with that load is probably extremely tight too, which means stoves with sealed combustion/ducted combustion air kits move to the head of the list, to keep backdrafting issues at bay.

    Either the Hearthstone Tribute or Bari (~35,000BTU/hr max fire) would probably work, but bigger stoves would have them throwing open the windows at 0F outdoor temps. The Bari takes bigger wood (17" max), but has the Euro-modern look, compared to the traditional boxy (but smaller) look of the Tribute. The Bari is also 475lbs compared to 318lbs for the Tribute, which is probably mostly in high-thermal-mass stone. (But it's 2-3x the cost too.)

    With a small higher-mass stoves that are ~2x oversized for the peak loads it's possible to run multiple burns that are allowed to burn to down to dark embers to manage the room temps. They continue to emit substantial amounts of heat hours after they go dark, but are lousy at raising the room temps quickly- there's a measurable lag, and a lower peak surface temp, but that would probably work just fine in this house.

  4. GBA Editor
    Martin Holladay | | #4

    I agree with most of Dana's advice.

    Concerning Option #1: Electric resistance boilers don't make any sense. If you want to heat with electric-resistance elements, just install electric baseboard.

    Concerning Options #2 and #3: I agree with Dana on the disadvantages of a propane boiler.

    The Daikin Altherma is expensive, and it probably isn't appropriate for your very cold climate.

    The Vertex or Polaris solutions probably rise to the top of the list. If you want to plan for global climate change, and you assume that winters will be milder in the future, you might want to go with cold-climate minisplits and a Marathon water heater. You'd need electric-resistance baseboard for backup if you go this route, as Dana wrote.

  5. jinmtvt | | #5

    Dana and Martin : i find it funny how often we recently end up thinking about mini-splits as a solution in many different situations :)

    Dana: may i name you "Sifu 2" ?? would make me a double apprentice!!

  6. JustHousing | | #6

    Martin and Dana, thank you. Regarding Martin's take on option 1, I don't think of electric baseboard as a substitute for an electric boiler, because that doesn't factor in the big (I think) differences in mode of distribution, plus the ability of the system to be easily modified in the future. If I go with a hydronic distribution system, an electric boiler could be replaced later with something else, or become augmented with solar thermal. Of course, the Polaris allows this too. But with PV, a 5 kW modulating electric boiler seems like an option, anyway, and putting heat into the slab means that we have a bit of heat "storage" which can cause problems but can also be a benefit. I'm placing the Polaris or Vertex high on the list for combined space/water heating option.

    Onto the discussion of minisplits plus a separate water heater (again, with drainwater heat recovery). I think the newest cold climate Fujitsu mini-splits don't have a shut off temp, and they are rated at full delivery to -15F, so even though they'd perform less well in -30F they'd produce something? I need to look more closely at this. I'm looking at the RLS2H by Fujitsu - one for the first floor and one for the second floor.

    By the way, regarding the wood stove. Dana, I've never heard of a sealed combustion wood stove. I"ve used both models you refer to, though, and clients love them. this client is looking at the Tribute. Also, I've pretty much given up on those outdoor air kits. We have stoves drafting fine even in houses testing at 0.6ACH50. It's not the wood stove that causes the problem, its the other exhaust devices and so for those we either install make-up air or tell clients they'll have to crack a window next to the exhaust device.

    Again, thank you Dana and Martin, and GBA for providing such a useful forum.

  7. Expert Member
    Dana Dorsett | | #7

    It's the backdrafting when a major high-duty cycle exhaust blower such as a clothes dryer is running that I'm most concerned about. I personally never count on user instructions for opening windows as the solution to the backdrafting issues. I've used the Hearthstone outdoor air kits in retrofits- you can probably do it somewhat better in new construction using steel/iron hard-piping rather than the crummy flex I usually see. While not exactly "sealed" combustion, it's close enough in this context.

    It's worth digging up more engineering info on the Fujitsu AOUxxRLS2H series to see what they have to say about operation at -25F. As long as they don't self-destruct it could be a real solution. IIRC the Mitsubishi FEs shut down at lower temps because the dimensional tolerances on some components become sufficiently out of spec to run into wear issues.

    I'll agree with Martin that the Altherma is less than ideal for Duluth's peak-coolth, but net efficiency is about averages, and it handles the +16F binned hourly January average with ease:!dashboard;a=MN/Duluth

    This is definitely a "deluxe" way to go, and the client may not consider it worth the upcharge, but I throw it out there since it can be micro-zoned and provides the domestic hot water as well.

    Jin: A title of "Sifu" could only be considered sarcasm when applied to a Nufi like me! :-)

  8. JustHousing | | #8

    well, I must be a nufi, because I had to look that up, along with Sifu and IIRC .... :-)

    A month or two ago I called a Fujitsu tech guy on the east coast somewhere to discuss the RLS2H. He basically said it would work. ? Also, Mike Duclos in MA told me he's had Fujitsus (RLS2, before the "H" came out) working in -22F. So I'm feeling okay about this.

  9. jinmtvt | | #9

    Dana: well i'd sure like to be a newfi like you then !!

    Your posts are usually brilliant, and your line of thinking is somewhat in the same direction i am trying to orient my learning. I'll consider myself lucky to participate with 2 ( you and Martin ) wiseman on this board. ( not that there aren't other super pro here !! )

    Wagner: i've read stories also about the RLS2 working down to -20F and still outputting heat.
    Wonder how low the new H model really goes.

  10. heinblod | | #10

    It might make sense to heat the house directly or indirectly with electric (storage) heaters.
    The RE part in the grid is increasing fastly, the German parliament just lifted the ban on electric heaters, national electricity consumption is falling back and available renewable electricity is on the increase


    The Scots are following

    having calculated this issue very well over a few pints

    Solar City in the USA offers free PV installations (leasing).

  11. Expert Member
    Dana Dorsett | | #11

    Windcrofting is uneconomic in most of the US, due to comparatively low electricity prices, low turbine output, and high turbine first-cost. Unlike solar there is a very significant economy of scale when you go top 1megawatt+ turbines compared to 10-200kilowatt turbines, and wind developers will be far better off investing in more total capacity of the former than of the latter.

    There are multiple companies with rooftop leasing options for photovoltatics. The installed cost of PV in the US for <10kW that...

  12. KeithH | | #12


    Any chance you can share what your clients went with and how they did this winter (assuming the house went up in 2013)? I'd love to hear how a mini with backup worked out in Duluth.


  13. JustHousing | | #13

    hi Keith,
    The house was completed last month (September) and the owners have just moved in, so we have no data. Stay tuned - next year I will try to post data on our website ( - it is called the Ecologists' House. Here's what we went with: an all electric home with 6.6 kW roof-mounted grid-tied PV system. A 5 kW electric boiler, fully modulating with outdoor reset, for main floor radiant slab heat. Jotul Rangley F50TI wood stove also on the first floor, so they can play with wood heat vs radiant slab. Second floor has a Fujitsu 12RLS2H mini-split for heat (and cooling if they need it). We mounted the indoor cassette high on the wall above the stair, there is an open area across the south side of the upper floor, with 3 bedrooms on the north. We installed a louvered grill high on the wall in each bedroom, to help encourage warm air transfer from the cassette. The stair is open to below, so we're feeling confident that if the lower level is warm the upper level is warm (stacked floor plans). We went with the Marathon for water heating. With a slab on grade main floor and our long cold winters, it is hard to get folks to let go of the desire for a radiant floor. The evenness of the heat is appreciated and the clients know the floors won't be "warm to the touch" but a 73 degree F concrete slab feels better than the alternative. We have the Venmar EKO1.5 HRV, and had a make-up air divert system installed so that when the clothes dryer comes on, the HRV goes into a dedicated recirc, dumping all the recirc'd air into the laundry area. This should prevent backdrafting of the wood stove, which is on the other side of the house.

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