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Heating a cold-climate off-grid superinsulated house

user-1072251 | Posted in Mechanicals on

The house will be in New England, northern zone 5, high elevation, open to north winds. Good southern exposure but lots of windows for north view. Cathedral ceilings up to 18-20. Proposed insulation R40 walls, R65 roof, triple glazed European windows; very tight. (<1.0ACH50) There will be a PV system with battery back up and possibly solar thermal. Do minisplits work for off grid situations? Solar thermal with radiant wall heaters? Anyone have experience with superinsulated off grid homes in this climate?

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Replies

  1. Kevin_in_Denver | | #1

    You don't ever want the water lines to freeze up. Therefore, you should plumb it with PEX because it is freeze tolerant, and also slope all the lines so they can be drained.

    Too bad you seem to be leaning away from passive solar. It is very cost effective compared to solar thermal and PV.

  2. jinmtvt | | #2

    Anything is possible. Why would mini splits not work off grid ?? feed them energy and they run..
    Get the latest models and as small as possible .
    Problem is still that when u need heat, there is not much power to be drawn from the sun.
    ( night + winter )

    You will definitely need to work on the building design for passive solar, with some thermal
    mass to be able to extend that temperature rise.
    You may think about some discomfort due to interior temperature rise that will be required
    to be able to go through the winter nights.

    Solar thermal is a not go with current PV costs in usa/canada.
    Even more if you can manage to purchase and install the panels urself.
    ( some good panels sell for near 1$/w currently )

    Anyhow you will need large south exposure to winter sun to gather directly as much solar passive
    as possible during daytime.

    The rest is only possible through careful design of all parameters.

    I'd consider propane or wood stove if you have access as backup.

  3. GBA Editor
    Martin Holladay | | #3

    Bob,
    I have lived off the grid in northern New England for almost 40 years, and many of my friends also live in off-grid houses.

    Before you make any decisions on heating systems, I strongly suggest that you talk to people in your area who have lived off the grid for at least four years.

    Between Nov. 1 and February 15, almost all of your electricity will come from a gasoline-powered generator or a propane-powered generator. You will not be getting any significant electricity from your PV array.

    It is extremely inefficient to power any kind of electrical heating system (including a minisplit) with a gas-powered generator. So you won't be using electricity to heat your house.

    Nor do you want any type of heating system that includes pumps or blowers. Electricity is very precious and expensive in November, December, and January.

    You have two basic choices: a wood stove -- one that does not require electricity -- or a propane space heater with a through-the-wall vent -- also one that does not require electricity.

    Many off-grid homeowners have both types of heating system. They use the wood stove when they are home, and they use the propane space heater when they have to go away for the weekend.

  4. jinmtvt | | #4

    Martin : how much energy can be gathered daily from the PV
    is it normally enough to regenerate batteries for lighting and small appliances ??

    wood and propane is a must ... comfort compromises also!

  5. GBA Editor
    Martin Holladay | | #5

    Jin,
    The amount of sunlight in northern New England during Nov., Dec., and Jan. is highly variable but not very significant. I remember one November when we had sunlight on only three days during the entire month.

    During these months, when the sun does come out, it reaches the southern part of the sky at about 9:30 a.m., and it barely skims the tops of the trees, because it is so low in the sky. At my location, it sets behind the trees during the winter at 2:00 p.m. or 2:30 p.m. The angle is so low that it is hard to gather any energy unless you adjust the angle of your panels to a steep winter angle.

    The other problem is snow accumulation. A vigilant off-grid homeowner will be on his roof every day at 8:00 a.m. with a broom, but that isn't always possible. Fortunately, a steep winter angle for one's panels helps with snow accumulation.

    It is wildly unrealistic to size a PV array and battery bank to meet your lighting and small appliance needs for Nov., Dec., and Jan., at least in this climate. While it is theoretically possible to do so, the money is better spent on a good generator and gasoline. The generator and gasoline cost less, in the long run, than the huge PV array and huge battery bank that would be required to get through the month of November.

  6. Expert Member
    Dana Dorsett | | #6

    As enamored as I am of mini-splits, Martin has it dead-right: The lifecycle cost per kwh of battery backed PV and the absolute sizes of both the battery & PV array required to make a substantial impact on even tiny heat load is an order of magnitude (or even two!) beyond all reason.

    For tiny off-grid heat loads, tiny off-grid biomass burners make the most sense, and the less they need rely on power, to operate the better. (Tiny high-thermal-mass wood stoves with dedicated ducted combustion air make more sense for off-grid than pellet or corn stoves with electric augers & blowers etc.) Save the precious PV/battery power for higher-value energy uses that don't have many viable alternatives, such as ventilation, lighting, and power tools, etc., not low grade uses such as space heating.

    When it's all said and done with all carbon bits accounted for heating with a mini-split off a propane or gas generator is dirtier and worse for the planet than heating the place directly with a 75% efficiency coal stove. (There's nothing quite like the smell of burning anthracite in the morning, eh? :-) ) But heating with a biomass is fairly carbon-neutral, though the localized air pollution issues are still real.

  7. user-1072251 | | #7

    "Tiny high-thermal-mass wood stoves-"
    Dana: what do you consider a high thermal mass wood stove? The owner was thinking originally about a masonry heater and hot water radiant wall panels with hot water supplied by solar thermal; I first brought up minisplits. Sounds like the big drawback is not enough sunlight at the appropriate seasons to supply the electricity for either minisplits or the walls panels (which would need electric pumps) so that leaves the masonry heater or a woodstove. I think the next step is to get some accurate heat loss calculations, determine the load, review the plans, then make the final decisions on heating. All of these answers are helpful to me as I haven't been involved in off grid projects before, so thanks!

  8. GBA Editor
    Martin Holladay | | #8

    Dana,
    I don't always keep up with the last technical jargon -- I'm an old-fashioned kind of guy -- and I'm not exactly sure what you mean by "tiny off-grid biomass burner."

    However, I'm sure I know the type of heater that Bob needs. It is a wood stove. (That is, a steel or cast-iron box connected to a stovepipe.) The latest models all meet EPA emissions requirements. Get a simple model without a catalytic converter, whistles, or doo-dads.

    To use the device, you put sticks of wood inside and light them.

  9. ncmeter | | #9

    Martin,
    Back in 2013 you wrote:
    "It is wildly unrealistic to size a PV array and battery bank to meet your lighting and small appliance needs for Nov., Dec., and Jan., at least in this climate. While it is theoretically possible to do so, the money is better spent on a good generator and gasoline. The generator and gasoline cost less, in the long run, than the huge PV array and huge battery bank that would be required to get through the month of November.
    Answered by Martin Holladay
    Posted Aug 12, 2013"
    My question is: given the lower cost of pv panels, is it still overly optomistic to hope to use mostly solar? I am building a near passive house off grid, at the foot of the White mountains in NH. We've got 8900 watts of panels and around 1700 amp hours of batteries. I have also purchased an 12k LP generator. I was planning on using radiant with a gas hot water heater as my primary "automatic" heating system along with a basement woodstove and an eventual Russian fireplace / masonry heater. I've incorporated passive solar designs as well. My second question is do you still think it's unrealistic to use radiant heat with D.C. Circulators? Thanks, Scott in NH

  10. GBA Editor
    Martin Holladay | | #10

    Scott,
    I suggest that you read two articles:

    How to Design an Off-Grid House

    Batteries for Off-Grid Homes

    Be careful when determining the storage capacity of your batteries. Batteries can't be fully discharged if you want them to have a long lifetime. Discharging them down to 80% capacity (using only 20% of their capacity) is a good way to guarantee they'll last a long time. So if you think you have 1700 amp hours of storage (204 kWh), check to find out whether that storage capacity assumes 100% discharge or 20% discharge.

    If you actually have batteries that can store 204 kWh without discharging the batteries to the point of battery destruction, that amounts to 6 days of storage for an average American family. But winter lasts 90 days in the White Mountains of New Hampshire -- 90 days of clouds, when a sunny day means only two or three hours of full sun.

    I know several off-grid families who installed hydronic heat with in-floor tubing. They have all abandoned their hydronic heating systems (because these systems drain their batteries at the time of year when there is no sun). They now heat with wood stoves, supplemented by propane space heaters with through-the-wall venting.

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