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Community and Q&A

Best hot water heating system for off-grid year-round cabin (zone 4c)

Cynnergy | Posted in General Questions on


Newbie here so please be gentle!

My husband and I are currently renovating a 1950’s 1 1/2 story 750 sq. ft. cabin in coastal BC for use on most weekends and holidays. It’s off-grid, but has access to the community’s diesel-powered 15 kW generator. The fuel bills for electricity are enormous and I doubt the generator will be around 10 years from now, so I’m trying to minimize electricity use when looking at what to do with the cabin.

At the moment, our hot water heater system consists of an indirect 20 gallon storage tank (in the bathroom), heated by a coil in the oil stove (in the kitchen next door) using thermosiphoning. The oil stove is a McClary Charm converted from wood to an oil pot-burner at the factory. We’ve got it going and are experimenting to see how much it burns – so far it’s about 1/2 a jerry can (10L) per day. With stove oil at $1.47/L, not cheap, efficient or environmentally-friendly! And while we haven’t got to use the shower yet, apparently when my parents lived there in the 70’s, they could each have one short shower per day – one in the morning and one in the evening.

I’ve thought of just swapping out the oil stove for an Esse Ironheart wood cookstove with boiler, but at 800 lbs for the unit, I don’t think the floor would support it (the cabin is suspended on log skids). And I’m not sure how cold the cabin would get overnight because I think the fire would die down (did I mention that the insulation is non-existent at the moment and I’m not sure how good we can make it?). At least the oil burns all night :).

I guess we’ll live with it for the moment, but does anyone have any other suggestions? I’ve been thinking of propane on-demand linked to a solar system (but that’s probably the most expensive option, at least in up-front costs), or else swapping out the storage tank (when it goes) for a propane hot water heater. Can you get those with two coils in them so that I could hook up the oil stove and a solar system, and then just turn on the propane an hour or so before I wanted a shower to top up the heat? I’ve also read on here that propane hot water heaters are prone to backdrafting, so would it be a safety issue if I’ve got a woodstove in the living room? The house is ridiculously leaky at the moment, but we’re hoping to reduce that at least a bit in the future.

Any ideas/suggestions welcome!


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  1. GBA Editor
    Martin Holladay | | #1

    Can a propane delivery truck reach your cabin to fill a stationary tank, or do you have to haul small propane tanks in and out?

    A propane water heater is certainly the simplest solution. By the description of your cabin, the wood stove will not cause any backdrafting problems if you install a conventional propane tank-style water heater.

    You can install a propane tankless water heater if you want -- as long as you choose one that doesn't require electricity -- but of course the "endless hot water" feature just encourages longer showers and burns up the propane faster than ever.

  2. Expert Member
    Dana Dorsett | | #2

    If you can dig a 1.5 meter deep crawlspace to accomodate it you can cut the fuel use for showering roughly in half (or double the showering time) with a drainwater heat exchanger:

    Payback is pretty slow for on-grid apps, but probably not for you- even propane is a pricey way to heat water, at any HW heater's efficiency!

    Tankless propane isn't great backup for solar, since the minimum fire outputs are too high for good temperature control at low flow when the incoming water is above 35C. The off-grid versions tend to have even higher min-fire than the powered versions, or use standing pilot ignition, burning propane 24/365. The Bosch Therm 520 HN tankless has a flow-powered ignition, but a roaring ~30kbtu/hr min-fire input.

    Marey makes/sells some inexpensive smaller tankless units with D-cell battery powered ignition, but I can't really recommend them. They're pretty popular in the Caribbean and in Mexico, where they compete with the low-end Bosch units on price. Not sure if they carry sufficient safety certifications for residential use in Canada. If you went that route the 10L (~80KBTUhr) version would deliver enough for a decent shower at B.C. wintertime water temps, but that's probably the minimum burner input rate you'd want in a tankless.

    You're probably better off with a propane fired RV or trailer-sized tank water heater, lighting it up only when you want a substantial slug of hot water for showering. That way flow isn't a problem, only total capacity is, but the recovery rate would also be pretty fast if someone else just took a shower.

    But in a zero-insulation drafty cabin your first priority would be tightening it up and insulating, which would reduce your fuel consumption FAR more than any water-heating hacks. It's unlikely that the floorboards (or any other surface) has an air barrier, and digging in a crawlspace for a CMU or concrete foundation that's possible to reasonably air seal and insulate is probably in the top 3 projects to tackle first. Also on the top 3 list, if the windows have reasonable flashing &/or big roof overhangs blowing the wall cavities full of cellulose (at any density) would be a very good expenditure of that very expensive electricity for a day.

    If you're going to tighten up the place, only consider woodstoves that have an option for piping in the combustion air, which eliminates the backdrafting issues. The smallest woodstoves out there would still be plenty to heat a cabin that size after it's been even moderately well sealed, even if uninsulated, but post-insulation your fuel use would drop to near-nuthin' compared to the amount of oil you're burning now.

  3. Cynnergy | | #3

    Thanks for your quick response Martin!

    Nope, we can't get a delivery truck to the cabin (it's on an island). We could probably get the fuel barge to make a bulk delivery, but I don't really want to get into that, so we'll be packing it in.


  4. Cynnergy | | #4

    Hi Dana,

    Thank you for your advice. I think I'll rule out an on-demand system then! I hadn't thought of an RV-type water heater, that's a great suggestion.

    Yes, insulating and air sealing are definitely on the to-do list - I just didn't want to make my question any longer than it already was. We don't actually have to build a crawl space to somewhat insulate the bottom, as the cabin conveniently(?) sits on log skids over a big hole in the ground (each corner of the logs sits on the ground, but the middle of the logs are suspended - you can stand up and walk around underneath most of it). To make that into an actual crawlspace that we could insulate would be a huge project though I think.

    After some advice from my father-in-law (who's a building surveyor in the UK), we are probably going to try stapling some foil-bubble-type insulation to the underside of the floor joists to create an air barrier, and hopefully get some insulation from the air pockets in between the floorboards and the insulation. I know from poking around on the site that Martin doesn't think very highly of the stuff, but I figure it's worth a try given the low cost. If it doesn't work very well, then I was thinking of trying a layer of rigid foam insulation attached to the underside of the floor joists. The roof also needs replacing, so again, after poking around on this site (it's great!), I was thinking of trying a 'hot roof' construction using rigid foam under the new roofing material. It's expensive, but we're trying to avoid using fibreglass wherever we can - the place was absolutely infested with mice who had made dozens of nests in previous ad-hoc fibreglass insulation attempts that we had to rip out. I think I also read somewhere that in damp climates (like ours), an unvented attic is actually preferable.

    I was thinking of doing cavity wall insulation as well, as you suggest. Do you know if I can I do that myself, or do I have to hire someone in to do it? We could probably get a contractor's truck to the island using a small barge, but it's much easier to do it ourselves if at all possible.

    Now I just need to get my dad on board! He's been a great help, but his response anytime I bring up insulation is 'don't worry about it, you can just put another log on the fire!". I like my sleep, so I'm hoping insulating will help to keep the house warm enough so that I DON'T have to get up at 3am every morning to put more wood on the fire :). It will be a major bonus if I can turn off the oil stove too and only use it for cooking.

    Thanks again!

  5. user-757117 | | #5

    What island are you on?
    My family maintains a cabin on Nelson Island access is only via personal boat or water taxi, so I know about some of the logistical problems you're up against.

    I'm not sure I fully understand how your cabin is built and laid out, but...
    If you can gain access to the inside of the wall cavities, Roxul batt insulation would be easy enough to bring over by boat, is easy to do yourself and is much more resistent to rodent damage than fibreglass.
    A vented attic will be more forgiving that a "hot roof" assembly in your climate.

    At our cabin on Nelson Island, we heat with a woodstove and also use it for cooking in the winter.
    In the summer, we cook outside on a small propane fired stove - large wood-fired cookstoves are beautiful, but you may find it uncomfortable cooking in the cabin in the summer.
    We get electricity from a small PV system and use 12v lighting - a small inverter provides 120vAC for charging small batteries and a small radio.

    Also, don't forget the power of extra sweaters and blankets.

  6. davidmeiland | | #6

    Aside from tightening and insulating, what about a heat pump water heater inside the cabin, using the community electricity? When the generator falls from favor, everyone in the community is going to have to deal with it, and you may all choose to install PV at that point.

  7. Cynnergy | | #7

    Hi Lucas,

    Hello nearly neighbour! We are on Hardwicke Island - personal boat access only. My cousin runs a Sealander barge though that can transport vehicles and stuff up to 5 tons which is our lifeline. I don't like asking for favours too often though :).

    Roxul is definitely a good option. My father-in-law recommended prying off the siding at strategic locations and stuffing fibreglass in. It's good to have a vote of confidence though for the rodent-resistant qualities of Roxul, so that might be the best option.

    I mentioned the 'hot roof' to my father-in-law, and he seemed to think it would be OK (but expensive). I'm basically treating it like a cathedral ceiling because the 'attic'/second story needs to be heated (the second bedroom for guests is upstairs and I don't want to give them hypothermia). See here for a description of what the house is like:

    Just replace 'basement' with 'hole in the ground' :).

    I am a bit confused by all of the HVAC stuff, but it makes sense to me that in coastal BC there is more water trying to come into the house (in winter) than is trying to get out, especially with the drying heat of a woodstove, so a 'hot roof' would work. I guess we could put Roxul in between the attic joists with appropriate venting between the Roxul and the roof (that would certainly be cheaper). I'm just worried about mould/mildew/damp etc if we don't get the venting right.

    Even with a small woodstove, I don't think we'll be cold in the house when it's burning. The problem is with a small woodstove, I don't think I'll be able to get it to burn overnight. With a good duvet sleeping shouldn't be a problem, but it's what to do in the morning! I suppose the best option is to just kick hubby out of bed to start the fire while I doze :). But better insulation will hopefully help make it easier.

    Another reason why I want to get rid of the hot water/oil stove connection - I don't want to have to have the oil stove running in the summer with the doors open just to be able to have a shower! I'm planning on cooking by BBQ in the summer, and maybe a solar oven.

    I like your electricity system! Any more details? I am trying to make the place use as little electricity as possible, but it would still be useful to have some for email, internet, phones, lighting, etc. Maybe even a fridge and freezer and a washing machine, although I know that is a lot of power. I'd like to look into a microhydro system for the Island, which might be possible in winter to combine with PV in the summer. Wind maybe too? We won't get 15 kW, but we might be able to at least reduce the generator use and convince some community members to alter their ways. There is so much to do though! And entrenched views from others along the way! One step at a time...


  8. Cynnergy | | #8

    Hi David,

    I don't know much about heat-pump water heaters. Would it work in the winter on PV? How much electricity do they use? Lots of clouds and rain here in the winter. Microhydro in the winter might be a possibility, but only a vague possibility at this point (we might be able to use the overflow from a spring, prob at about 50m elevation, but it only overflows during the winter and I haven't measured the flow rate yet). I originally looked into a ground-source heat pump for heating the place, but quickly dismissed that when I saw how much electricity they used.


  9. user-757117 | | #9

    It is nice to have neighbours - especially when you are on an island and one of them has a barge.
    Sadly, I can't really count myself as one of your neighbours.
    I live in Ontario now and only make it up to Nelson Island on my now rare visits to the coast, though I do miss it.

    An off-grid PV system can be as simple or complex as you like, it really depends on what you figure you need to get by...
    It is certainly possible to power all the amenities of modern life this way, but beware the costs, design challenges and maintenance involved with large, complex systems.

    For hot water, you may want to consider solar thermal heating as part of the system.
    Solar thermal has recently taken a back seat to solar PV, but for an off-grid application in an isolated setting it may make some sense to look into it.
    Flat plate solar collectors are very simple and durable - there is a guy up the road from me who has been running the same two collectors as part of a closed loop system with glycol for the last 25 years.

    Some good general advice for this type of cabin might be to keep everything as simple and maintenance free as possible - that way you can spend more of your time enjoying your surroundings ;-)

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