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Closed loop woodstove powered water heating loop

Greetings from Alaska!

I am in the midst of building a 200sq ft cabin, mounted on a mobile truck chassis. 1.5 person fulltime occupancy. fully offgrid, solar/wind as primary electric power.

I purchased a new woodstove with a 1litre boiler factory installed.

I intend to use this boiler to heat my potable tap/shower water.

I hope to accomplish this by building a simple closed loop glycol system that uses the woodstove as the heat source. I want this primary heat circuit to be pump-less and rely wholly on thermo-syphoning for circulation.

The loop exists only to feed a Heat exchanger (likely sidearm) that will heat tap water.

The primary circuit total line length of 40', giving a capacity of less than 2 gals. I could also mount the exchanger directly above the stove, cutting the total pipe length to 14'.

i have many questions, not being familiar with hydronic systems.
(though I do have a 4 zone hydronic system installed in my home, with a navien boiler/waterheater).

First, I just received the specs on the woodstove boiler, and was suprised to see it expressly state "Solid fuel water heater must only be fitted to open vented hot water systems".

Why on earth would this be? the manuf. has very little info.

generally speaking, are there going to be issues arising from building such a small capacity primary circuit? (ie. overheating, evaporation).

My target input water pressure thru the secondary will be 1-5gpm, 60-20psi. (higher psi means lower gpm), and 35-60 deg water.


  • kcopp
    kcopp Member Posts: 4,386
    The reason they have it on an open loop set up is temperature and pressure.
    Solid fuel has no way to be regulated. Its not like electric, gas or oil. In those cases you turn off the fuel and no more heat goes into the system...cant do that w/ wood solid fuel.
    If you close up and pressurize the system in effect you have created a serious bomb.
    Water when heated and pressurized is a VERY powerful force.
    Just look up watts water heater explosion videos on YouTube.
    Please dont do this...

  • Big Ed_4
    Big Ed_4 Member Posts: 2,687
    Agree , You will learn how much or little to heat the stove to heat what as needed for efficiency over time. . Leaving it an open system is a must for safety when throwing in logs .

    Saved an old shovel a day from a basement for my dream cabin for hot water . It was thrown out durning my second marriage when I bought my cabin . I Just in stalled AC in the cabin last week with a low temp heat pump ... You get soft with age ...
    I have enough experience to know , that I dont know it all
  • joegood
    joegood Member Posts: 5
    edited April 2021
    perhaps my understanding of open vs closed is skewed. Could someone tell me how the system in this photo vents to allow it to be open?


    Is it using the storage tank as its overpressure vessal?
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,228
    Hi, @joegood , What you're talking about can sorta be done, but it's tricky and dangerous. Glycol may not be able to take the temperatures you get with a stove, so it could turn to sludge and fail to move heat, in addition to corroding through metal tubing pretty fast. That leads to overheating. You must have some sort of storage in the system or the likelihood of your being satisfied with the results are slim. It would need a relief valve set to just over working pressure and a temperature relief that could discharge a lot of water fast.

    As your truck will have this big, well regulated, fuel burning thing called an engine, why not scavenge heat off the exhaust, store it an a small but well insulated tank and get your hot water that way? Another approach may be to use a small diesel burner like they use in trucks for space heating. Also, they do make 12 volt electric elements, which could do this job pretty simply.

    I think the point others have made is that controlling solid fuel heating is really hard, if possible at all and controlling gaseous or liquid fuels is easier and safer. Electric is simpler yet. There's two cents! ;)

    Yours, Larry
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,228
    Hi, I just saw the diagram of an "open" system. It doesn't look that way to me. Open would have a vented pipe or tank at the top, so that pressure could never build up over the pressure created by the vertical column of water.

    Yours, Larry
  • joegood
    joegood Member Posts: 5

    Hi, I just saw the diagram of an "open" system. It doesn't look that way to me. Open would have a vented pipe or tank at the top, so that pressure could never build up over the pressure created by the vertical column of water.

    Yours, Larry

    My thoughts exactly. been trying to find a diagram of a typical open system, and that one threw me a bit.

    My backup option is actually to use a diesel fired coolant heater to heat the primary loop, similar to what you mentioned.

    On an open loop system, is there simply an atmospheric vent? would it always be releasing steam and heat? in a glycol system, I cant imagine how that would work.
  • Jamie Hall
    Jamie Hall Member Posts: 22,888
    That diagram is a catastrophe waiting to happen. Not only is it a closed system -- I can't even find a pressure/temperature relief valve on it -- but it mixes the radiant loop heating water with the domestic hot water. If it doesn't blow up the bacteria will make you very sick.

    Now. That is to say that you can't do what you want to do, but not exactly in the way you describe it.

    First place, what exactly is an open system? As the name implies, it is a system which is, in fact, open to the atmosphere -- the easiest way to do that, assuming there is a tank on the system somewhere (there will be -- see below!) to have a good big vent on that tank -- for a wood stove as the heat source, I'd be inclined to use 2 inch black iron pipe going straight up, then turning over at the top with the outlet -- unobstructed -- pointing down. The purpose of the vent is simple: when the wood stove gets carried away (which it will, trust me) and starts boiling the water, you don't want any pressure to build up at all, and the vent will allow the steam to escape without building any pressure.

    Now I mentioned a tank. Your primary loop water capacity is way too small. So, the next thing you need -- on the part of the system connected to the wood stove -- is a tank big enough to absorb all of the output of the wood stove for a reasonable time -- which I would say would be the maximum burn time of a fully loaded stove. There should be some water left over at the end. So... take the rated output of the stove (which you can likely find somewhere stated as BTUh) times the maximum burn time in hours and multiply -- that gives you the number of BTUs which you are trying to absorb. Divide that by how much heat you can get rid of in terms of steam, which is about 7,000 BTU per gallon of water. That gives you gallons. Double that, and that's the tank size you need. That tank should be set up so that it is above the wood stove (any water left below the wood stove won't be usable -- don't count it) and can be hooked up, as you suggested to thermosiphon.

    You can tie your sidearm domestic hot water heat exchanger into that thermosiphon loop very handily.

    Do not use any glycol in the system. As has been noted, the stuff cokes when it gets too hot -- and it will. Use only water. If you are concerned about freezing -- and living in Alaska, I can see you might be -- arrange your piping with a drain for the tank so you can dump it if you anticipate freezing for some reason.

    Someone mentioned using the heat rejected from the diesel engine, assuming you plan to use it from time to time. And why not? There is no good reason why you can't hook it's cooling system up to that nice big tank I mentioned above. You could have a conventional radiator hooked up in parallel, with the circuit to the tank. To prevent overcooling the diesel, though, both those circuits need to have the usual bypass and thermostat arrangement. That would take a little ingenuity, but would work; the only down side is that most automotive diesels these days are designed to run at around 195 Fahrenheit, so you would need to arrange your engine thermostats to maintain that. I can't say I'm much of a fan of the idea of a heat exchanger on the exhaust, though. First place, there really isn't that much heat there (yes it's nice and warm -- but there aren't that many BTUs available) and unless you bought a marine dry exhaust exhaust cooling heat exchanger -- which would be pricey -- you have a very real risk of creating an unwanted carbon monoxide source for your building.

    Keep asking questions!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • joegood
    joegood Member Posts: 5

    Wow Jamie, thanks for taking the time to type that response!

    Ive considered the tank option, But I just dont have the room for it. Hence using the heat exchanger as an "on demand" of sorts.

    Interesting that the glycol cakes up. Sounds like i am underestimating the operating temp of the liquid. My homes hydro system operates at 180deg, so I assumed the woodstove would probably come out about the same.

    The truck will rarely run, once every few months or so. I dont see using the exhaust as a viable heat source, much more BTUs are to be had in the cooling system-but I digress.

    Using a non glycol based medium is a little scary for me, as temps regularly drop below -25F in winter. Its mid april for petes sake, and it cold snapped to -25 yesterday.

    A concern for the open loop system is water conservation. I cant afford to watch my water supply evaporate into steam. Ive got 80-100 gals onboard in winter.

    Maybe I need to consider Xing the wood boiler altogether, and relying on the webasto coolant heater to power the heat exchanger.

    Is it realistic to expect good heat from a sidearm style exchanger in a on-demand scenario?
  • joegood
    joegood Member Posts: 5
    I think my main concern with adding an open loop/tank,more than fitting a tank somewhere, is the system will always need to have water in it to keep from damaging components. It will just sit there, boiling away precious water 24/7, just for the luxury of having it hot a few times a week. additionally, Id then be adding another pressure pump to power the hot side of the tap.

    no, I need to keep looking for an on demand solution.
  • GroundUp
    GroundUp Member Posts: 1,863
    With only 1 liter in the jacket, you'll cook the glycol the very first time you fire the stove without a pump for circulation and a load to pull the heat out, atmospheric or not. Enough thermosiphon to prevent this isn't really an option here unless you use piping 2"+ and even then, highly unlikely.

    The way this is typically accomplished with outdoor wood fired boiler systems (which are atmospheric, just an open vent at the top of the water jacket) is to spin a pump 24/7 while the firebox is hot, through a heat exchanger (sidearm would be a good application for you here) which then loads a DHW tank via thermosiphon when properly piped with no heat traps. This way, your glycol shouldn't ever get above 216 degrees being open to the atmosphere, and the DHW tank won't either as the output from the glycol is under that 216 figure and the domestic water is under pressure. It's still not a great idea, but is done all the time with larger water volumes and constant circulation. Long story short, in order for this to work you WILL need a pump moving fluid when the wood stove is burning and I would strongly suggest a buffer vessel of some sort to raise system capacity a bit, to avoid cooking the glycol
  • lkstdl
    lkstdl Member Posts: 41
    A simple 'on-demand system' for domestic hot water is just a pot sitting on top of the woodstove. Compact, no risk of explosion, easy to monitor, easy to clean. Plenty for doing the the dishes or making tea.

    For baths & showers, you could just trickle water through the boiler attachment and directly into the tub, with no extra storage tank. Just make sure there isn't a second valve (after the wood stove) that could allow pressure to build up.
    Luke Stodola
  • BTU1Vermont
    BTU1Vermont Member Posts: 0
    In our camp, we had an Ashley wood stove with a FirePlate internal heat exchanger hooked up to a raised 40 gallon water tank.
    Gravity feed from a spring up the hill, and thermosiphon between the stove and tank with a pressure relief valve up top. Had all the hot water we needed as long as the stove was running.No pumps or expansion tank.
  • hot_rod
    hot_rod Member Posts: 21,805
    I put a Cubic mini wood stove in my TC. It heats nicely with a couple of dielectric fans, and can also be used for cooking. Tried a few versions of a water jacket without much luck. Even if you get the pressures under control, you end up overheating the space before you have enough hot water to shower :)

    The RV industry has quite a few options for heat and or hot water generation with LP, diesel, or gasoline fired units.
    Here is a company that supplies some of the RV manufacturers. For the amount of DHW you need I would consider the truck fuel as your DHW heat source.

    Some of the former long hairs here may remember the gasoline powered heaters in VW microbuses :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 22,888
    Worse, @hot_rod -- I remember the infernal heater in my first bug. A '55 convertible, no less, with all of 36 horsepower. The heat was from the engine and controlled by a hand wheel with about 90 turns stop to stop...

    No gas gauge, either.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England