Water heater as boiler

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Theojkett

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Hey gang,
Okay I know going into this there are going to be a lot of "we've done this topic already" and naysayers. Humor me.

I have a small 624 off-grid cabin in upstate NY powered by solar, wind and generator (when I'm there). I currently use propane and it is expensive, I'm not there all the time and I need to keep the place above freezing (at least 50 degrees) when I'm not there. SO, I have this 20 gallon electric water heater that I'm not currently using. So I thought, let me get a recirc pump, some pex tubing, some baseboards, create a closed system. Knowing it is only a 20 gallon tank and 1400 watts max, if I set the temp down about 50% I should be good. I'm going to do a test run with it first of course.

My questions are:
1. Will I need to install an expansion tank?
2. Will I need to hook the system up to the water system with a backflow valve for at least filling the system.
3. Do I need to install a pressure relief valve and guage somewhere in the run?

I don't see any reason why this wouldn't work. Again, I'm not looking to keep the place at 72 degrees whether if I'm there or not. Just need to keep things from freezing in there.

I've piped the whole place with Pex now and it's wonderful and easy to work with.

Your insight, professionalism and good old fashion DIY'er know-how is greatly appreciated.

Theo
 

Dana

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Yes you would still need to install an expansion tank.

Unless you have an unusually large PV array AND batteries you won't be able to get much mid-winter heat out of the thing.



Even if you could get by with 1400W of heater and have enough battery & PV to back it up, is your heat load with a 50F interior design temp really that low? Like anything other heating system, you need to start with a heat load calculation before you can come up with reasonable prospects for heating solutions. Do the math.

1400W= 4778 BTU/hr, which at 624 square feet of space is a ratio of 7.7 BTU/hr per square foot. A very tight code min "real" house would have a heat load that size at a delta-T of maybe 35F (indoor temp - outdoor temp), and I suspect your cabin isn't as well insulated or as tight as that, with crummier windows to boot, and your design temp is probably lower than +15F (which would be 50F indoors, less 35F for the delta-T.) Smaller buildings have higher exterior surface to floor area ratios too, and heat loss is a function of the exterior wall/roof/window/door area.

Even if it was that low, and you had enough PV & battery to cover you during the darker/colder weeks, the cost of a 1500W worth of electric baseboard is lower than the costs of the pumps & valves & controls needed to do it with an electric hot water heater, AND the glycol that would be needed to protect it from being destroyed in a freeze-up AND it would actually meet some level of safety code.

With a 99% outside design temp, and better description of the cabin we can probably come up with a pretty good heat load ballpark. What what are your insulation levels & windows like? We would need total wall area (less doors & window areas) & insulation level, attic insulation level, window type & area, door type & area, etc.
 

hj

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Regardless of the size of the system, it still needs all the appurtenances of a large one, i.e., automatic water filler, expansion tank, air removal device, air vents, etc. 1400 watts is about the level of a good toaster so it may only be enough to keep up with the heat loss of the piping. Then, too, you need PEX with an oxygen barrier, NOT the PEX you used for you water piping, to connect everything together.
 

Brian1949

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Can you get oil? It would be expensive . but some boilers can operate off the grid with gravity circulation and only a t pile for a control circuit. a tank with a containment area is also expensive ...Those Adirondack nights are cold. 1500 watts are not enough... Easier to buy a small electric space heater and keep it close to your pipes ...pex will reduce the chance of pipes freezing but not the fixtures ... does the cabin have water pipes or any plumbing?
 

hj

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quote; .pex will reduce the chance of pipes freezing but not the fixtures ..

PEX does NOT "reduce the chance of the pipe freezing", but since the pipe, but not the fittings, can expand, it MAY reduce the chance of it breaking.
 
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