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Thermosyphon radiant floor heating with wood stove?  RSS feed

 
D. Moonstone
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Location: Canada, Zone 3
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Is it possible? Would it work to put tubing in my floor and wrap it around my indoor wood stove's chimney, and have that heat my floors? I know domestic hot water can work with thermosyphon, so I thought I'd ask. I haven't built yet, but I'm planning to this summer and want to explore all options. Not a lot of money to spend on the project. Already might have a wood stove for free.

Building cheap & toxin-free
 
D. Moonstone
Posts: 34
Location: Canada, Zone 3
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I should also mention that the water table here is quite high (about three feet down), and the frost line is six to eight feet down. Just in case that makes a difference...
 
bob ellis
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Your stove is sitting on the floor you want to heat, yes? Seems to me that you'd only get good flow if the heat source is lower than the heatsink, the floor in this case. The DHW setups I've seen depend on the hot water rising out of the heater, cooling as it transfers its heat to the floor, and then sinking back to be reheated. It's a neat idea, though, but you might need a small pump, otherwise the hot water will rise to the top where the heat is, and just stay there. I'd be tempted to put a coil right inside the stove pipe, as well as around it, to get as much heat as possible out of it. You should include a temp/pressure relief valve somewhere at the hottest point for safety- If the water stops moving for any reason, it could quickly boil and burst a pipe.
 
D. Moonstone
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Location: Canada, Zone 3
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Thank you, Bob! I certainly appreciate that advice and know-how. ☺️
 
Roy Hinkley
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Location: S. Ontario Canada
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In this case I would thermosiphon the hot water to a tank at least 2 ft above the heat source. The tank would be open vented to atmosphere for safety.  Pump the hot water from the tank to your under floor pipes. If for any reason the pump fails you'll still be safely making hot water. The tank itself doesn't have to be insulated and will be a sizable bit of thermal mass in itself. Use a toilet float valve to control level.
You'll need large diameter lines for thermosiphon, 3/4" at least.
 
D. Moonstone
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Location: Canada, Zone 3
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Thanks, Roy! Any way you could draw a quick diagram explaining how that works?
 
Roy Hinkley
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Try this.
Hmm. After looking I think you could take the hot water off the tank a little higher up, like get the hottest water you can to the floor loop. Also, no joints inside the stovepipe, one bent piece of pipe, no kinks.



 
D. Moonstone
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Location: Canada, Zone 3
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Thanks so much!! So does that mean it works for the heat source to be on the floor? This diagram is like a cutaway?
 
Glenn Herbert
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It doesn't work by itself with the heat source higher than the floor you want to heat; that's why the diagram has a separate pumped loop for floor heating. As long as the pump works, this setup should work well enough. The caveat is that you are not going to get more than a small fraction of your stove's output into the water. Most of it will radiate off the stove's surface, and what goes up the chimney is needed to some extent to maintain the draft so you don't get smokeback, and keep the exhaust hot enough to not form creosote in the chimney. Personally, I don't think this kind of arrangement with a woodstove is likely to deliver enough heat to the floor to be worth the effort. If you do draw enough heat out of the exhaust, it will cool it enough that creosote is a serious danger.

If you want to get hot water from a woodstove, I would recommend wrapping collector tubing around the stove itself, not touching the stove surfaces. The larger the tubing, the safer it is likely to be, as larger volumes of water are harder to flash into steam and explode, and larger tubing lets steam bubbles escape to the open tank easier. You do want the collector tubing to have a constant upward slope so that heated water has a good path to rise to the tank.
 
D. Moonstone
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Location: Canada, Zone 3
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Okay, how do you wrap a pipe around a stove without touching it though?
 
Glenn Herbert
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Put metal spacers at the corners? My suggestion was generic, as it is difficult to give specific advice without seeing the actual installation. The gist of the matter is that you don't want to have 500 degree stove parts in contact with water piping, as it might cause local overheating and steam explosions. For some stoves (like soapstone covered ones) this might not even be an issue.
 
D. Moonstone
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Location: Canada, Zone 3
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Gotcha! Thank you for the advice.
 
Roy Hinkley
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I have much more to say but before I elaborate I would like you to post a pic of your stove Moonstone.
 
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