Thermosyphon radiant floor heating with wood stove?

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

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...

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.

Thank you, Bob! I certainly appreciate that advice and know-how. ☺️

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.

Thanks, Roy! Any way you could draw a quick diagram explaining how that works?

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.

Thanks so much!! So does that mean it works for the heat source to be on the floor? This diagram is like a cutaway?

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.

Okay, how do you wrap a pipe around a stove without touching it though?

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.

Gotcha! Thank you for the advice.

I have much more to say but before I elaborate I would like you to post a pic of your stove Moonstone.

Spitballing an idea - what if you put the tank on or along the side of the woodstove, and thermosyphoned the heated water to a 1/2 floor above?  (My house was built by wonderful hippies, who didn't think the back bedrooms required any warmth!)

Interesting idea!! Unfortunately, I had to move back to the city for health care though. I have some chronic issues that needed more attention than I’d been giving them. Will be going on my third surgery in February. Yeesh.

Before gas or electric water heaters were commonly available, wood stove manufacturers made accessories called "wetbacks" (or possibly other names), which were water tanks mounted on the back of the stove for a ready supply of hot water.

If you have a higher floor to heat, this would work fine and be very simple to do.

Oooooh!! That’s rad!!

We have wood stoves with wet backs here but then they've been certified with them. They usually feed hot water into a hot water cylinder.

The problem with them is that they steal heat from the combustion chamber which leads to less complete combustion, and thus increased pollution. So certified stoves are built larger to accommodate both functions of space and water heating.

Hi, I am planning on doing something similar to this.  Just thought id share with you.  So I have everything set up for my in slab heat. Tubing, manifold, circulator pump expansion tank, etc.. The only thing I am still working on is the heating source.  So what I plan to do is this. I have an indoor wood burning stove that I have installed a 1" Stainless steel hot water loop inside the fire box.  Beside my stove, I have a 55 gallon drum I plan to have sitting on some blocks to make it a little higher than the loop. From the bottom of the tank the "cold" line will go into the bottom of the loop, from there the top side of the loop will angle back over and come back into to tank at the top of the tank, but still remain under the water level.  This will hopefully heat up the entire tank once the stove has been running, via thermosiphon/natural convection.  Now, inside of the 55 gallon tank. I have 2 Radiators submerged that will work as a heat exchanger. Once the tank is hot enough, the pump will kick on and pump the closed loop fluid thru the two submerged radiators, pulling the heat from the "hot water storage tank" through my floor and returning back to the radiators.  I am confident this will work although I am sure I will have some tweaking to do once everything us up and running.  I welcome any comments or advice on this setup!  

I would like to try this! Could I build to heat a 12' x 12' piece of floor, which the woodstove will be standing on, using an outdoor (elevated) tank, thermosiphoning, and copper coils mounted (inside?) the woodstove?  Just beginning to explore - needing to make an off-grid forest cabin more liveable. All thoughts welcomed!

I saw this article on the topic and it addressed some of the issues raised. https://forum.heatinghelp.com/discussion/170960/homemade-wood-stove-hydronic-radiant-heat-setup