# Wood stove as hot tub heater?



## Gooserider (Dec 12, 2011)

I got the following as a PM a couple days ago...  I felt it was a question that belonged in the forums, so I've copied the original message and my response over to here...  If anyone else has advice for Frank S. I'm sure he'd welcome it.



> This is Frank S. I wanted to get yout opinion on an idea I have. I have a pleasant Hearth wood sotve that I would like to help heat the water in my hot tub. The tub itslef is a 350 gallon and costs a fortune to heat electrically. I am getting a wood stove for the sunroom next to the hot tub and would like to heat, or supplement the heating of the hot tub with the wood stove. I have about six foot of single wall 6 inch coming out of the wood stvoe to the ceiling and thought about using some of the exhaust gases in the flue to heat the water. I thought about using copper tubing 5/8 o.d. available at HOme depot. I am not sure wether to try the tubing inside or out of the piping. Will the water get too hot if I have the tubing inside the piping? THis is not a wood sotve that is super big (200 US stove) and I certianly would not be firing it all the time. Just to keep the sunroom warm.
> If you have time, give me some thoughts.
> Thanks
> Frank S.



This is the kind of question that is more appropriate for the forums, where you can get more than just one opinion...  I'm taking the liberty of copying this message to the Boiler Room area, as it seems more fitting there.

First off, I advise EXTREME caution, as a badly designed / operated water heating system can *explode *with devastating results!  At the very least, ensure that the system has a temperature / pressure relief valve on it, and that there is no way to shut the water flow off so that the stove is isolated in a closed section of piping.

I'm also not sure how good an idea this is, or how effective it would be.  Your stove is (hopefully) designed to radiate almost all the heat that it produces into the room, with just enough left to make the draft work so as to get proper combustion.  Putting a heat exchanger in or on the pipe will cool the smoke, possibly getting rid of your draft, and probably contributing to extra creosote buildup.  Putting the exchanger on the outside of the stove potentially could cause the firebox to cool enough that you lose the secondary combustion, again giving more creosote, and lower burning efficiency....  I don't know if you can pull enough heat out of the system to be worth the trouble and without causing negative side effects.

I would advise against putting anything inside the flue pipe, as that will increase the difficulty of cleaning the system.  There are also potential issues of acids in the exhaust attacking the heater materials. 

What I've seen that looked most effective for a stove pipe setup was a water jacket around one section of pipe - sort of like a double wall pipe with the water in the space between.  That way the smoke wasn't obstructed and you had maximal heat exchange surface.  Another approach I've seen is to make a serpentine pattern of copper tubing attached to the outside of the stove.  Make sure to have some way of regulating the flow through the exchanger so you can control how much heat you pull out.

Gooserider


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## Duetech (Dec 13, 2011)

Creosote will definitely be a problem in or out of the stove pipe but especially so inside of the stove pipe.  With the copper pipe in contact with the outside of the stove pipe it will generate cold spots on and inside of the stove pipe and it will cause creosote condensation. The known problem is of course potential plugged chimneys or chimney fires but your proposal would run amuck with creosote insulating your copper pipe and compromising its purpose. Safety issues that Gooserider has expressed are very real as a steam explosion can be just as forceful as gun powder and just as lethal only more so as gun powder gives a flash blast of heat and steam carries the heat medium to you and sticks. Evenso the Amish use a pipe in their wood stoves to heat water so what you propose can be done. Just do be careful.


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## ewdudley (Dec 13, 2011)

Here's an example of some purpose-built free-standing living-space boilers:

http://www.hydro-to-heat-convertor.com/models.html


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## willworkforwood (Dec 13, 2011)

Hey Goose, nice to see a post from you again in the boiler room  Not to hijack the thread, but how goes the battle for you?


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## Gooserider (Dec 14, 2011)

willworkforwood said:
			
		

> Hey Goose, nice to see a post from you again in the boiler room  Not to hijack the thread, but how goes the battle for you?



I really ought to post a long update in the main thread about my status...  In short, about as well as can be expected - no major change in medical situation, but am doing OK in other regards...

Gooserider


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## RobC (Dec 14, 2011)

Good to see your avitar again Goose...
There was another way that I saw that I liked to heat water. 
Take or make a pan that is about the size of the back of your stove. Have a way to attach pan to stove back. Get coiled copper pipe and lay as much as possible into pan. Once this is secure you were to fill the pan with plaster. The plaster was the only part I didn't like. It's being used as a heat sink but I was concerned that it might corrode the copper after time. Maybe, maybe not I never looked into it.
Other items would be, 1) have the water go from the outside of the pan to wood stove side and, 2) by all means have a pressure relief valve inline.
This set up was being used with a hot water tank installed overhead so it was being run by therm-o-siphon. My guess was that it never produced enough hot water in a 24hr cycle that 2 people wouldn't use up. 
Rob

( If that doesn't make sense I can try to draw a picture ? )


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## Gooserider (Dec 14, 2011)

It makes sense Rob, and is one way of doing what I suggested about putting a coil on the stove...  As long as the plaster stays dry, I don't see any major corrosion issues as a problem, and it would always be possible to use something else as a filler.

Long as you are careful not to pull to much heat out, it would probably work, but I don't know if it would supply enough heat to be worthwhile...

Gooserider


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