# Building wood burning boiler for spa



## burton420 (Dec 20, 2009)

I am from BC, Canada, and just came in from a good soak in the hot tub. I have always wanted to build a boiler out of an old wood stove, so as the hot water surrounded me in my tub, I figured what better way to save money than to heat the tub with wood. I am a journeyman plumber, so the piping and the construction would not be a problem, but I am wondering if there is anyone out there that would like to give me some ideas of how to construct it.

I imagined surrounding the stove with copper coils, maybe old ones recycled from somewhere. (ideas?) Then encasing the whole thing in a steel box. Now i wasn't sure if i could get away with circulating the same water from the tub through the coils, or if i would have to use a heat exchanger. At some point i would have to adapt to plastic and then connect that to the circ. pump already in the tub. I also thought that the water would be quite hot coming from the boiler, so i would need some way of controlling the amount of heat going into the tub. That was when i thought of building a second smaller tub that would act as a storage or a place to dump the heat. It would also be an excellent soak. 

Anyway, I am getting a little ahead of myself. 

thanks for any replies.


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## nhmaster3015 (Aug 5, 2008)

Your biggest issue is too much heat. You don't want to make steam in the copper coils so you will probably need a dump zone of some sort.


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## rombo (Jul 17, 2008)

I have also thought of this many times. What I would do is drill trough the sides if the stove and run a single piece of 2" copper. You have a very high flow rate on a hot tub, so your heating element would not need much surface area.


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## ROCKSTARPLUMBER (Dec 14, 2008)

how bought a mixing valve to control that tempature?


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## Bill (Jun 17, 2008)

How about posting an intro first?


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## Ron (Jun 12, 2008)

burton420 said:


> I am from BC, Canada, I am a journeyman plumber,





Bill said:


> How about posting an intro first?


He did mention where he was from and that he is a journeyman plumber.

A better intro would be nice but not needed at this time.


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## Miguel (Aug 10, 2009)

Welcome to the Zone, Burton420! Intros outside of the intros forum does seem a bit odd but if the mods don't mindit then it's a perfect way to segue into an interesting topic for discussion. 

To avoid issues with pressure vessels inspectors and regs make your boiler atmospheric. ie: esentially a kettle over a fire. If you put a lid on your kettle to keep your HW from evaporating then make sure it has a sufficient vent.
If your fire is contained in an airtight woodstove then you can control the temp with an aquastat and a damper motor or solenoid connected to the air damper on the woodstove.

Once you have that worked out it should be no big deal to circulate hottub water thru a heat exchanger, homemade or otherwise, and use a mixing valve or other tempering device to keep the hottub temp nice and comfy.


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## ToUtahNow (Jul 19, 2008)

I have an old wood burning stove sitting around waiting to be restored. It has a tank on the side of it to heat water. I think the biggest problem in wood burning would be to generate enough heat as a primary heater.

Mark


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## burton420 (Dec 20, 2009)

WOW!! thanks for all of the great repies.

i was thinking about it last night some more and realized a heat exchanger is a must since this system is outside and if there isn't a fire going then the system could freeze. 

but before i go any further, i better appease the forum moderators with some details...well, like i said i am a journeyman, but only a first year. i like long walks along the beach... lol.
i live in kelowna, bc which is about a 4hr drive north east of vancouver. it's a smaller town of about 120 thousand. i have been lucky to be able to do most of my apprenticeship working new construction and commercial. right now i am working on the medical services building at the university which should have some interesting piping to do. other than that i am in my mid 30s with a wife and a kid due in may. 
so i am going to have to get this project done before then if i am ever to finish it.

so i will run a glycol system around the fire place and put the heat exchanger inside the skirt of the hot tub. (all of my plans are open to suggestions, thank you) the stove i have is good quality, and very simple. just a couple of air vents in the door. i wanted to connect a couple of old coils together from say a fan coil unit, but i don't know where i could look to get one. a scrap yard? or just phone around to mechanical companies that might have swapped an old one out? i would like to run a mixing valve as well to control the heat, but would that mean i would have to supply a domestic cold line from the house? i am a little concerned with the freezing issue.

thanks again for all of the replies. i am excited to get started.


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## TheMaster (Jun 12, 2009)

burton420 said:


> but before i go any further, i better appease the forum moderators with some details...well, like i said i am a journeyman, but only a first year. i like long walks along the beach... lol.


 That cracked me up:thumbup:


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## breid1903 (Feb 8, 2009)

google "wood heated hot tubs". there is an abundance of reading. i don't know how cold you get, with wood you will be firing 2 to 4 times a day if it get below freezing. it would appear to me that it would be easier to do this with conventional fuels. i looked you up on wunderground. your warmer than us, but your tub will still freeze if it is not heated or drained during the winter. i'm an old hippy. i looked at this awhile back. the savings from running the electric heater are negated by having more pumps. definitely doable. also you might google "wood fired water heaters". breid.................:rockon:


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## burton420 (Dec 20, 2009)

well the hot tub is already running off an electric heater. so i was thinking of building the system in such a way that when there wasn't any wood being burned the electric element would kick in. i would also like to build a second smaller tub (2 person seat) that would be much much hotter and act as a sort of 'dump' for excess heat. what i need is a schematic that describes all of the components needed for this and how they would all interact.


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## Redwood (Sep 8, 2008)

This project sounds familiar....








:laughing:


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## Airgap (Dec 18, 2008)

Redwood said:


> This project sounds familiar....
> 
> 
> 
> ...


 Dang Red, I was just getting ready to go look for this...


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## ironandfire (Oct 9, 2008)

Here's a little sumpin' sumpin.


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## Protech (Sep 22, 2008)

You will need to run a glycol system. You can use a wood fire as a primary heat source but the trade off is that you will need several radiators with fans to dump the heat when tub reaches 100-104F. You will need some kind of thermostat and a 3 way zone valve to divert the flow of heated glycol away from the tub and into the heat sinks. The tub it self can be used as a heat regulator to some extent if it is a spa. By turning on the jets you can dump some heat via the increased evaporative cooling. You will need a stainless steel (titanium would be even better but very pricey) shell and tube heat exchanger. You cannot let copper come into contact with the spa water as it is almost impossible to keep the PH under your thumb. Any time the ph crashes you are dissolving copper. Don’t forget your expansion tank and relief valve on the glycol loop. Circulation pump. Heat exchanger for the fire._


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## burton420 (Dec 20, 2009)

*Protec* that is some great info. Do you know how what i have to do to use the electric element currently heating the tub as a backup for when there isn't a fire going? i must need some kind of switch or relay to that is connected to a thermostat that will tell the electric heater to turn on. 
thank you for the great info, i am grateful.


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## gladerunner (Jan 24, 2009)

Your electric heat is already on it's own thermostat, just make your set point lower then your heat exchanger aquastat.
I don't think you can control it from the hot tubs controls.
PS. sounds like alot of work. I use my hot tub all the time year round, but i can't see me building a fire so that I can go in it in an hour or two.


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