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A wood fire to heat the hot tub.

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A wood fire to heat the hot tub.

Postby I Think » May 5th, 2015, 8:13 pm

Just finished, today, a wood fired water heater for the hot tub, been heating it with propane for the last couple of years at around $100'month. The water is now about 40 deg C and about ready for warming the buns.
Years ago heated it for a couple of seasons with used veggie oil in a babington burner I built, SWMBO thought it was too messy so hopefully this is the final reiteration. Off to soak.
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Re: a wood fire to heat the hot tub.

Postby Poindexter » May 7th, 2015, 2:18 pm

Sounds awesome. Now you just have to take it out on the lake.

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Re: A wood fire to heat the hot tub.

Postby I Think » May 7th, 2015, 7:40 pm

Yeah,saw that one while I was doing research for the one we built.
Gonna have to improve the efficiency, it is a firewood hog.
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Re: A wood fire to heat the hot tub.

Postby Bleach » May 7th, 2015, 9:34 pm

What kind of wood are you using? I use apple wood with pine for kindling, for my fire place. I might have to try heating my hot tub.
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Re: A wood fire to heat the hot tub.

Postby I Think » May 9th, 2015, 2:48 pm

Am burning softwood, fir pine hem etc.
Yesterday cut up some old scrap lumber, will try that tomorrow.
Applewood makes a lot of heat when it burns (so does all fruit wood), so if you have enough it is great.
By positioning the firebox below the hot tub, we do not have to pump the water, the heat of the fire circulates it.
Anyone that wants to come and look can PM me, but we live a little way from Kelowna, bout 2 hrs drive.
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Re: A wood fire to heat the hot tub.

Postby I Think » May 17th, 2015, 8:49 pm

So have an interesting phenomenon and wonder if any one has any ideas.
When we used to heat with propane, the water would cloud up and get yuckie in about 5 days. we use no chemicals. exceeeept.
When we filled the tub we used cold water and added 1/2 cup of 30% hydrogen peroxide. The water sometimes boils inside the firebox, as we heat it.
The water is clear after 7 days. Swmbo ( a retired microbiologist) says it is the boiling of the water that is keeping it clear, her feckless husband thinks it is the h2o2.
Could try leaving out the h2o2 next fill but really like the 7+ days of clear water.
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Re: A wood fire to heat the hot tub.

Postby Poindexter » May 18th, 2015, 6:43 am

Hydrogen peroxide converts to it's gaseous phase (boils) in contact with water, perhaps it's just floating to the surface and giving the impression it's boiling. If you're trying to avoid a chlorine or chemical smell while you're soaking just use a couple table spoons of chlorine shock treatment after you're finished and let it run for a while. Should kill whatever you brought in and help keep the water clean but dissipates so there should be little chlorine left by the time you're ready for another soak.
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Re: A wood fire to heat the hot tub.

Postby I Think » May 18th, 2015, 7:46 am

Poindexter, thanx for the reply.
The boiling is happening inside the firebox, am familiar with the outgassing of h2o2 . When the thermo syphon is working there is a steady flow of almost boiling water coming into the tub, when the fire is first started, and when the fire is allowed to cool, the thermo syphont is irregular and water in the firebox boils, the steam created pushes out a flow of hot water.
Am really happy not to use chlorine or bromine, and since our bodies create h2o2 feel this is much more natural.
Have never used chemicals before just chucked out the water when it got cloudy 4/5 days, but now after 7 days the water remains clear.
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Re: A wood fire to heat the hot tub.

Postby Boda » May 18th, 2015, 8:21 am

I Think wrote:Am burning softwood, fir pine hem etc.
Yesterday cut up some old scrap lumber, will try that tomorrow.
Applewood makes a lot of heat when it burns (so does all fruit wood), so if you have enough it is great.
By positioning the firebox below the hot tub, we do not have to pump the water, the heat of the fire circulates it.
Anyone that wants to come and look can PM me, but we live a little way from Kelowna, bout 2 hrs drive.




When I was a young lad my folks installed a 36' long X 4' deep above ground pool in the back yard , so quite a large volume of water.
After the second season my dad built a wood stoked fire box with coil's. He plumbed the water from the pool to the wood box and convection circulated the water. No pumps required.

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Re: A wood fire to heat the hot tub.

Postby I Think » May 18th, 2015, 6:55 pm

Boda, do you remember how much wood it ate, and how much diff it made to the water temp?
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Re: A wood fire to heat the hot tub.

Postby Boda » May 20th, 2015, 3:42 pm

I Think wrote:Boda, do you remember how much wood it ate, and how much diff it made to the water temp?



Other than I remember the water feeling significantly warmer when we dove in, I don't recall measuring the temperature differential.
The wood box itself was an airtight design so we could control the fire but if memory serves correctly it was used more as an open door fire pit family and friends would sit around roasting weenies or whatever.
Firewood conservation was not a target.
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Re: A wood fire to heat the hot tub.

Postby Jflem1983 » May 22nd, 2017, 10:18 am

Boda wrote:
I Think wrote:Am burning softwood, fir pine hem etc.
Yesterday cut up some old scrap lumber, will try that tomorrow.
Applewood makes a lot of heat when it burns (so does all fruit wood), so if you have enough it is great.
By positioning the firebox below the hot tub, we do not have to pump the water, the heat of the fire circulates it.
Anyone that wants to come and look can PM me, but we live a little way from Kelowna, bout 2 hrs drive.




When I was a young lad my folks installed a 36' long X 4' deep above ground pool in the back yard , so quite a large volume of water.
After the second season my dad built a wood stoked fire box with coil's. He plumbed the water from the pool to the wood box and convection circulated the water. No pumps required.


Have the remains of one in my yard . Pool is gone . Old stone fire box with pipes to nowhere is still here . Man they had it good back then .
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Re: A wood fire to heat the hot tub.

Postby TylerM4 » May 28th, 2017, 5:05 pm

Sounds like a nifty project!

I'd love to see some pictures as it's a project I've considered doing myself.

What type of design are you using? Coil of copper in a firebox?

My experience has been that H2O2 will evaporate/react away quickly. I'd bet that the microbiologist is right - high temp in the heating loop is killing the bugs.
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