Wood fired tub conversion, it works!


Old 07-22-11, 12:29 PM
Thread Starter
Join Date: Oct 2008
Location: USA
Posts: 29
Wood fired tub conversion, it works!

A couple years ago I purchased an old 350-400 gallon Apollo spa that was in so-so shape for very cheap. It was in a wooden enclosure that was rotting, so I took the sledge to that and dropped it into my deck instead.
We have electric and wood heat in our home, so I decided that I would make the tub wood fired as well by building an outdoor fireplace/woodstove for it.
I did not want the stove IN the tub like some of the units you can buy, so I opted to build my own stove with an heat exchanger inside(I have a 250A mig welder and some basic fab equip at home). It sits about 6' away from the deck and is connected with foam wrapped hoses running under the river rock.

I ended up having the laser cutter cut and form the pieces of plate for the stove body and door, which I simply welded together to form a 30x20x18 firebox. I supplied a sheet of 3/16" steel plate, and they cut all the parts for $100...can't beat that.
The door uses a couple of strap hinges and has a BBQ style draft control with three 1-1/2 holes in it, and a round bar door handle using a large plate washer with a flat on it to "camlock" to the body.
The exchanger element I tig welded at a friends house using 2ea 1-1/2" Sch 40 stainless pipes, with 9ea 1/2" Sch 40 stainless pipes spanning between the two about 10" long each. Think of an H with 9 horizontal pipes...Drilling the 18ea 7/8" holes in the sides of the large SS pipes was the hardest part...go slow and use lots of oil. The unit has a plate flange that bolts into the stove from the inside so it can be removed and serviced if needed.
I also layed some lengths of scrap square tube on the floor, this allows air to get under the fire, and is also drawn to the rear of the stove thru the tubes and gets a nice swirl effect going which burns very clean. I asked my neighbors if the smell was bothering them and they didn't know I was burning it

Don't forget the firebricks. Mutual materials sold me enough fire bricks to line the bottom and sides for ~$45, and you can touch the sides without burning yourself when it's wide open.

I bought two 4' lengths of 8" Heatfab stainless chimney liner and a chimney cap kit and used that for my stack.

On my first test run, the pump circulated the water too fast for it to pick up any significant heat thru the unit. At the 3hr point with the fire roaring I had only brought the water to around 80*'s.
I ended up inserting a 1/2" fender washer into the hose where it connects to the exchanger unit, to restrict the flow. The pump "sucks" the water thru the unit so there is no risk of a hose blowout from the restriction.
This worked almost perfect. If I turn the pump off for more than 15 seconds the water starts to boil in the pipes, and it's a hot but safe temp at the outlets in the tub.
The 2nd test with the restriction went much better, up to ~100 deg in 2 hours, close draft halfway and it maintains itself very nicely.
My plan next time I drain the tub, is to install a 1-1/2" ball valve with a 1/2" hole drilled thru it in the closed position, so once I get the water to temp I can open the valve to let the jets have some more flow.

I have a new appreciation for the energy req'd to heat water. This stove burns hot and clean, and it takes an entire heaping wheelbarrow of firewood to heat all that water. Once heated, it only needs around 4 sticks of wood each hour, I'm using smaller seasoned alder about 4-5" in diameter.
I have a free source for wood so it sure beats the 5hr heat time and ~$20 cost incurred at our kw/h price each time heating the tub.
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Old 05-31-13, 10:36 AM
Join Date: May 2013
Location: USA
Posts: 2
Older Jacuzzi brand hot tub conversion

Hi, Carguy..your conversion sounds interesting. I have a similar older fiberglass tub left in my yard when we bought the house. Where does your heat exchanger connect to the tub?

I assume you removed all the 'guts' of the tub from the outside of it....so, its just the tub > heat x > foam wrapped hose > stove?? I do not have the tools to create my own stove but I can buy a ready made one. Can you walk me thru what I need to do to connect it?

Thanks!! sounds like you have a great setup.

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