# Wood fired hot tub



## awoodman (Oct 4, 2009)

I got a 4 person hot tub free off graigslist this summer.

In the process of building another small wood burner (out of a compressor tank
this time.) The first experiment was 1/2'' ridged copper soldered into a back
and forth grid laid flat ontop the fire and circulated with the pump on low
speed. It worked but took about 4hrs to bring the water up 30 deg. plus uses a
lot of elec.

I want to build the next one with the tank in the upright position and operate
totaly with the thermosyphon system. What would be the best (cheapest) copper
tubing configeration to use? *And would a larger ID tubing size increase the flow
rate?*
What about a small stainless steel tank (old fire extinguisher canister)
centered inside the fire box with an infeed and outfeed lines welded to it?

Has anyone tried something simmilar?


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## raven (Oct 4, 2009)

I'm no engineer and can only guess what your doing. but this may be food for thought. i have an instant water heater built by Rudd in 1906.
they used a vertical coil of 1 inch copper with the gas burner centered at the bottom of the coil. i would think a large diameter coil on the vertical rise would beat the small horizontal any day. lol but hey like i said im not a engineer.there coil is about 16 inches in diameter and  24 to 26 inches high.
 i have not looked at it in some time so im guessing on the width and height of the coils. anyway it may be something to think about.


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## CarbonNeutral (Oct 4, 2009)

You need to find out what temperature the water is entering the coils at, vs what it's leaving at (just use your hand, carefully, if you have no other way). 

Very hot in, still hot-ish out = you're not dumping enough heat out  - you need smaller tubes (smaller diameter = more surface area per volume = better heat flow), and/or more length - think of a car radiator (in fact, could you use a car radiator, or some of those baseboard heating fins?)

Very hot in, much cooler out = you are successfully dumping the heat, so you need to find a way to get more heat from the fire to the water, so flow rate improvements would work, assuming that can work with the heat exchanger over the fire

Medium hot in, whatever out - you're not getting the heat out of the fire -again thinner tubes, more length....

And, at the end of the day, it takes a massive amount of energy to heat water, so your limit may be the BTUs you can extract from your size of fire....


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## woodgeek (Oct 5, 2009)

you could spoof what these guys do.... http://www.dutchtub.com/


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## begreen (Oct 5, 2009)

Or these local folks: http://www.snorkel.com/


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## awoodman (Oct 5, 2009)

I have checked out these and the coils on the dutch tub seem to be 1 1/4'' to 1 1'2 ''.





I have also checked out the Chofu heater and it has a water jacket surrounding the fire box. The specs are............

Features
• High grade stainless steel stove body
• 1/8-inch steel stove front
• Heavy cast iron door and grates
• Hot Tub Connection kit (see photo)
• Generous size firebox, 18"L x 14"W x 10"H
• Vented smoke-outlet for secondary combustion.
• 3/4-inch drain for freeze protection.
• An ash drawer for easy removal of ashes.
• A long handled ash rake

Specifications
• Dimensions: 16" wide x 23" long x 18" high
• Weight: 59-lbs.
• Stove body: Grade 316 stainless steel, inner wall 20-gauge, outer wall 22-gauge
• Stove Front: 1/8-inch steel
• Firebox door and grates: cast iron
• Firebox dimensions: 18"L x 14"W x 10"H
• Heat exchange surface area: 9 sq. ft.
• Smoke outlet: 4 5/8-inch (reduced to 4-inch)
• Circulating pipes: 1 3/4-inch O.D.
• Drain: 3/4-inch I.D.


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## CowboyAndy (Oct 5, 2009)

just do this:


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## awoodman (Oct 5, 2009)

This is my tub I got free.............


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## SteveKG (Oct 5, 2009)

I don't have any experience with the Chofu, but I've researched them and from that they do look great...depends on whether you want to spend the money. I believe they are in the neighborhood of $650, might be more. 

A friend lives near Santa Fe and his neighbors have a Snorkel. I don't know the volume of the tub or, really, any other technical figures, but the neighbors told me it takes 4+ hour burn to get the water hot. In their case, they work from home and it was no big deal to let it burn all afternoon in prep' for an evening soak. I didn't see a cover for the tub, and if they don't have one, maybe it would heat up faster with one. By the way, later and back at my friend's house, he said he'd shared the tub with them once and his opinion was to get the water really warm took 5-6 hours. [He does like it hot, though.]

But they all take some time to heat up.


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## colebrookman (Oct 5, 2009)

awoodman said:
			
		

> This is my tub I got free.............


How cold does it get in KC?  Will you have to empty the tub every time in winter?  We checked on them but would waste a lot of water at 32 temps.  Be safe.
Ed


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## awoodman (Oct 5, 2009)

It has been getting in the 40's at nite latley. But the winters can be bitterly cold. 
I found these pics of a stove made from a propane tank, real nice........
I want to make something like this only closer to the ground. 
Just trying to figure some sort of heat exchanger to build.


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## SteveKG (Oct 5, 2009)

Few months back, looking to replace our wood-fired water heater, I ran across a series of how-to clips on YouTube for making a water heater out of propane tanks, the small ones. The guy gets real detailed. Pretty darned slick. Wood-fired. If I recall, there are seven clips to watch. His stove could easily be adapted to heat a tub of water. It is possible I came across the reference on this site, but in any case you can easily find the videos. The stoves are made from scrap plus an old gas bottle. He removes the valve and fills them with water first to be darned sure there's no gas left, as they require cutting and welding on the bottle.


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## Stevebass4 (Oct 5, 2009)

here 

http://www.instructables.com/id/How-to-build-a-wood-fired-hot-tub/


and here 

http://www.youtube.com/watch?v=zhD-7pw2Pv0


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## begreen (Oct 6, 2009)

awoodman said:
			
		

> It has been getting in the 40's at nite latley. But the winters can be bitterly cold.
> I found these pics of a stove made from a propane tank, real nice........
> I want to make something like this only closer to the ground.
> Just trying to figure some sort of heat exchanger to build.



Looks like an ancestor of Bender on Futurama. LOL


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## rowerwet (Oct 6, 2009)

on the instructables.com site there were posts about using an old deep freeze as the tub, it is an insulated box and is water proof, also you close the lid to keep in the heat. from the chofu site "the average heat output of the Chofu CHS is 32,000 BTU’s. A 200-gallon tub can be heated at approx. 20ºF per hour." that will take some patience. 
You still run into the same problem of how long it take to heat the water unless you have a wood boiler already, you need something that can convert a lot of wood into a lot of BTUs. the other issue with using thermosyphoning to heat the water is that you get stratification, (something the guys in the boiler room will go on about for days)  you will need to stir the water to mix the hot and cold water or it will be hot on top and cold on the bottom until it is almost all hot.


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## Frostbit (Oct 6, 2009)

BeGreen said:
			
		

> Or these local folks: http://www.snorkel.com/



My neighbor at camp has a wood hot tub with a Snorkel (above) stove. He built the hot tub himself, and purchased the Snorkel brand stove and installed it. The stove is built and welded of aluminum, and its a direct submersion stove. I have soaked in it many times. Know this:

It uses a horrendous amount of wood. The heat it makes is directly exchanged to the water. I surmise a TON of it goes up the stack. The stove works well, but its design is more like a blast furnace. A damper in the stack helps, I guess. On a given day you would like to soak, its filled with 35 degree well water. To bring it up to 100 degrees, it takes AT LEAST 8 hours, tending and filling the stove often....big emphasis on "often". Have a lot of wood handy, its going to need it. So, planning a soak means starting the fire early in the day. My friend has a 120v. sump pump in his to help "stir" the water and allow some circulation, but its a slow process. If you have a lot of wood, lots of time to tend, you can enjoy a nice soak. It works. Just be prepared to work, too.


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## awoodman (Jun 29, 2010)

Well this year I utilized the wood fired canner I have had for a few years. The tub heats up by thermosyphone, and works great.

What the heck can't upload photos off photobucket???

I also added an attachment to get double duty (multitask) the fire to smoke meat while the water is heating...

Finally got a decent cover for it off craigs-list and had to build a frame around the tube to accommodate it from free wood off craigs-list.....


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## northwinds (Jun 29, 2010)

Very nice.  Where are the party pics?


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