Anna; Are you putting the tub in a Big Box filled partway with sand? Then fix in the tubing for the Hot exhaust to travel through that heats up the sand. Then top off the box with more sand?

Then you cover with a clay layer to hold in more heat and make it easier to sit on.

I'm trying to picture this tub in my head.

http://tinygogo.blogspot.com/2009/11/doug-and-erins-wood-fired-hot-tub.html scroll down to "dutch tub" image. Looks simple and would avoid the hot seat problem. Dump coals when desired temp is reached.

PS. I love your site. It's like having zany neighbors that you can drive by several times a day and say to yourself."Look what they're up to now!!!"

Keep it up Tom

Personally I'd build a gazebo place it under. Then I'd work out how to close it in for winter use. Perhaps old style wooden storm windows for the winter, or just plastic as storm windows. You may find other designs at the permies.com wood stove forum.

I'd also locate it to fit in with the new grey water system.

I hope to build a donkey boiler at some point for outdoor wood fired hot water. Then one could fill the tub (appropriately insulated, so that it wouldn't drop temperature too fast) with the hot water; no burned bottom.

They are reasonably common among a certain segment in Southern Africa and I got to take photos and speak to the owner at the beautiful Mukambi game lodge in Zambia (http://www.mukambi.com/.) They have solar hot water heaters for each chalet, but in speaking with the owner (about my Oregon place), he recommended I look at their donkey boiler which is used out at the camp sites. I'm not finding them now, but I had found a few separate sites with instructions on how to modify a used propane water heater tank to build a wood fired water heater/donkey boiler. Mukambi folks told me it takes them about 20 minutes from firing (with a relatively small bundle of sticks) to a tank full of super hot water.

Here was one model that I had saved that didn't have the fire directly heating the water tank: http://milkwood.net/2009/07/12/the_rocket_powered_shower/. But the traditional donkey boiler model has the fire directly under the water heater, with the chimney going through the middle of the tank, to heat the entire water reservoir along the way. Then you could have a shower alongside AND the bath option - a full tank of wood fired hot water to do with as you wish.

http://kasinhunter.tripod.com/plainpics/FarmPics/outdoorTub.jpg

If I knew how to post a pix I'd have done that. As in this picture I think the tub should be set in to a full cob surround. It would insulate the tub and keep it hot longer.

As long as you only light the fire when the tub is filles with water, you cannot get a burn-through. Since the metal is in direct contact with water and both conduct heat very well, the temperature of the metal can never become much higher that that of the water. And the latter is limited by physics to the boiling point.

But I think a rocket stove powered hot water heater is a much better idea, for several reasons.

• A shower uses much less hot water (and energy) than a bath. A shower uses between 2 and 5 L/min. So for a rather decadent 15 min shower that would be between 30 and 75 L. A bathtub will hold anywhere from 140 to more than 300 L.
• If the water circulates through the heat exchanger the heat transfer is much more efficient, even without a pump (thermosiphon)
• You can heat the water in the boiler to a sufficient temperature to kill legionella pneumophila and other harmful bugs. A standard thermostatic mixing shower valve mixes hot and cold water to give you a nice temperature without burns.
• The insulated holding tank (which you probably can find on a scrapyard, maybe add some extra insulation) can keep the water hot for a long time.
• You can use the hot water for other things (like washing) as well.
• You can combine this system with a solar water heater for the summer, saving you some wood in the process.
• You could probably add the whole system as a bolt-on alcove to your trailer.

One thing I never understood about the rocket water heater Roland just mentioned is why they didn't run the exhaust through the center of (what would've been an otherwise) gas water heater. A gas water heater typically has an exhaust pipe through the center for the gas flames and exhaust to go. If you just placed the otherwise gas water heater on top of the secondary combustion chamber, you would no longer need the water jacket/heat exchanger and the design would become much simpler. I can understand, however, that placing a heavy water heater some distance above ground level may take a bit of work ...

Caveat: I am not a hot water expert, just asking simple questions.

Dan, the chimney up the middle of the water heater is the design of the wood fired water boiler I am familiar with called a donkey boiler in Southern Africa. I found a link with a limited discussion that indicates some experience that a slightly larger chimney is needed than found in a typical gas water heater (though I know I've seen photos and plans around the internet for a straight modification job.) http://www.pssurvival.com/PS/Hot_Water/How_To_Build_A_Safe_Effective_Wood-Fired_Hot_Water_Heater_2001.pdf.

But in Southern Africa they simply weld a 4-6 inch chimney pipe through a large steel drum (water reservoir) and set that on top of a cob insulated fire box and call it a day.

@Dan: The separate heat exchanger has a relatively low volume compared to its surface area. That heats the water in it up quickly. Since hot water want to rise, you get circulation (the thermosiphon effect). The higher the water flow, the better the heat exchange. And the water circulation can also help to increase the temperature difference between hot gasses and water, which increases the energy transfer.

A single pipe through such a big water heater is not that efficient. Steam locomotives used to have many "fire-tubes" in their boilers, see all the horizontal pipes running from the firebox on the left to the chimney on the right in the picture below. This design maximizes the (indirect) contact area between the hot gases and the water.

What a wonderful idea <3 Yes I think, that some firebricks assembled directly between the bathtub and the fire, could make a great difference! The water will stay warm for a while, too! OR: You could simply install a caldeweil woodstove beneath the bath tub - that would make everything very very easy!

Have you resolved you water heating issue? We made a very basic donkey boiler (no welding) and it works perfectly. If you are interested I can give you the details.