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My brother and I built a wood burning, convection based, thermal circulating hot-tub. (With and oxyacetylene torch, lots of 1.5" pipe, a brake drum, and a thirty year old jacuzzi). Our design is similar to that of what is manufactured and sold for $7000 by the name of a Dutchtub:

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Now As I'm sure you can figure out from the picture, The wood burns inside the coils, heating the water, cold water is sucked in from the bottom and hot water is pushed out the top. Our Version is using a fiberglass luvtub from the 80s or so, that holds approx. 500 gallons of water. After completion of construction and filling we decided to go ahead and test our creation. We knew it would be slow. (given not only that we were heating close to 1,900 litres by convection but that it also happened to only be a couple degrees above freezing. We Started the fire at 17:00 and kept it burning until after 21:00 in the first 20 minutes we already noticed a difference, after an hour or two, the top was steamy and the first six-eight inches or water was ready. however at 21:00 when it was make or break time, we had to call it day. Unfortunately the tub was only up the temperature about a foot down. My brother feels we need to install some sort of pump to increase circulation because he feels the tub temperature is reaching a plateau and our heat loss is too great. I think that install a pump defeats the whole purpose of the tub in the first place. Here's my question(s).

Would Stirring the tub help? Based on my understanding, this would slow the circulation as the difference in temperature would be lower. However the average temperature of the water would be consistent. Therefore the stove would not only not have to change the temperature of the incoming water as greatly, but the warmer water would not cool the stove as quickly as the frigid water sitting in the bottom waiting to be sucked in. There's also the joule effect to be considered which, would make an extremely minimal impact in this instance I would think. Nevertheless, would this, stirring, not make a huge difference in the overall heating of the tub?

A few things to also consider.

1. The Lid Our lid is made of 3/4 plywood that does not make a perfect cover. The wood is also perforated with 3/8" holes every square foot.

2. The Tub It's dug into the ground but only about half way. That is to say that only half the of the tub is subterranean with some dirt packed around the sides. I am under the understanding, correct me if I'm wrong, but packing dirt all around the edges of the tub would help retain heat. As the dirt will act as a heatsink at first but in the long-run it will help maintain the temperature of the tub. As opposed to bleeding that heat out the side of the tub, for the cold air to wisp away.

3. The fire We started with a pretty solid fire inside the stove, then we surrounded the coils with brick (with hole opens so the bottom of the fire/coalbed would still get oxygen and could still be billowed) There was also a chiminea top that we set on top of the coils/bricks, to hold more heat in. (At any given time there was close to a 8-18inch flame shooting out of the top) After some time we changed tactics. We pulled the top and set the bricks about 6 inches away from the coils and made a bigger fire. We knew we wouldn't be retaining as much heat, but there would be so much more produced and the coils would be heating from inside the stove and from out. *Our Stove is less than a yard from the tub but we installed a sheet of galvanized steel to act as a heat shield so the tub won't catch on fire.

In conclusion, how dramatic would our gains be if we, packed dirt all the way up to the lip of the tub, started with the style fire we ended with, and I think, most importantly, stirred the water in the tub?

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How does getting a pump defeat the purpose of the hot tub? Every (working) hot tub I ever saw and sold had one driving the water through the filter. –  Kyle Kanos Jan 31 at 2:18
    
Stirring should help, but perhaps not enough to notice :( –  Mark Rovetta Jan 31 at 2:37
    
@KyleKanos, the reason a pump would defeat the purpose is because the entire purpose of having a hot tub that functions off of thermal circulation is that you don't have a pump. So if you are going to add a pump you might as well just drag a generator out there and add an electrical heating element. And if you've never seen a working Hottub that doesn't have a pump you should try a quick google search of Dutch Hot tubs. –  DougtheCuriousPhysicist Jan 31 at 3:36
    
I am trying to think how a siphon might help, and where it could be located. Maybe this amazon.com/TERAPUMP-2EQK-Siphon-Pump/dp/B0007VYMUQ used upside down somehow, or an intermediate small tank that would heat up fast and siphon from it to the bath? –  anna v Jan 31 at 17:20
    
A siphon won't work as they rely on a difference in height between two bodies of fluid.In this case there is only one body of fluid. –  Fergus Jan 31 at 20:24

3 Answers 3

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As you're aware, thermal stratification is occurring. Once the upper surface of the tub is hot the driving force (temperature and therefore density difference) for the thermal siphoning effect is reduced. Mixing the tub will definitely help here.

All the tubs I've seen are very well insulated (they take days to cool down..) so burying it will not have a great reduction in heat loss. Your lid sounds like the weak link as far as reducing heat loss is concerned.

Any fouling inside the steam pipe can significantly reduce heat transfer to the water. In extreme cases by an order of magnitude.

To maintain the 'off the grid' aesthetic while still improving heat transfer a non-electric pump could be used. A water pump from a dishwasher or washing machine could be rigged up to be powered in another way. Some ideas: A wind up mechanism, pedal power, a wind mill (or a 'fire mill' powered from the fires exhaust flow!?), or any other harebrained scheme you can come up with, it won't take much to dramatically improve flow rate from the current thermal siphon rate. Motorcycle speedometer cables are a really flexible (pun intended) way to get rotational energy from one place to another.

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Referring to the engineering toolbox page on thermal conductivity, the thermal conductivity of dry earth is $1.5\ \frac{W}{mK}$ while that of air is $0.024\ \frac{W}{mK}$. Meaning that the thermal conductivity at the bottom of the tub is 60 times greater than at the top! This admittedly doesn't account for stirring of the air due to wind and convection next to the hot tub, but I don't think those factors will increase the thermal conductivity by more than an order of magnitude.

So, if you want the tub to work better you probably need to insulate it from all sides; particularly the part that is now underground.

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I might add that the "thermal circulating" part is generally referred to as a thermosiphon. The more flow, the better. You are working with gravity, and against friction and there is a balance. The longer the pipe coil , the more heat. But longer pipes have more internal friction. You want a relatively short fat coil. Notice how fat the Dutch Tub pipes are. Also you really want the tub above the fire. I can only presume your tub is below the fire considering is partially underground. And lastly water is about the best heat sink around. Allowing the heat to escape the system into the dirt will only work against you. Insulate that thing on all sides.

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protected by Qmechanic Mar 19 at 17:50

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