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:
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?