We built a homemade wood-burning hot water heater to use in an outdoor spa. It was fun to make and even more fun to use. We got to soak in a hot-tub by the side of the river. It was our first try though, and not very efficient. Here is a short guide to how we made it, what we did right, and what we would change on the next hot spring machine that we make.
To begin with, we got 50-feet of 1/2-inch copper tubing and coiled it around a frame. We used bailing wire to tie the pipes in place. Initially we had it on legs above the fire, but later removed the legs and lay it just on top of the flames. This worked because we had a portable fireplace. Were you to do it just over a regular campfire (which would allow you to use bigger fires) you will want to have to legs to raise it away from the wood a little bit.
We covered the copper tubing with a lid to deflect the flames towards the pipe, and prevent all the heat from blowing away in the wind. This made a big difference but was only partially effective, as many of the flames just went out the sides. A lid with a hole in it to act as a chimney, or perhaps an upside-down funnel, will be a feature of our next hot-water heater. I will also try to have centimeter wide spaces between the copper tubing next time, to allow flames to slip between them.
Because our system left little room for air to get in, we attached a small air pump to a metal pipe and blew air into the base of the fire. This worked, but I think that a regular campfire is a better system than the portable fire pit. On a larger, more open fire, it wouldn't be necessary.
We used a large tarp to set the pool. We filled the pool with hoses from the river with a fast pump, and also by siphoning from upriver to conserve power. We then used a used a slower 14-watt bilge pump to circulate water from the pool to the heater and back. With a slow pump the water temperature rose about 15C per pass. We insulated the circulation hoses by covering them with dirt.
Initially we were very ambitious and used a 3000+ liter swimming pool to hold 10 people at a time. However, our system was only able to heat that pool up to around 35 degrees. Despite the water coming out very hot from the heater, there was too much heat loss from such a large pool to keep the pool hot, so we decided to tone it down a little. The next day we made the pool smaller and let out more than 500 liters of water. With a smaller 2500 liter tub for six people, the system was able to heat the water to at least 40 degrees for comfortable soaking.
Post-project thoughts. Even with our crude, first round design would be more than enough for a 2-person hot-tub. For large several-thousand-liter pools though, heat loss quickly becomes a factor. Instead of making a significantly larger fire, it may be better to just find a way to insulate the pool. A larger fire without a larger pump would also heat the incoming water too much. Gaining more than 15-20C on a pass is dangerous. To reduce the temperature of the incoming water, but still raise a very large pool to 40C+, I will need a faster pump, or good insulation.
I hope you enjoyed this hot-spring-anywhere project. Feel free to ask me any questions and please share your ideas, thoughts, and your own designs in the comments.