Entry #29 – Building a woodburning hot tub

A couple of years ago Nancy and I spent two weeks together at our family cabin.  After a day or two of hanging out we decided we needed a project that would be both fun and productive. We decided to construct a wood burning hot tub that could serve the whole family. Our thought was that if it worked it could provide a therapeutic bath after back country skiing or mountain biking in the winter and a warm swimming pool for our granddaughter and her friends in the summer. The project turned out to be a great success so I’d like to share it with you.

Our hot tub design was based on the thermal dynamic principle that heat rises.  This meant that the tub itself had to be located higher than its source of heat.  The first challenge was to pick a location that would accommodate both the design and the amount of space we would need. We chose a spot just outside the cabin next to a huge bolder.  The massive rock provided a perfect wind break as well as a beautiful backdrop and a bit of privacy.  Because we wanted to make a pool large enough for a group of people (up to six) we knew our source of heat would have to be significant so the next step was to construct an oven-like-furnace out of fire brick.  We just happened to have a pile of fire brick that I had scrounged a couple of years before.  We built a form out of plywood so that the oven could have the shape of a dome and we began mixing mortar and laying brick.  I put together two coils of black ¾ inch pipe using elbows and nipples.  These coils lined the inside of the oven walls with their ends sticking out of the top and bottom of the back brick wall.  As the fire in the oven heats the pipe coils the water inside them rises through the coils and exits out of the top of the oven into the top of the outside tub.  This warmer water forces the cooler water to sink to the bottom of the tub.  As the hot water enters the colder water sinks in the tub and exits through a bottom pipe and is transported again to the bottom of the oven coils.  This convection process keeps the water circulating over and over again through the oven coils and into the tub until the water starts to reach a desirable warm temperature.

The next challenge was the tub itself. I decided to purchase a six foot diameter galvanized stock tank (or horse trough.)  I plumbed an inlet and an outlet pipe fitting on the top and bottom wall of the steel tank. This enabled it to be connected to the ¾ inch oven coils.  We built an elevated platform to set it on so that the water level would be higher than the oven. (Remember the principle that heat rises.) I wrapped the galvanized tank with cedar to give it a more natural look and built a cedar lid to cover it when it was being heated and when it was not in use.

When it was all finished we filled the stock tank with cold water, stoked the oven with wood and started a roaring fire.  As the coils heated the water began to inject powerful spurts of hot boiling water into the tank. It came out with such force that I attached an elbow on the outflow fitting and aimed it down into the tank.  It was amazing.

It took awhile to heat a non-insolated body of water of such volume, but within a couple of hours the top twelve inches of surface water became almost too hot to touch.  We stirred the tank with a paddle until it was a consistent temperature from top to bottom and climbed into the steaming pool.  It was like having a hot springs in our own back yard.

A wood burning hot tub isn’t for those who desire to have things instantly, but it has been a great blessing to our family for several years now.  Preparing it for use takes foresight and planning so that it can reach its optimum temperature at a desired time.  When the kids go mountain biking or cross country skiing someone has to stay behind to keep the fire burning.  For us, spending part of the day heating up the tub is part of the fun.  It requires a kind of a ritual that causes us to really appreciate the end product of a hot luxurious soaking bath.