A Great Wood-Fired Earthen Hot Tub
By Kyle Chandler-Isacksen
We just completed an amazing hot tub project that is already drastically improving the quality of our lives. It’s a wonderful combination of fire, earth, water, and metal.
There are few pleasures in life, I think, as luxurious as bathing in large quantities of hot water. Like naps, hot water makes me a better person. I relax, I breathe, I smile, I think good thoughts when I am in a hot tub. Christopher Alexander in his masterpiece, “A Pattern Language” under the “Bathing Room” pattern says there is evidence that cultures with widespread use of hot water soaks are more peaceful. I am happy to take that as fact! I absolutely love to immerse myself completely in a tub that’s large enough for my 6’ frame to fit and to float blissfully while I blow bubbles or just enjoy the quiet of submersion. It’s a shame, but for years I have avoided baths because the standard crummy bathroom tubs are small, uncomfortable, and shallow and lead to bent knees and exposed skin that becomes chilled to annoyance. In short they are abominations! With so many decades of this as the norm is it any wonder that the USA is such a war-mongering state? Thankfully, there is hope…
In June I traveled to Taos, New Mexico, with two friends to learn more natural building with Carole Crews. Carole is an amazing builder and person and our week with her was fantastic. In addition to learning about Aliz (clay paints), finish plasters, adobe, and casein washes, we got to soak in her great hot tub. I had been very skeptical of wood-fired hot tubs previously because I assumed they took a lot of wood to get up to heat and were generally burdensome in use. Was I ever wrong!
Carole’s tub was built by Dafyd Rawlings and other friends a couple years back and is both very efficient and easy to use (the first natural building workshop I ever took at Cob Betty’s in Ojai, CA was led by Dafyd. Both he and his wife, Yolanda, are wonderful people and great builders). The first time we fired it up we didn’t listen too well to Carole and used four logs instead of three. We wound up with water too hot to enjoy that night but covered it with two yoga mats and it was a delightful 100 degrees the next morning thanks to all the cob serving as thermal mass! After two more soaks that week I was a convert and determined to build our own once I returned to Reno.
Once home I went looking for used stock tanks. I searched and posted on Craigslist and spread word to friends but had no luck. So, last month I splurged and bought a new 6-foot x 2-foot x 2-foot stock tank from one of our local feed stores for $170. Buying new things, especially a luxury item, is a bit unusual for us as we pride ourselves on simplicity, the use of salvaged materials, wise water use, frugality, and an all-around low carbon footprint. So, here’s how I justified it: the used water goes to our plants, we normally take one to two-gallon showers with the use of a watering can with water heated over our multipurpose mini-masonry heater (or cold water in the summer), baths are restorative (as mentioned above), our family of four fits in all at once, this should last a long time, and I am hooking up a solar assist thermosiphon system to limit the wood-burning further.
I placed the stock tank atop old bricks stacked three high on a concrete slab in our backyard that is bordered by our cob/cardboard cabin, our “Bed Shed” cabin, and our greenhouse to further create a great outdoor space. We have grapes growing up an arbor and some measure of privacy given by the surrounding cabins. I also built a privacy wall with old pallet wood and an old window (see pic) and put it on castors so we have a wall we can move into place when the tub is in use and we’re frolicking about naked in our backyard.
The tank was level on the bricks because the slab was level. Each end of the tank was open – a tunnel was created with the bricks on each side. I then slathered a wet cob mix over the exterior of the bricks to cover the gaps between the bricks. If I did this again, I would slather the same cob on the interior as well to protect the bricks more and further fill in any gaps. Next, I gathered some large rocks and urbanite and placed them around the tank. This served as a foundation for the cob and raised it off the slab and away from rainwater – like a stemwall. From there I started piling on cob. I filled gaps between the big rocks and layered the cob up the tank with plenty of small rocks placed in the mix to take up space (my kids collected most of these). I went about half way up and took a break until the next day to let the cob set and harden a bit. Cob will “splooge” or sag if too much is piled too high at one time.
The next day I continued by rewetting yesterday’s cob to ensure better adherence with the new stuff and went just about all the way to the rim of the tank. The cob starts at probably six or eight inches wide at the bottom and tapers to maybe three inches wide at the top. The cob I used was old stuff salvaged from an old wall and reconstituted with water and then mixed with a hoe to make it usable again. Talk about sustainable building materials! I love this stuff. I knew I wanted some artistic flair so made the flames and wave reliefs at this stage. I added a lot more chopped straw to this sculptural mix of cob so there’d be less cracking and more stability – more straw allows the cob to extend out further, it acts like so many little bridges. As I built, I smoothed the cob and then, at the end, made some cross-hatched scratches because I only wanted to apply one layer of plaster and I wanted it to adhere well.
I also left an opening in the front for the future fire starting area and attached a stove pipe elbow at the back end using some cob, or course, for the exit.
Once the cob dried (a few days), I mixed a batch of lime plaster (three parts sand to one part soaked lime) and put that over the cob (always rewet surfaces with a spray bottle or sponge for adherence). Lime is water resistant so perfect in this hot tub application. On top of the lime plaster I then painted a lime wash with some yellow iron oxide pigment (concrete/stucco coloring available at hardware stores). This gave the tub a light yellow color all around which is nice – much warmer than the cold white lime color. As soon as I get some other pigment I will make the flames darker orange and paint the waves blue or green. It looks great already and I think it will be dy-no-mite with the two more vibrant colors.
Note the space I left around the tank outlet. I wanted to be sure I had easy access to the outlet hole and the 8” pipe I secured into it with a “Y” valve at the end.
The tub drains well with a hose I have attached to the y-valve but leaves an inch or so of water below the outlet hole. For the remainder I place the end of a thin plastic hose in the bottom and syphon the rest to our grapes. That still leaves a bit of water which mostly evaporates in a couple days. Before I refill it I sponge out the leftovers and give the bottom and the walls a once over.
The Solar Assist
I have not hooked this up to the tub yet but have a couple years of experience with a simple thermosiphon solar hot water setup and am confident it will work well. How much it will heat the water I am unsure but it will definitely give it a boost. To use the solar heater I will fill the tub and then open the valve on the “Y” to the bottom/intake of the solar water heater. Once the sun heats up the panel it will pull water into its thin tubes and start the cycle. The outlet is on the top at the other end and the heated water will be pushed into the tub. Hot water high, cold water low and the cycle continues, almost magically, heating water from the sun for hours and hours. This water gets steaming hot!
Our First Uses
The first night we fired up the tub I filled it with about 9 inches of water. We started the fire and added wood until it was about 120 degrees Fahrenheit. This took a little over an hour. My sons used a compost thermometer to keep track of the rising temperature. Then, we added a lot more cold water from the hose until the temperature was down to about 101 degrees – not too hot for our young sons. This left it at about 18 inches high – a little lower than I had wanted but still great for the first use. All four of us climbed in and had a good soak while a cool light rain fell for an hour or more. Lovely!
The next time I started with more water in the tank (maybe 12-14 inches high) and got it again to about 120 degrees. We topped it off and were left with a hotter temperature of about 103 and very high. It’s a learning curve without much chance of going really wrong and by soak number 3 we had it pretty much figured out. For me, getting to know the idiosyncrasies of a new creation is part of the fun and leads to deeper connection and appreciation.
The lime plaster is holding up great even with spillage and the rain we had.
We look forward to a this winter of soaks and years to come of more peaceful living.
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