January 5, 2006 A project to add to your “not to do” list Ethan @ 2:56 pm I like building stuff. I spend much of my life doing “work” that’s decidedly cerebral - writing articles, papers, blog posts, computer code or emails; talking with folks face to face or over the phone. When I get the chance to take some time off, I like to build things, preferably out of wood, preferably things that have a practical purpose. My favorite person to build with is my friend Nate. Like me, he spends too much time staring at computer screens. (In his case, he’s trying to make computers do things they’ve never done before, whereas most of my computer wrangling is, at root, interpersonal…) Like me, he’s good with his hands and likes making stuff that works. Like me, he’s a little crazy. To accompany our cosy, warm ger, Nate thought we needed an outdoor hot tub. Specifically, Nate was interested in building a hot tub that could be constructed almost anywhere, with minimal materials investment, so that a hot tub could be part of any rural outdoor gathering in the future. It’s a worthy goal, and both easier and harder than it looks. (To be very clear - I had basically no design input on this project. My role was to prevent Nate from burning down my house or woodshop, and to document his success and failure. I claim no credit or blame for the solution he pursued, and I strongly encourage you NOT to try repeating our experiment without some major modifications, for reasons that will become quite clear.) The first step in building a hot tub is building a vessel that can hold a few thousand liters of water. Nate did this by using a length of stone retaining wall in my back yard, and building three other walls from plywood. The sheets of plywood are held in place with cheap and massive stuff - in our case, bales of hay and stacked cordwood, which is what we had handy. Used tires would also be great, as would auto parts. The box was lined with a large (4m x 4m) green tarp (for those of you not familiar with tarp, the building material of choice here in New England, green tarp is the “intermediate” grade of tarp between the cheap blue tarp and the luxurious and strong silver tarp.) The tarp held water surprisingly well, but not well enough, so Nate relined the tarp with 4mil vapor barrier. A 6m x 7m sheet of plastic vapor barrier cost $20, the only money we actually spent on the project. Filling the enclosure with water, we now had a very large cold tub - the water comes from my well, and comes out of the ground at about 5C, a temperature not very conducive to relaxation. The goal was to heat the water in the tub with firewood, hoping to bring the temperature close to a comfortable 40C, perfect for a relaxing soak in the snow. Nate commandeered an old ash can, attached two thick blocks of 4×4 to the bottom, borrowed about 70kg of weight plates from my gym and used them to sink the can to the bottom of the tub. Without wood in the “stove”, it had neutral buoyancy. A few kilos of cordwood and it sunk securely to the bottom. A little lighter fluid later, and we had a (dysfunctional, wholly inadequate) pool heater. Starting a large wood fire in an ash can and leaving it unattended in a tub of water seems like a dumb thing to do, on initial consideration. On the other hand, if the pool walls gave way or the can tipped, the fire would likely be doused by the 2000 liters of water in the pool or by the several cm of wet snow on the ground. At the very least, I figured the house - 20m away - was probably safe. It also seems like a poor idea to climb into a pool of water that’s also otherwise occupied by a hot metal barrel filled with flames. While that’s true, it’s not true for the reasons you think it is. If you brush a limb against the can under water, it doesn’t feel warm at all - it’s transferring heat to the water very quickly, so the metal isn’t warm enough to burn you, unless you jam your hand against it and hold it there, preventing the metal from transferring heat to the water. (The can above the water line is another matter entirely. You don’t want to touch that.) No, the reasons it’s dumb to get into a pool of water heated by an open woodfire is smoke. Wood fires release a lot of wood smoke, and that smoke gravitates directly to your eyes as you’re attempting to relax in the water, making said relaxation harder than you’d think. Future designs of a stove will require us to add a chimney that pulls the smoke away, and captures more of the heat to warm the water, instead of warming the air above the fire. Ah yes, the heat. After four hours of a roaring wood fire, the temperature in our tub had warmed up to a balmy 11C. This isn’t quite the failure that it sounds like - after all, the water began at 5C, the air temperature was -5C and it was snowing. Somewhere in the heating process, Nate raided my shop for excess extruded foam insulation, hoping to hold more of the heat in the water. It didn’t work, but the pink insulation adds some nice color to the photos, I think. (That’s Nate pictured above, enjoying the balmy water. Yes, photos of me in the tub exist. No, I’m not posting them. Consider yourself lucky.) A future outdoor hot tub would have some major design changes. It would be much, much smaller, giving us less water to heat. It would have a thorough insulation layer on top. (A thick layer of ping-pong balls would be especially fun, as you could get into the tub, displacing the balls, and keep it insulated even as you soaked in it.) It would have a radically redesigned stove, likely an ash can with two chimneys, one for air intake, another for smoke output. And it would probably be built some warm summer evening, not during a freaking snow storm. As should be clear from this post, we’re idiots.