Build Your own Hot Tub

Build your own solar hot tub

By Robert C. Herman

Any homesteader knows that among the many rewards of a self-sufficent lifestyle are a sore back and aching muscles. Recently I realized what I needed to ease the aches and pains after a long day of chopping wood and moving soil: a hot tub.

Not one of those party-size, fancy marble pools with jets and bubbles and surround-sound stereo, but a comfortable place to soak away the knots and contemplate my place in the universe.

Since I haul water, generate my electricity, and basically live by my wits, the design criteria for my hot tub were: cheap to build, free to use, and frugal with water.

With about $100, some recycled hardware, and a bit of ingenuity, I built a thermosyphoning, solar heated tub that uses no electricity, no fuel, and less than 60 gallons of water, which is subsequently re-used in the garden.

The principles that make this system work are specific but simple. The skills and tools required to build the tub enclosure, and to plumb the system, are rudimentary. The satisfaction of soaking in my tub as the sun drops over the Rockies is priceless.

Your tub can be made of any suitable container that will hold water to cover your body. I chose a 100-gallon poly stock tank made by Rubbermaid Agricultural Products and available for about $70 where ranch supplies are sold. This tank is oblong, about 2 1/2 x 4 x 2 feet deep, which is large enough for one person or two very close friends. It is strong and durable, won't rust, and its rounded edges make it comfortable to sit in. Rubbermaid makes these tanks in other sizes. If you are extremely long-legged or plan to share the tub frequently, you might want the 150-gallon size. For my purposes, though, the additional expense and water requirements were not justifiable.

The thermosyphoning water heater is elegantly simple and effective. Basically, it works like this: the solar collector is filled with water and pointed at the sun. Sun rays penetrate the glass (or fiberglass) face of the panel, strike the heat-absorbing plate covering the water pipes, and transfer heat to the plate, the pipes and the water. Since hot water is less dense than cold water, the heated water rises to the top manifold, up into the "hot" pipe and to the tub. At the same time, cooler water from the bottom of the tub drains down via the "cold" pipe and into the collector's lower manifold to replace the hot water that is rising. As long as the water collector is being heated and the water at the bottom of the tub is cooler, hot water will circulate to the tub and cooler water will return to the collector. This system works well with no moving parts, provided that you take a few simple steps to help gravity do it's job (see the drawing).

Back in the mid '70s, after OPEC taught us the fragility of our dependance on foriegn oil, the federal government offered tax incentives to encourage the development and use of alternative energy technologies. As a result, thousands of solar heating systems were built and installed on houses across the country. Some of these systems worked better than others and when the tax credit program expired, the solar heating fad went the way of the leisure suit.

Depending on where you live, it is very likely that there are abondoned solar collectors nearby, patiently waiting to be rescued from the scrap heap. Ask around, or advertise in your local newspaper that you are looking for used solar collectors and associated hardware. Prices will be negotiable, but I would not expect to pay more than $20-40 for a good 4 x 10-foot collector (you only need one), and for that price would hope to get a truckload of pipe, valves and fittings thrown in. Some folks will even give away their collectors and all the associated plumbing and hardware, just to get it out of their barn or off the roof.

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If you really can't find a free or cheap used collector in your area, you can build your own. My own heating source is a 4 x 10-foot flat plate solar collector from the late 70s. It's what I had around, but a smaller collector would do the job, especially if you insulate the tub well.

One caveat: make sure the solar collector you rescue has not been damaged by water freezing inside its works. The flat plate collector is made up of a series of parallel, small diameter copper pipes with a larger diameter manifold at each end. If water is allowed to sit in these pipes at subfreezing temperatures, the pipes will burst. You can check for damage either by removing the glazing and visually inspecting the pipes, or but running water through the collector and watching for leaks.

Once you have collected the basic components, you need to site your tub and solar collector. As noted in the drawing, the bottom of the tank must be at least one foot, preferably two, higher than the top of the collector. A level spot at the top of a south facing slope is ideal; the tub sits on the level, with the collector tucked into the hillside below.

Alternatively, you can site the tub on a platform or deck, with the collector located below. Make sure, though, that the deck is strong enough to carry the weight of the full tub (including 500 pounds of water, plus your own wieght).

There are several considerations to address when siting your collector. Ideally, it should face south (within 15 degrees of due south) and have full exposure to the sun between 10am and 2pm. The collector can be oriented on its horizontal or vertical axis, and should be inclined at an angle of at least 15 degrees off horizontal (30 degrees is better; for year round use, the optimal angle of inclination should be equal to your local lattude plus 10 degrees). Finally, the collector should be tilted a few degrees so that the lower corner where the return (cold) pipe attaches is the lowest point in the system and the "hot" pipe comes out at the highest corner. This helps with the thermosyphoning and with draining the system down.

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