Build Your own Hot Tub

Build your own solar hot tub

By Robert C. Herman


Secure the collector in place by attaching it to posts or rods driven into the ground. The exact method will depend on your circumstances, but need not be fancy. Just make sure the collector is well supported and stable.

Once you have selected your tub site, level the area and set your tub in place. It is a good idea to raise it off the ground in order to reduce heat loss and moisture problems. I used a hardwood pallet about a foot longer and wider than the tub, and covered it with 3/4-inch plywood. This insulated the tub from the ground and provided a base for framing the enclosure.

Plumbing the tub is relatively simple. I used 3/4-inch copper pipe because I already had it around. If I had to but new pipe, I might have chosen CPVC (PVC won't take the heat) for reasons of economy. In choosing your pipe, remember that smaller diameter pipe is more restrictive and thus will reduce the performance of your thermosyphon system. I would not use pipe smaller than 3/4 inch diameter.

The stock tank I used already had a fitting near the bottom with a 1 1/2-inch drain plug in it. I simply removed the plug, replaced it with a 1-1/2 inch to 3/4-inch male adaptor (MIP) and I was ready to attach pipe.

For the "hot" (inlet) pipe, I had to cut a hole in the wall of the tank. I located the inlet pipe at a height equal to 2/3 of the minimum water depth of the tub when filled. The inlet must be located low enough to be submerged when the tub is filled, or the thermosyphon will not work. A 1 1/4 to 3/4-inch bushing, silicone caulked and secured on the inside of the tub with a 1 1/4 inch flare nut, formed the hot side inlet.

Actual routing of the pipes connecting the tub with the collector will be specific to your installation. A few general guidelines apply, though:

  • Be careful to avoid any high spots in the pipes where air pockets can become trapped.
  • Where the "hot" pipe comes out of the collector, route the pipe vertically, then nearly horizontally to the tub, rather than creating a long, steeply diagonal rise to the tub.
  • Install a gate or ball valve on both the "hot" and "cold" pipes to control the flow of water.
  • Install a safety (pressure relief) valve in the "hot" pipe to avoid dangerous pressure buildup.
  • Install a drain valve at the low point in the system.

Keep pipe runs as short as possible. Try to minimize 90-degree turns and other restrictions, and install threaded unions in both pipes near the tub so that the system can be easily assembled and taken apart.

Befor building your hot tub enclosure, test the integrity of your plumbing. Fill the tub and check for leaking joints and fittings. Any leaks a the the tub will be easier to correct before it's boxed in; leaking pipes must be fixed before they are insulated. Once you are satisfied that your plumbing is leakproof, you're ready to close everything up and put the tub into use.

Because the tub was intended as a stand alone stock tank, it needs no structural support, other than a firm, level base. All you really need is some insulation around the sides and a well insulated lid to keep the heat in. Beyond that, your tub enclosure can take whatever form you choose, based on materials available, your carpentry skills and aesthetic considerations.

My scrap heap was long on weathered 2 x 4s from an old deck, so that's what I used for my enclosure. The result was a rustic, handsome box that blends well with the landscape and cost almost nothing to build.

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I first framed the box around the tub, then insulated the inside of the box with fiberglass batting and wrapped it with 4mil poly sheeting. Then I sided the box with vertical battens cut from the 2 x 4s. With scrap pieces galvanized steel flashing, I covered the box using silicone caulk wherever the pieces overlapped to form a waterproof layer. Finally, again from the old 2 x 4s, I covered the flashing with a deck surface. Using scrap wood I made a two-piece lid, split laterally and hinged in the middle.

Insulating the top of the tub is important. A very efficient way to keep the heat in the water si to cut a slab of styrofoam to fit inside the tub and float on the surface of the water. More convenient to use, but not quite as effective, is a layer of foam glued to the underside of the lid.

Once you have enclosed your tub, insulate all exposed pipes. Standard foam pipe insulation works well. Pay special attention to the "hot" side pipes, as heat loss from the "hot" side will reduce thermosyphoning efficiency. But insulate the "cold" side to maximize heat retention.

Preparing the hot tub for use couldn't be simpler: you put water in it. Open valves 1 and 2 (see drawing) and fill the tub within 8 inches or so of the top. If you fill the tub during the hot part of the day (and the sun is out), the collector should immediately begin to heat the water. Within 15 minutes heated water should begin to flow into the tub through the inlet pipe. If not, you may have an air pocket somewhere. The easiest way to flush out an air pocket is to open the drain valve. Once the heated water is circulating, the water in the tub will gradually warm up.


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