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