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You will need....
*
a water source
* a large tub
* fuel and heating system
* friends to enjoy it with
The Tub
Wooden tubs are great.
You could use large whiskey barrels etc., but they are expensive
and hard to come by. If you have the tools, the skill and the
time you could build a wooded tub. We have seen a very effective
hexagonal design (used for a birthing pool) that uses plywood
and could make great transportable hot tubs. Wooden tubs don't
need to be water tight as you can use pond liner etc although
that may cause difficulties with plumbing.
Pond liner and a
dozen or so straw bales could also be used to make an effective
hot tub (or perhaps even a small swimming pool). Using bales
provides in-built insulation and an all round ledge. It also
makes it easy to include steps, and seats.
We've heard of people
using the large plastic trade waste skips to make unattractive
(but highly portable) hot tubs. There are many large plastic
containers that could be given a new lease of life as a hot
tub.
Our tub is made from
a 400 gallon orange juice concentrate container. These black
plastic tanks are often use by farmers for water storage and
are available from some agricultural supply places like Mole
Valley Farmers and often from agricultural and free ads papers.
The container is cut in half to create a tub almost four feet
in diameter and about four feet tall (one tank could provide
two tubs). These tubs hold upto seven people, although four
or five may be more realistic.
Fuel
Wood is the tradition
fuel of choice for hot tubs. It requires between one and two
wheelbarrows full of wood (one to two smashed up pallets) to
bring the average hot tub up to temperature. It seems to take
between four and five hours to heat up the average tub depending
on size and climate. Less fuel would be required during warm
summer months that in the winter when both the air temperate
and the temperature of the water are lower. If the hot tub is
used on consecutive days using the same water, it will still
be warm so heat up again more quickly and use less wood.
Heating
An ordinary household
radiator placed at an angle over a fire pit provides one method
of heating. The fire and radiator are placed so the the top
of the radiator is on a level below the bottom of the tub. This
enables a thermosyphon to be created which circulates the water
without the need for pumps. A pipe comes from a fitting at the
bottom of the tub into the bottom of the radiator. Another pipe
goes from the top of the radiator to a fitting about halfway
up the side of the tub. The lower pipe carries water down from
the tank to the radiator where it is heated. As heated water
expands and rises, it forces hot water up the top pipe and into
the tub. It is important to stir the water before getting in
as the hottest water will stay at the top. It is also import
to stay clear of the hot water inlet because it is very very
hot where it enters the tub.
One method of avoiding
having scalding hot water entering directly into the tub would
be an indirect heating system. This could involve a second radiator
or coil of pipe placed inside the tub.
A tub made from metal
(or incorporating a metal base) could be heated with a fire
directly under it . This would avoid plumbing issues but some
kind of slated wooden flooring would be needed to prevent burns.
Smoke could also become a problem.
Another plumbing
free method is to put the fire inside the hot tub itself. This
is not as stupid as it sounds, there is a commercial stove called
the snorkel stove which is designed specifically to go inside
hot tubs. It is very efficient as almost it's entire surface
area is heating the water. It would be fairly easy to build
such a stove, perhaps using a old gas bottle which you could
then hang from beams into the tub and remove when the water
was heated.
Another possibility
is to not to heat the water in the tub at all but instead fill
it from a separate hot water tank and add more hot water to
the tub when needed.
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 This
photo shows a 1500 litre (400 gallon) contain previously used
for bulk shippment of fruit juice concentrates.
The photos
beloww show a commercial system with a barrel like wooden
tub made from tongue and grove.
It is
heated using a commercial wood burning stove (shown below)
that sits directly in the water.
 
The photo
below shows a home built system using a plywood and timber 'tub'
with a plastic liner. A 55 gallon drum with a fire below acts
as both the heating system and as a hot water storage tank.
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Alternative
Fuels
Wood
obviously isn't the only possible fuel with which to
heat a hot tub. While not wanting to encourage the use
of non renewable such as coal, gas or petroleum etc.,
you might consider a combined heat and power system
with the cooling water from an internal combustion engine
being used to heat the hot tub while the engine is providing
mechanical power or electricity. Other possibilities
include compost heating systems and solar.
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Insulation
A
lid of some sort will reduce heat loss and help to ensure
the water heats up quickly. A good method is the use of sheets
of plastic bubble wrap layed on the surface of the water.
It is a good idea to insulate any pipe work involved in the
heating system. Insulating the tub is not essential but sensible
if you want to use the tub on consecutive days. It may not
be so important if you intend to refill the tank with each
use. Wooden tubs are probably pretty well insulated already.
Other possible recycled insulation includes old duvets and
blankets, polystyren, foam or even straw. Whatever is used
it is important to try to keep the insulation material dry
because it will be less efficient when wet.
Hygen
and safety issues
The
water will get pretty murky quite quickly with heavy use.
It helps if people wash first in a shower or at very least
using a foot bath before entering the tub. We would strongly
suggest that you never consider using chemicals to keep the
water 'hygienic' as is often the case with commercial hot
tub installations. It is a good idea to drain and refill each
time the tub is fired up although it is tempting to use the
same water the next day as it will still be fairly warm and
heat up much more quickly than with fresh water. Floating
stuff like leaves can be removed with a kitchen sieve. When
the tub is drained (you can use a syphon to do this) take
the opportunity to remove any grit or sludge that has accumulated.
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