How to build a hot tub
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.
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.
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.
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.
Useful links to help give you further ideas for your own design:
www.dte.org.au/ConFest/HotTubs – Another DIY constructors guide
www.buildatub.co.uk – Great DIY guide for a more grand design. Uses an electric heater but i’m sure it could be converted to be woodfired
www.dte.org.au/ConFest/HotTubs – Commerical site for submerged stove
sweetwatertubs.com/index.html – Commerical site for timber tub
