Our hot tub is made from one half of a used 400 gallon orange juice concentrate container. These huge black plastic barrels are often use by farmers for water storage. The tank is cut in half to create a tub almost four feet in diameter and about four feet tall. We have had seven people in the tub although four or five is more comfortable. It is trimmed with poly pipe that has been split along its length and then hammered around the cut edge of the tank to provide more strength and a more comfortable finish.

The tub is heated by a wood fire (of course). It requires roughly two wheelbarrows full of wood (about two smashed up pallets) to bring the hot tub up to temperature and keep it going all evening. It takes between four and five hours for our tub to heat up. This is based on our experience during the winter and using fresh cold water. It would require less wood in the summer when both the air temperate and the temperature of the water are higher. If the hot tub is used on consecutive days with the same water it is much quicker to heat up again and uses much less wood.

The heating system consists of a household radiator (found in a skip) that is placed at an angle over a fire pit. 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.

While the water is heating up we keep a 'lid' on the tub (a 4ft square sheet of plywood) and have a sheet of plastic bubble wrap floating on the water surface. This helps reduce evaporation and heat loss. The pipe carrying the heated water to the tub from the radiator is also lagged and we have insulated the tub itself by tying a length of rope around its diameter from which are hung several old duvets and blankets which are covered in plastic sheet in an attempt to keep them dry (wet insulation is very inefficient).

The water gets pretty murky with heavy use so we have a foot bath which we uses before entering the tub. When the tub is drained (we use a syphon to do this) we take the opportunity to remove any grit or sludge that has accumulated.

We've become aware of a number of safety issues. It is important to stir the water occasionally while it is heating because otherwise the hottest water stays at the top and it is difficult to judge when it is warm enough to get in. If it is not stired there is a strong chance that the first person in will be burnt. Another problem is the hot water intake. The water is very very hot when it enters the tub. Everyone using the tub needs to be aware of this and stay well clear of it.

The future... We plan to install a better and more attractive system of insulation at some time and hope also to building decking around the tub. We will probably also have a go at using solar heating next summer.

See also : How to build a hot tub