Lounging in a hot tub is a wonderfully relaxing way to end a difficult day. Unfortunately, its extremely expensive to buy a hot tub and heat it using gas or electricity. Build your own wood fired hot tub instead, saving you money and earning you bragging rights among your friends.
Plan the layout
Plan where you're going to place your hot tub and bring in sand to create a level base for it to sit on. Place it on the sand, then use a level to make sure that it's not slanted.
Buy a metal or plastic bulk liquids or stock tank that suits your needs. You can get the tank at a local feed shop.
Choose a heating apparatus for your hot tub. You can either make your own heater or you can buy one. The Chofu heaters are an excellent choice for your hot tub and can be found online. Any wood burning heater, that is designed for heating hot tubs is a good choice.
Adapt the tank
Unbolt the crossbars holding the plastic tank in the cage, then remove the tank.
Shear off the top of the cage using a hack saw or grinder. Shorten the cage to a comfortable height when sitting in the tank. Smooth the steel edges using the grinder or a file. Sharp edges may tear your plastic liner.
Replace the top ring of the cage. You may weld, bolt or wire this ring into place, but the results must be smooth so they don't tear the liner.
Put the tank back in the cage. Draw a horizontal line with a marker just below curve at the top of the tank. Use the cage's lip as a reference when you're drawing the line.
Cut off the top of the tank at your marked line with a hand or hacksaw. If you set the top aside, it can be used as a lid for keeping out debris later.
Slice deep slots into each corner of the tank, stopping approximately 1.2 cm (1/2 inch) above the top of the cage.
Heat the top portion of the plastic with a heat gun, then bend the plastic flaps over the rim of the cage. Clamp the flaps in place until they've cooled.
Build the heater
Buy a commercial snorkel stove or build your own. A snorkel stove is simply a small, water tight wood stove with an access port for adding fuel, and a stove pipe for directing the smoke. You can build one out of any heavy metal container that will withstand fire burning directly within it.
Build a fire under a suspended household radiator to create a thermosyphon. Place the top of the radiator, below the bottom of the hot tub. Run piping from the bottom of the tub to the bottom of the radiator, and another pipe from the top of the radiator, to a point half way up the tub.
Creating a thermosyphon as described in the previous step, but force the water to run through an piping system located at the bottom of your tub. This will indirectly heat the water in your tub, heating more slowly, but preventing possible burns.
When you buy a stock tank, be sure to actually sit in the tank before you buy it to ensure it's a comfortable size and shape for you. It's easiest to construct the tank on a warm, windless day or in an enclosed space, protected from the elements. When using a stock tank, or heavy duty stand alone plastic fruit or juice container, all of this work is done for you. All you have to do is add the heat. A thermosyphon draws in cold water from the bottom of the tub, and as it heats up in the radiator, the hot water is forced back into the tub.
If using a thermosyphon, do not sit near the inlet for the hot water, as it may scald you.