I have always wanted a cedar wood fired hot tub. After checking prices I realized I needed to find a more realistic dream. So I began the research process and found a few people who had done what I wanted to attemp. I combined the best ideas from those and came up with a solid plan. This tub cost under 300 and requires no electricity. All materials were easy found at the home improvement store and farm supply chain.
Step 1: Procure Materials
Gather your materials. You will need a tank around 100 gallons or so. Bigger the tank the harder to heat. I went with a rubbermaid 150 fits 2 people great under 2 hundred dollars. 20 feet of 1/2" copper tube. Diameter and length will effect heat time. The proprr fitting to secure copper line to tank. I used 1/2" compression fittings to 1/2" pipe thread then found fitting made to seal to a tank with matching thread sizes. Then a few rims or similar metal scrap to build a burn chamber.
Step 2: Placement
If the area you want your tub on isn'ta hard level surface make one. I used a pallet the tub fit perfectly on.
Step 3: Making the Thermo-siphon (Chofu) Heater
Your copper tubing should come in a roll, unwind it till it looks thusly and fits in your stove snuggly. For the thermo-siphon to work one end needs to be higher than the other. This will be the hot water out. Be careful it comes out boiling and often shoots out quickly. Yes I burned myself while testing it out. The coil must never be run dry or it will be damaged. It should have a lid so it can be dampened down if it gets out of control or to control smoke while soaking. Due to the popularity of creative stoves here, I decided mine needed character. So I added elephant ears a trunk, eyes and glasses. I set my stove body on a rim, it fit perfectly. Notice the holes in the bottom of the stove, these were there already and will allow air in to allow a good burn. I used a rim for a lid welded up the rim holes, added a hinge, handle and stove pipe. The stove pipe diameter is a little small.
Step 4: Attaching Heater to Tub
Line the leveled stove up to the leveled tub and mark the spot. Drill then carefully cut or hole saw a hole for the gasketed tank fitting. Telfon tape on any non gasketed non compression connections. Many connection styles will work, possibly even a length of hose to a hose barb in place of the compression fitting. My lower piped leaked a bit at first.
Step 5: Inspect and Fill!
After inspecting all connections, checking tub for level-ness, etc. Fill your tub, and wait to fire it up till the coil is filled completely. The tub will not function unless the coil is fully below the water level. I takes me 8 hours or so of burn time to get my tub up to temperature. Also be careful of the 120+ degree water coming out of the coil. It can scald.