Hot Tub Pallet Tank, Wood Fired Using Copper Coils

All you need is:

- pallet tank

- tank connector pieces (we used 8 brass compression fitting tank connectors)

- soft copper pipe

- clean fine grained sand (for pipe bending)

- waterproof insulation material

- Tools, such as metal drill, angle grinder, clamps, metal saw, wrenches

- Optional: wood stove (using an open fire is also possible, but more heat is lost)

- Optional: PTFE-tape (to make an easy waterproof connection)

We used 10 meters of 22mm diameter soft copper pipe

Find a wood stove with walls thin enough so you can cut holes in them. We used a small stove with two sections of iron chimney on top (see photo). Even a small stove like this gets hot enough to heat the water in the pipes.

No pump is needed, the water circulates from the pallet tank, into the copper pipes, up through the stove and back into the tank again by heat convection.

We cut the pipe into four sections of 2.5m using a hacksaw. Next, we closed off one end of each pipe section with a piece of bike tube and filled the pipe completely with fine grained, clean sand (from a sand blaster). When the pipe was completely filled with sand we also closed off the other end with bike tube. Next, we bent the pipe around a cyllinder as shown in the image. The sand inside reduces the chance of kinking the pipe. We mainly used our hands for this, but we kept the pipe in place with a weak clamp. We wrapped the pipe 2.5 rounds around the cyllinder in such a way that there was enough space for 3 other pipes to fit in. Repeat with all four pipe sections and rotate them into each other.

Using an angle grinder, we cut open the lowest section of the chimney and cut holes to fit the pipes in. After that, we merged the chimney back together using a metal drill and some bolts and nuts.

We drilled 8 holes in the sides of the tank to form the inlet and outlet of our 4 copper pipes. The outlet (from where water flow out of the tank) was made at the bottom and the inlet was drilled at the top so that the water passing through the pipes always flows upwards, the way it naturally wants to go when it is heated.

We cut the pipes all to the same length using an angle grinder to make a clean cut. After that we filed off some impurities and used a wrench to carefully bend the pipe into a circular shape (it had turned a bit oval during the bending process). The pipes need to be round at the ends so that the nut and olive of the compression fitting can slide over it. With the nuts and olive slid over the pipes, it is a possibility to wrap PTFE tape around the olives to form a water-tight connection.

We used wooden cladding made of leftover floor laminate and stuffed bubblewrap that we found in our factory behind it as our insulation.

After filling up the tub with 700 liters of cold water (around 5 degrees C), we lit the stove with bits of wood and built up the fire all the way through the copper coils. We insulated the top of the tub by leaving a piece of bubblewrap floating on the surface. After 3 hours of burning, the water reached a very comfy 40 degrees C and it was time to jump in! Even as the fire died down, the tub stayed above 38 degrees C for at least one hour (the outside air temperature was around freezing that evening). The next day, approx 12 hours after heating the tub, the water was still 28 degrees C. Pretty good insulation!