You can adapt a portable outdoor fireplace to make a homemade firewood heater for your pool and extend the swimming season. The parts are readily available, and it's a low-tech do-it-yourself project you can complete in a short time. You can either use the pool's circulation system or add a separate submersible pump to run the heater on its own.
Understanding the Principles of a Firewood Pool Heater
The firewood pool heater works on the principle of convection heating. Water flows from the pool through the copper coils suspended over a wood fire. Because the water circulates through dozens of coils as it runs through the tubing, it remains over the fire long enough to get a good transference of heat before returning to the pool. The larger the fire in the firebox, the greater transfer of heat, so it's important to keep feeding the fire when you're trying to heat the pool.
Gathering Parts and Materials For the Firebox
The basic materials include a portable outdoor fireplace with a removable lid, enough fire bricks to stack inside and enclose three of the fireplace walls, a 50-foot coil of flexible copper tubing (either 3/8 inch or 1/2 inch diameter), two U-shaped pipe hangers, a length of 1/2-inch diameter garden hose and two hose clamps.
Assembling the Parts
Remove the fireplace top, turn it upside down and drill two holes, each one near the edge of two opposite sides. Make the two holes slightly larger than the copper tubing diameter. Lay the tubing coil inside the lid, drill two smaller holes on each side of the coils and install the hangers so the coil is held up against the inside of the lid when it's in place over the fireplace. Run about four feet of tubing through one of the larger holes for the inflow side of the heater and 8-10 feet of tubing out the other hole for the outflow side.
Stack the bricks up on the inside of the screen along the back and both sides. This focuses the flames and heat directly up to the copper tubing and protects the firebox from breezes that will lower the unit's heat efficiency. Put the lid back on after stacking the bricks.
Connecting the Heater to the Pool
Connecting the return line to the pool is simple. The 8- to 10-foot section of copper tubing is the outflow line carrying the heated water back to the pool. Measure a length of garden hose long enough to reach from the end of the copper tubing over the edge of the pool and into the water. Slide one end of the hose section over the tubing and clamp it with the stainless steel hose clamp.
To run water from the pool to the heater, you can do one of two things:
One option is to install a "tee" in the pool's return line, glue on a hose adapter and attach a garden hose from the fitting to the inflow side of the copper tubing using a hose clamp to attach the hose over the tubing. This way, the heater can operate any time the pool pump is running.
The other option is to buy a 1,000-gallon-per-hour submersible pump, attach the hose fitting to the pump and drop it into the pool. With this method, you can run the heater and heat the pool whether the pool pump is running or not, as long as the submersible pump is plugged in and running.