Convert a fire pit or old grill into a wood burning pool heater (water heater).

I bought a 16' x 4' pool, but found that a gas or electric heater would cost more than the price of the pool itself. I also had a lot of limbs and split wood from a fallen tree in my yard. This made for a very easy and effective combination. Now, when I burn wood in my fire pit, I also heat my pool.

Step 1: How It Works

Cold pool water is pumped from the pool via the submersible pump into a 50 foot coil of copper pipe mounted above the fire inside the pit/ grill. By the time the water makes it out of the pipe, the fire has heated the water. This is similar to how a standard hot water heater works. The heated water exits the pipe back into the pool.

Step 2: Materials

The parts you will need are all common parts that can be found at most local hardware stores.

One old, but useful, fire pit or grill
60 foot coil of 3/8 copper tubing
10 feet 1/2 water hose
plastic 1/2 female water hose adapter (recommended) and zip tie
6+ feet black foam pipe insulator
1/2 hose clamp (comes with female hose adapter above)
4 wire copper pipe hangers
1000 gph Submersible sump pump with water hose adapter
scrap brick or stone

Step 3: Installing the Copper Tubing

You will need to first place your coil on the inside of the lid to check for size. The tubing is flexible, so you can adjust to fit. **be careful when bending copper tubing as it kinks easily** Drill two holes through the lid for the ends of the pipe.

I would recommend feeding at least 2 feet of tube through one hole for the intake and 10 feet for the outtake. Drill small holes for the u-shaped wire pipe hangers and install, bending to fit the bulk of the coil. The coil cost about $75. A little costly, but it is much more efficient and easy to install than using a series of elbows and straight pipe, as in my original design.

Step 4: Installing Pump and Tubing

Cut the 25' water hose in half and cut off the metal female fitting. Replace with the plastic fitting and a plastic zip tie. Screw the hose into pump, the shut-off valve is optional (not really needed). Place the pump into the pool. **Do NOT plug the pump in yet**

Slip the other end of the water hose over the shorter piece of copper pipe and secure it with the clamp that came with the female hose fitting.

Carefully bend the 10 feet of pipe to shape until it reaches the pool. Place the black foam insulator, used for hot water pipes, over the pipe. Be sure that the foam covers the outlet into the pool to protect anyone from hitting their head on the pipe.

Step 5: Improving Efficiency

I tested both the pool temperature and the temperature of the heated water regularly to asses the efficiency of the system. I found that the water heated from 60F in the pool to 75-95F from the outlet of the heater. The temperature fluctuated in the range of 75-95F directly with the size of the fire. Keeping the fire at full flame quickly used up wood. Most of the heat was lost through the grate sides and not transferred to the pipes.

Modifications:

I added brick and stone to carefully insulated the three walls and leave the door open for easy access to add more wood and clean out of ashes. I also moved the grate up 6-8 inches to allow a smaller fire to heat the pipes more efficiently.

Step 6: Stats and Tips

I checked the temperature of the water with a temperature sensor that came with my multimeter. I bought and filled my pool in May. In my climate (outside Washington D.C.) the days were usually in the 70's, unusually cool this year. My tap water filled the pool in about 8 hours to about 5000 gallons of the approximate 6000 gallon capacity. The water temperature before I began was 58-60F, much too cold for kids.

In the original design, with the open pit, the fire heated the water coming through the pipe from 60F to 75-95F, depending on the size of the fire. The problem was that this required constant addition of fuel to keep the temperature constant above 80F. At this temperature it took approximately 1 hour to heat the 5000 gallons 1 degree Fahrenheit. It took me 3 days to get the pool from 60F to 72F, only to have 6 inches of cold rain cool it back down to 64F a few days later.

After letting the fire die out and cool, I added the brick and stone to insulate and direct the heat. I also raised the grate for the fire to bring it closer to the pipes. This was much more efficient and allowed me to maintain a smaller fire to keep the water output in the range of 95-104F. When I added charcoal to the top rack the temperature remained in the upper range longer and did not fluctuate as much.

At long last I found the out-of-season fire logs and starter bricks. I built a wood fire on the top rack and lit and Enviro-log on the bottom. This gave me a nice roaring fire with only the occasional need to add wood to the top rack. These logs last for about 3 hours and are a very efficient way to fuel the heater. They also dramatically boost the water temperature coming out of the heater.

I noticed the water was hot to the touch under the pool surface. When I pulled the pipe up, I noticed a little steam and got my meter. The pool water was 68F and the heated output was now 137F!!! I added the second half of the water hose to the outlet. With the end still weighted by the metal fitting it came with. It sank to the bottom and heated the water from the bottom up. This water was much hotter, so you will need to be careful if people are in the pool.

Best of luck if you try this out for yourself. The project was relatively easy and worked very efficiently, although all projects have room for improvement. Please leave your comments and suggestions. Thanks for reading.