We got a small above ground pool for the kid's and so naturally Dad decided to find a way to heat it. Solar was the first step and a wood burning heater the second step. Following is how I built the "Fire Pool Heater".I had an old gas water heater that I had replace. After I had removed the metal cover and insulation I was left with a tank that had an exhaust pipe through the center. This gave me an idea to make a water heater that can use various fuels to heat the water. I have tested propane; wood; waste oil (old cooking oil); charcoal and (waste oil gasoline mix). All work well and I get the pool up to a nice temperature. I only use it on week ends and days when we are going to swim. It is connected in line with the Solar heater, hot water from the solar heater gets pumped into the bottom of the tank and pushes the hot water out the top of the tank into the pool. The pump is on an interval timer and only pumps water for 10 min every 1 hour during the day. The tank is also black and in the sun most of the day so gets a little extra solar heating. SAFETY : Always use the correct safety equipment. For this project I used, safety eye glasses; ear plug; gloves and protective clothing and shoes.1. The first thing I did was cut the bottom of the tank open. The tank has a concave dome at the bottom that is welded to the outside wall. I used an angle grinder with metal cutting disc to cut the weld off. ( I use about 4 discs and had to wedge the cut open to prevent the discs from binding)2. Next I cut the pipe free from the top of the tank. I put the cutting disc flat on the pipe and gently cut the weld moving all around the pipe till it was free.3. I pushed the pipe out the bottom of the tank and shaped the concave dome ( used a large hammer ) so it would slide into the tank. 4. Now I pushed the pipe and dome assemble about 24" into the tank the bottom area will be the burn chamber and the top will be the water tank. 5. Using a metal chisel and hammer I crimped the top of the tank and dome. This will make welding easier. ( you also need to grind the inside of the tank and out side of the pipe at the weld points, because most tanks are lined with glass "very difficult to weld I found this out the hard way" 6. I welded the tank at top and bottom and tested for leaks with water. ( sorry I neglected to take photos of this part of the project)Now o make the burn chamber. My original idea was to make a rocket stove type setup but I have not built that part yet, so the door is the same measurements as a cinder block.1. To get the size of the block about 8"x8" on to the curved tank I made a simple tool from a piece of wood and 2 nails.2. I transferred the measurements to some masking tape on the tank and then cut it out with the angle grinder and cutting disc.3. The next step was to make a base. First I used some long self drilling screws along the base to hold the concrete in place.4. Then filled the base area with dry concrete to get the correct amount I would need. Next I mixed the concrete in a bucket and cast the base in the burn chamber.5. While the concrete was setting I cut the fittings off the door and painted the door and heater mat black this was to help absorb some sun light. To get water in and out of the tank I used galvanized fittings and plastic pipe fittings to connect to the solar heater pipes. I also installed an aluminum inlet pipe on the inside of the inlet port. The aluminum pipe serves 2 purposes first to get the colder water (still hot from the solar heater) into the bottom of the tank and second as a sacrificial anode to prevent rust in the tank.1. I used a galvanized reducer; nipple; 45 elbow and a plastic barbed fitting put together with thread tape for both inlet and outlet ports.TIP: A good method to wind thread tape on to a fitting without getting it all messed up. Hold the fitting in your left hand with the fitting threads facing out of your hand. Place the thread tape with the end of the tape at the top of the spool and the spool under the pipe threads, pinch and hold the tape at the bottom of the threads with your left hand. At the same time use you right hand to wind the tape up and over the fitting. ( in the pictures I only use one the other was holding the camera)2. Next I got an old crutch from my local thrift store for the aluminum sacrificial anode, this would give me 2 pipe I'm hoping 1 will last a full season. After cutting the pipe to the correct length. I cut 4 slots into the top end and crimped it to fit into the galvanized reducer that I had bored out with a step drill and file.3. I pushed the pipe into the bottom of the fitting assembly and then sanded the coating off the pipe so I would have a bare surface to attract ions and help prevent rust.4. Now I screwed the fitting onto the tank and connected the outlet of the solar heater to the inlet of the tank ( the one with the aluminum pipe). The other fitting connects the the pipe going to the pool.5. I used some old retaining wall blocks I had to insulate the burn chamber and hold the heater in place.For Wood burning I left an opening in the retaining wall blocks to load wood and clean out ash. The door is held in place with a pin in the wall of the heater and hole in the door, leaving an opening at the bottom for air inlet. This is covered with paving block.1. I stack old off cut wood from construction sites in my area and some waste oil ( old cooking oil ) in a steel bowel in the burn chamber. I add some ( about 80 ml ) gasoline or white gas to the oil to get the fire started.For oil gas mix ( biodiesel ) burning I use an old pot with holes drilled in the sides, an old barbell weight for heat retention. There is not much ash so I close up the opening and installed a fill pipe, the pot holds 1 liter that I fill with a funnel and light it by dropping a lit piece of paper down the chimney.1. I fastened an L shaped 1/2" pipe to the pot and side of the burn chamber with wire putting the weight at the bottom of the pot.2. I stacked the wall block around the opening leaving gaps for air flow and flame inspection. 3. For insulation in the gaps of the blocks used some gravel so it will drain any water.This project is still a work in progress But this is as far as I have come I hope this gives you some ideas on how to make a pool heater from an old water heater.