I just got my heater up and runnning. Here's how I built it:
It's a wood fired, u-tube hot water boiler. Don't let the word "boiler" scare you, it doesn't boil the water, it just raises it about 3 to 4 degrees C on each pass through.
A 45 gallon drum lays on it's side. I made some quick feet out of 1/4 x 1 flat bar stock, self tapping screws used to attach the feet to the drum. This thing is top heavy, so you don't want it rolling over.
The bundle is made of 1/2" thin wall copper pipe. An inlet header of 1 1/2" PVC pipe and an outlet header of the same.
I made each header out of three short straight pieces, and two 45's, so the header sort of follows the round shape of the drum. Put a cap on one end of each header, and nipple to attach the hose on the other end. Drill each header to receive the pipe, mine has 20 holes in each header. There is a total of 40 tubes in the bundle, each the length of the drum, (appx. 3'), so there is 120' of copper pipe exposed to the fire. Copper 90's are used to make the u-tubes. Water flows from the inlet header, through the 20 u-tubes and out the back into the outlet header, and dumps back into the pool. Soldering is required to make the tubes. I glued the tubes into the holes that were drilled into the pvc headers. A white two part epoxy called Wet Bond is what I used, but it softened up and blew a few holes after the boiler was fireing hard. I ended up useing Copper Bond copper pipe epoxy to cover the Wet Bond, and it works quite well. It is half the price of the Wet Bond. Four units of Wet Bond were used, at $20.00 Canadian each, compared to Copper epoxy at half the price. It adheres to the pvc very well, so I would recommend to use only Copper epoxy.
Water is pumped from the pool with a submersible sump pump, through some cheap, flexible black sump pump hose, into the bundle, and dumps back into the pool. The pump inlet is covered with a lady's nylon stocking and a nylon door screen to filter the water.
The bundle is a semi circle shape and fits up in the upper half of the drum. The fire is made in the bottom half of the drum, and heats up the tubes. Cut a tabbed hole for a 6" wood stove chimney pipe. Cut a hole for a door, and bend all four edges out a half a inch. Make a door out of the heavy bottom of a second drum. Build it about 1 inch larger than the door opening, so you can affix some gasket rope around the edge for a door seal. Cheap hinges, and a door clasp are screwed on. Be sure to put the door opposite the pvc headers, or you will have trouble fitting a large enough door opening below the bottom header. Also, put the door opposite the bung holes, and keep the large bung hole on the bottom, so it can be used for a air inlet for good draft.
Use wood stove cement to seal the slots or holes drilled into the drum ends where the tubes go through. You don't want heat getting outside to the pvc headers. They could be insulated with a combustible proof insulation, but then you want to be able to access the glue connections to touch up any leaks.
After you have circulated water through the bundle, checked for and repair any leaks.
Let the stove cement dry thoroughly.
Put a layer of sand on the bottom of the drum to insulate the drum, and provide some thermal mass.
Be sure to have water circulating through the bundle before the fire is built. Never shut off the pump if there is any heat in the drum, or the tubes will be overheated, and worse yet, the glue and/or pvc headers could be damaged.
I am going to put an air bleed fitting on the outlet header of my heater. I will drill a 1/4" hole in the top center of it, glue a nipple in and attach a rubber tube to it with a clamp. The tube will be tie wrapped along the discharge hose to vent off any air in the bundle into the pool.
I warmed the pool 10 C degrees in a day and half of casual attention. Its cool, rainy September here now, so my goal is to get the pool up to 30 C, have a good swim with the family, brag about my heater, and take it all down for the winter.
I have some pics of the project that I could e-mail if anyone is interested.
Heat up your pool everyone!