Inside the heat exchanger
Three spiraled 1/8" ID HDPE tubes with 1/16" wall take up a little less than half the cross-sectional area inside a 1/2" PVC water pipe. So pressurized warm water and returning cold water can exchange their heat. This counter-flow design helps to conserve heat in the main tank, as outgoing water gives up its heat to warm returning water. Because I'm not too thrilled with the thermal conductivity of HDPE I made my heat exchanger about 8 feet long, and enclosed the whole thing with foam insulation outside the PVC pipe. The ideal material for the inner pipes would be soft copper perhaps, but I heard rumors that copper was not good for fish.
Inside the heat exchanger
Three spiraled 1/8" ID HDPE tubes with 1/16" wall take up a little less than half the cross-sectional area inside a 1/2" PVC water pipe. So pressurized warm water and returning cold water can exchange their heat. This counter-flow design helps to conserve heat in the main tank, as outgoing water gives up its heat to warm returning water. Because I'm not too thrilled with the thermal conductivity of HDPE I made my heat exchanger about 8 feet long, and enclosed the whole thing with foam insulation outside the PVC pipe. The ideal material for the inner pipes would be soft copper perhaps, but I heard rumors that copper was not good for fish.