Todd Harrison hacked a pool heater out of his backyard barbecue grill:
Normally you can't swim in our pool until June because it's just to cold (68F - 75F). I like swimming in about 80F to 85F myself, if it's sunny and warm out. I created a prototype heater coil that seemed to work for its size. I then create a large heater coil out of 180 feet of copper tubing that connects to my pool pump using a garden hose and fits inside the grill. The hose runs from the pump through the grill heater coils and then into the pool. I heated my pool from 68F to 89.4F in 48 hours using 3.5 tanks of propane.Link (via MAKE: Blog)
The device heats water 30 to 35 degrees at 3 to 4 gallons per minute flow. I find I get a 7 degree increase in my 8000 gallon pool for each 5 gallon tank of propane. Each tank will burn for about 13 to 15 hours on high. A pool cover is required...
The coils come out and the grill converts back to cooking in less then 3 minutes which includes cool-down and draining the coils.