I'm well below Canada's latitude, and it costs me $250/mo to heat a relatively small swimming pool (about 10,000 gal) ... in the summer. If I keep it warm enough to swim in October, it'll be at least twice that. The capital cost of the heater is practically zero compared to the fuel. I can totally understand janky DIY setups to try and defray that cost.

There’s a bunch of YouTube videos showing how to make them. It’s really easy, actually, and you can get like 250-500W of heating from what seems to be a reasonably small device.