External heater for hot tub

Heater to be placed outside the hot tub, as the advantage is that there will be a lot of space inside the hot tub. Self-circulation without electricity. The hose goes from the bottom of the hot tub into the heater, heats up the water and is pushed out from the top into the hot tub again, creating self-circulation without electricity. We have chosen to have the wood inlet on the top. This way, embers cannot fall out and ignite the surface underneath.