Im interested in designing a rockety fuel stove to a hot water heat exchanger suitable for a spa or hot tub. The tub holds approx 5000 litres and its enjoy it at about 36c. We expect to have a small pump to cycle the water. Ive looked at and visited Geoff Lawtons system and seen various systems on line but most are set to heat water for a few showers. I imagine it should take about 8-10 hours to heat this volume of water from 22-36c and as the spa is well insulated it could be maintained at this temp easy enough. Does anyone out there have experience in such a thing.