Rocket stove water heater for our copper hot tub

My rocket stove with water heater attachment and copper hot tub. Rocket stove uses wood pellets and heats 1000 litres of water from tap cold to 40C (100F) in under 4 hours. Only uses about 10kg of pellets to do this so very cheap to run (about £2.50 per heat up from cold). Pellet stove is 'fire and forget' so no need to tend the fire. simply light and return 4 hrs later for a bathe. My rocket stove has adjustable heat output, so once tub is up to temperature simply set stove to idle to maintain temp with very low fuel burn rate. Copper hot tub designed and fabricated by www.metalandbutter.com