“Wood fired hot tub coil heat exchanger of stainless steel” is an outdoor tub heater kit to heat water by burning firewood pieces.It utilize natural hot circulation: hot water goes up and forces into bathtub through the fire burning spiral coil. At the same time,the coil’s bottom entrance draws in cold water from the tub,because the cold water always sinks.During this cycle,the water in the tub becomes more and more hot without electricity power or gas to drive. The heater coil is a basic element for you to DIY an off-grid wood fired hot tub or swimming pool.