A domestic hot water coil is used to supply on demand domestic hot water in larger boilers or gradual heating using a thermoshipon in hot air stokers and hand fired coal stoves.
Domestic hot water coils in boilers will usually be provided by the manufacturer of the boiler as an option. The coil typically consists of a long length of ribbed copper tubing that is coiled so it can fit into the boiler compartment. The ribbing increase the surface area substantially so the coil can absorb more heat. Installation may be from the factory or they can be retrofitted into older boilers by removing a plate intended for that purpose. The coil sits inside the boiler water completely submerged. They are rated by GPM (gallons per minute) and may range from 3 GPM to 8 GPM but can be higher. Properly set up they can provide an endless supply of hot water.
Domestic hot water coils in hot air stokers and hand fired stoves are usually aftermarket products or even homemade. These are usually a U shaped stainless steel pipe about 2 to 3 feet in length with threaded ends. Installation is achieved by drilling two holes in the side of your stoker or stove and mounting the two ends of the pipe into the holes. Domestic hot water coils of this type do not provide on demand heat but instead create a thermosiphon. The hot water in the pipe will naturally gravitate into an existing domestic hot water tank or simply a storage tank. This is slow process but if properly managed can meet the demands of most households.