In hydronic heating systems, such as those using radiant floor and/or baseboard heat, domestic hot water can be provided by an indirect-fired water heater. There are many designs, but typically this type of water heater uses the hot boiler water to heat the consumable water in its storage tank.
The boiler's hot water never comes in direct contact with the consumable water in the storage tank, but is circulated through looped tubing inside the tank. The heat from the boiler water in the tubing is transferred to the consumable water. As the heat is transfered, the "cooled" boiler water then flows back to the boiler to pick up more heat and the cycle continues until the consumable water in the storage tank reaches a set temperature.
Indirect systems can be indirectly fired by gas, oil, propane, electric, solar energy, or a combination of any of these. In general, indirect-fired water heaters require minimal routine maintenance, can be located near or far from the boiler in an unvented area and are relatively inexpensive to operate.