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Indirect Water Heaters

Indirect water heaters are another form of combination heating/water heating system. In contrast to tankless coil models, indirect water heaters utilize a storage tank.

How It Works
An indirect water heater uses the main furnace or boiler to heat a fluid that is circulated through a heat exchanger. Unlike tankless coil units, the heat exchanger in the indirect water heater is located in the storage tank, not the furnace or boiler. An alternate configuration runs the boiler water through a jacket which surrounds the domestic hot water tank. The energy stored by the water heater tank allows the furnace to turn off and on less often, saving energy.

Indirect Water Heater

Indirect systems without a storage tank run a coil of the plumbing for the domestic hot water through the boiler or through a tank next to the boiler (the tank is filled with boiler water or steam).

Semi-instantaneous indirect systems have a very small tank of domestic water which is not heated until hot water is called for.

Indirect Water Heater with Tank

Since boiler water is often treated with chemicals to prevent corrosion, scaling and bacterial growth, some local regulations require that indirect water heaters use double-wall, vented heat exchangers to prevent contamination of the potable water with the boiler water in the event of a leak inside the heat exchanger.

Tankless Indirect Water Heater

Efficiency
An indirect water heater used with a high-efficiency boiler and well-insulated tank can be the least expensive means of providing hot water. Combination systems are measured by their CAE, combined appliance efficiency rating. Combination appliance efficiency ratings range from 0.59 to 0.90.

Energy Types
Indirect systems can be fueled by natural gas, oil, propane, electricity, solar energy or any combination.

Maintenance
Like tankless coil water heaters, indirect water heaters have few moving parts so maintenance and service is less than other types of water heating systems.