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Indirect Water Heater Basics

Indirect water heaters consist of a single boiler, a stand-alone storage tank for both domestic hot water and space heating, piping, a pump, and other associated components. Water is run in a piping loop from the boiler (where it is heated) through a coil in the storage tank. In the storage tank the coil of water exchanges its heat with the surrounding storage water before returning to the boiler. Domestic hot water is usually controlled as its own ‘zone’ of the house. In warm weather, the boiler is used only to produce domestic hot water. In cold weather, very little extra energy is used for domestic hot water since energy is used to heat water for space heating anyway. Boilers can be heated with a variety of heating fuels.

Costs and Benefits

The main drawback of these systems is the upfront cost: even though the space heating and domestic hot water heating needs are combined into one unit, the purchase price can be more than buying a space heater and water heater separately. Installation costs, however, may make up for at least some of this difference since only one heating unit (a boiler) is being installed. The nameplate efficiency of indirect water heaters is referred to by the combined appliance efficiency (CAE), which can vary from 0.59 to 0.90. While these units can be very efficient in winter when water is heated for both space heating and domestic hot water, they can also be somewhat inefficient in summer since the temperature of the relatively large volume of water in the storage tank needs to be maintained.

Because indirect water heaters need to accommodate both space and domestic hot water needs, they should be sized carefully by qualified contractors. Maintenance requirements are similar to the requirements for conventional boilers and storage tanks.

Last updated: October 18, 2017 at 10:51 am