The indirect system uses a heat exchanger to separate the heating fluid from the domestic hot water. An advantage is that an antifreeze solution may be added to the heating fluid, and snow melting can be added to the system. Some code administrators may require the heat exchanger. A disadvantage is that the cooling benefit is eliminated. Also, cost and complexity are increased with the addition of the heat exchanger.
When heat is called for by a thermostat, the heater pump comes on and water flows from the water heater through 1 side of the heat exchanger, heating it up in the process. At the same time, the zone pump comes on and causes water (or anti-freeze) to flow through the other side of the heat exchanger to pick up the heat and then travels through the heating zone. When the thermostat in the zone is satisfied, it shuts the pumps off.
1. Allows for the use of a domestic water heater as the source for heating and domestic hot water in locations where separation of the two is required.
2. Allows for the use of two different types of fluids in each part of the system (anti-freeze for snowmelt and water in the heater, for example).
3. Provides absolute protection from oxygen diffusion in boiler applications.