Indirect fired water heaters have
a similar appearance to the traditional tank water heater and store a volume of
water for use. The key difference, however, is that while the traditional tank
models have a burner system and a vent (if a combustible fuel is being
utilized), the indirect fired water heater does not contain these components.
The unit instead relies on the transfer of heat generated externally and piped
to the water heater. Most often, the heating unit is a boiler that is being
used to generate hot water for hydronic heat within the building. An additional
zone is added to the boiler, which includes a circulator pump or zone valve, as
well as backflow protection. Hot water is pumped from the boiler supply through
piping into a heat exchanger coil that is situated at the bottom of the tank.
The circulator pump flow is turned on or off by the aquastat which has a
thermocouple seated in a well in the tank wall to sense the water temperature.
Once the desired water temperature is achieved, the circulator pump switches
off, shutting down the flow of feed water from the boiler. If hot water is
drawn from a tap, the water temperature in the tank drops as cold make-up water
enters the tank and the drop in temperature causes the aquastat to restart the
circulator pump.
One
advantage of indirect fired water heaters is that they do not require a vent,
as there are no combustion sources in the unit itself. Traditional tank water
heaters that have a vent are subject to standby losses, meaning that there are
natural convection currents which will draw heat from the tank and send that
heat up the vent even when the unit is idle and there is no demand for hot
water. The elimination of the vent makes the efficiency of the indirect fired
water heater higher as there is less heat loss. They do, however, require a
boiler to generate the feed for the heat exchanger, and therefore are not
suitable for use when the building’s heat source is an air furnace. And as the
boiler is essential for hot water, that unit must remain in operation in order
for hot water to be available, even in seasons where there is no call for heat.