Indirect
Fired Air Heaters
This is an extract from our Domestic Gas Fitting Handbook.
Basic Operating Principle
In its simplest form, when heat is called for, the burner will light, firing directly
into the heat exchanger. When the heat exchanger is hot enough, the air circulation
fan will switch on. Air is filtered and blown across the heat exchanger, into the
plenum chamber, where it is distributed through the duct system to the registers
or diffusers in each room requiring heat.
The room thermostat controls the room temperature, the fan control and limit thermostat
monitor the temperature of the air within the heater, switching the burner on or
off as required. When the room thermostat is satisfied the burner goes out but the
fan remains on for a short while to distribute the useful heat left in the unit.
If a water heater (circulator) is incorporated within the unit, it will work independently
of the warm air unit and will have its own set of controls.
Warm air heaters are made to suit a number of applications. Dependent on system
design, the heater type can suit the location.
Two types are shown opposite.
The upflow heater: Filtered air is drawn in at the base of the heater by the air
circulation fan, passing across the heat exchanger. It exits through the top
of the unit and is distributed by the ductwork.
The downflow heater: The air circulation fan on this unit is sited above the heat
exchanger and blows filtered air downwards to exit the unit at the base, this is
then distributed by the ductwork.