Installation, Operation & Maintenance Manual
IM-895
8
4. Control Relay
– Includes double-pole, double throw (DPDT) contacts for sequence of operation control switching.
5. Air Flow Proving Switch
– The switch monitors the pressure drop across the burner to ensure that sufficient air flow exists
before allowing the burner to operate. The switch is electrically interlocked with the flame safeguard control (#3).
6. 24V Isolation Transformer
– A 24V to 24V transformer is used to electrically isolate sensitive controls from the rest of the
control circuit.
7. High Temperature Limit Thermostat
– The high temperature limit control prevents the burner from firing if excessive heated
air temperatures are experienced. The limit control is mounted on the blower housing and is electrically interlocked with the
flame safeguard control (#3). The switch requires a manual reset if tripped.
8. Temperature Control Amplifier or Signal Conditioner
– The amplifier converts the temperature control signal from the
discharge air temperature sensor.
9. Regulating Valve
– All.
10. Main Gas Valve
– All units are supplied with redundant automatic main gas shut-off valves to control gas flow to the modulating
gas valve (#11). These valves may be a combination gas valve as shown, which have two valve seats in one valve body or two
separate gas valves on higher MBH capacity models.
11. Modulating Gas Valve
– The modulating gas valve is controlled by the temperature control amplifier or signal conditioner (#8)
to vary the flow of gas to the burner.
12. Gas Pressure Test Port
– Used during start-up to easily connect a manometer to measure profile pressure drop during unit
balancing.
13. Inlet Gas Pressure Gauge (not shown)
– The inlet gas pressure gauge provides a method to easily determine if the gas pressure
entering the unit is within the range required, without having to connect a manometer.
Control Cabinet Features
(cont.)
1. Direct Drive Axial Fan with Inlet Guard
2. Blower Motor
3. Direct Fired Burner
4. High Temperature Limit Thermostat
– The high temperature
limit control prevents the burner from firing if excessive heated
air temperatures are experienced. The limit control is mounted
on the blower housing and is electrically interlocked with the
flame safeguard control (#3). The switch requires a manual reset
if tripped. Photo to the right shows high limit sensor.
Unit Features