Detroit Radiant Products Company QTD Series - Manual (Page 35)

QTD Series
35
4.0 Operation • Sequence of Operation
WARNING
!
This heater must be installed and serviced by trained gas installation and service
personnel only.
Do not bypass any safety features or the heater’s built in safety mechanisms will
be compromised.
Sequence of Operation
When operating under standard configuration, two voltages (120VAC supply and 24VAC control)
must be supplied to the QTD series unit.
Starting Circuit: Upon a call for heat the dual pole relay sends 24V through the post purge timer
energizing the fan relay. The relay is closed sending 120VAC to the blower. Once operational static
pressure is achieved, the differential pressure switch closes, sending power to the ignition module.
After a seven-second pre-purge, the spark igniters and the solenoid of the gas valve are
simultaneously energized. The trial for ignition is 15 seconds.
Single Stage Running Circuit: After ignition, the control monitors burner flame through the flame
sensors. If sense of flame is lost, the control immediately sparks (identical to the starting sequence).
If flame sense is not established within 15 seconds, the heater will attempt two (2) additional ignition
sequences before proceeding to lockout mode. The control can be reset by briefly interrupting the
power source.
Two Stage Running Circuit: High fire operation is actuated by the thermostat, switch or controller
sending a 24VAC signal to the high fire relay. NOTE: Low fire must also be energized to fire the unit
in high fire. The energized coil closes a contact sending 24VAC to the high solenoid of the gas
valve, increasing the manifold pressure.
Shut Down: When the thermostat is satisfied, the fan will enter a 90 second post-purge cycle.