Installation, Operation & Maintenance Manual
IM-895
13
Sequence of Operation – General
The following describes the general sequence of operation for the unit. However, each unit may be slightly different based on
unit configuration and application. Each unit includes a laminated job specific Sequence of Operation affixed to the inside of the
control access door. Refer to that document for the actual unit Sequence of Operation.
Fan On/Off Switch = "Off"
The unit is shut down with no blower operation.
Fan On/Off Switch = "On"
1. With the blower motor control circuit energized, the motor starter contacts will close.
2. The gas and temperature controls are locked out to prevent heating during the Fan "On" mode of operation.
Burner On/Off Switch = "Off"
The unit is shut down with no burner operation.
Burner On/Off Switch = "On"
1. Before the ignition control sequence can be initiated, the following safety devices are checked for closure:
a. The airflow pressure switch ensures the airflow is at or below the maximum allowable to ensure clean combustion and
reduced flame disturbance.
b. The high temperature limit control ensures the supply air temperature does not exceed the maximum allowable limit for
safe operation.
2. With the safety devices described in the above steps verified, the flame safeguard control is enabled to initiate the ignition
sequence. After 10 seconds of pre-purge time, the ignitor is energized to produce spark ignition as follows:
• The main valve(s) opens and with the modulating valve at minimum position, the burner is allowed to light (direct spark
ignition).
3. The burner remains on low fire momentarily, then the temperature controls take over and fire the burner based on load
demands.
4. If any of the devices discussed in step #3 open, the gas circuit is disabled, the unit goes into lockout and the burner lockout
light is lit. Before resetting, a service person must inspect the unit, determine the cause and take corrective action.
Sequence of Operation – Blower
The blower control circuit is configured from the factory as initiated. The motor starter contacts will close and the blower motor
will operate at rated speed. The speed is fixed and not adjustable.
Final Step
With Start-Up and Commissioning complete, set the temperature controls for automatic operation if the unit is to be put into
service immediately.
If the unit is to be left off, set the On/Off switch to the “Off” position and turn the power off at the unit disconnect switch.