Crest Audio FBN0751 Service Manual page 12

1
Service
Table 1A Sequence of Operation
Sequence of operation
Note: This unit is equipped with two (2) gas train systems. Gas Train 1 will fire first. If the demand cannot be met by the first
gas train, the second gas train (Gas Train 2) will fire.
Upon a call for heat, the control turns on the appropriate pumps (system and boiler pumps for a space heating call,
1.
HWG pump for a hot water generator call).
The control confirms that the low water cutoff contacts are closed and energizes the louvers (optional) and damper
2.
(optional) relays.
The control confirms that the gas pressure switch, blocked drain switch, limits, louver proving switch (optional) and
3.
damper proving switch (optional) contacts close. The Pre-Purge cycle begins.
4.
The control confirms the blower is up to the desired speed, and the air pressure switch is closed.
5. Once the Pre-Purge cycle is complete, the control lowers the blower speed, closes the air metering valve, initiates
sparking of the ignition electrode, and opens gas valve 1.
After a short wait, the control stops sparking and checks for the presence of flame current through the flame sense
6.
electrodes.
If flame is not detected after the sparking ends, the control will perform a postpurge, then start another prepurge cycle
7.
and try to light the burner again.
If the control detects flame current, the control will hold the blower speed constant for a few seconds to allow the
8.
flame to stabilize, then begin modulating the firing rate in order to maintain the controlling sensor to the desired set
point temperature.
If the current call for heat is for space heating and a HWG call for heat becomes active, the control will turn on the
9.
HWG pump relay output, then turn off the boiler pump. It will then modulate the blower speed in order to maintain
the outlet temperature to the desired HWG outlet set point temperature (Normal Mode Only).
If the small gas valve in the boiler is unable to maintain the desired set point temperature, the control will transition
to the large gas valve. The blower will modulate to a set speed, the transition gas valve will be turned ON, and the
small gas valve will be turned OFF. The air metering valve will be opened, and the large gas valve turned ON. Once
10.
the main burner reignites, the transition valve is turned OFF, and the fan speed is held constant for a few seconds
in order to re-establish the flame across the entire burner surface. If the heat load should decrease sufficiently, this
process is reversed, and the small gas valve is used.
Once both the space heating and HWG calls for heat are satisfied, the control will turn off the gas valve and begin
11.
the Post-Purge cycle. Any pumps that are running will begin their respective Pump Delay cycles. The air metering
valve will be opened.
12.
At the end of the Post-Purge cycle, the louver relay contacts will de-energize.
13.
At the end of the Pump Delay cycle(s), the pump(s) will be turned off.
12
Service Manual