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10 Operating information (continued)
57
Installation & Operation Manual
Sequence of operation
1.
Upon a call for heat, the control turns on the appropriate pumps (system and boiler pumps for a space heating call,
HW pump for a hot water generator call).
2.
The control confirms that the low water cutoff contacts are closed and energizes the louvers (optional) and damper
(optional) relays.
3.
The control confirms that the gas pressure switch, blocked drain switch, limits, louver proving switch (optional) and
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 the gas valve.
6.
After a short wait, the control stops sparking and checks for the presence of flame current through the flame sense
electrodes.
7.
If flame is not detected after the sparking ends, the control will perform a postpurge, then start another prepurge cycle
and try to light the burner again.
8.
If the control detects flame current, the control will hold the blower speed constant for a few seconds to allow the
flame to stabilize, then begin modulating the firing rate in order to maintain the controlling sensor to the desired set
point temperature.
9.
If the current call for heat is for space heating and a HW call for heat becomes active, the control will turn on the HW
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 HW outlet set point temperature (Normal Mode Only).
10.
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 air metering valve will be opened, the large gas
valve will be turned ON, the small gas valve will be turned OFF, and the spark igniter will be turned ON if the flame
current is low 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.
11.
Once both the space heating and HW calls for heat are satisfied, the control will turn off the gas valve and begin 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.