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Sequence of operation
Table 1A Sequence of operation - space heating and HW
Note: This unit is equipped with two (2) independent, but synchronized combustion systems. Heat Exchanger 1 (top)
combustion system will fire first. If the demand cannot be met by one (1) combustion system the same sequence of operation
will be followed to bring the Heat Exchanger 2 combustion system online.
1.
Upon a call for heat, the control turns on the appropriate pumps (system and boiler pumps for a space heating call,
HW pump relay output for a HW call).
2.
The control confirms that the low water cutoff and flow switch (optional) contacts are closed.
3.
The control starts the blower and closes the louver contacts to begin the Pre-Purge cycle.
4.
The control confirms that the blower comes up to the desired speed, the flap valve opens, and the air pressure switch,
gas pressure switches, louver proving switch (field installed), and blocked drain switch contacts close.
5.
Once the Pre-Purge cycle is complete, the control lowers the blower speed, 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 spark and
flame sense electrodes.
7.
If the control does not detect flame current, the control will lockout, until the RESET button on the touch screen
LCD is pressed.
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 pumps. It will then modulate the blower speed in order to maintain the
outlet temperature to the desired HW outlet set point temperature.
10.
If the first heat exchanger in the boiler is unable to maintain the desired set point temperature, the second heat
exchanger in the boiler will be started, using much of the same sequences as described above. Once both heat
exchangers are firing, the controls will work in synchronization to maintain the desired set point temperature. If
the heat load should decrease sufficiently, the second heat exchanger will be shut down, much like the sequences
described below.
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.
12.
At the end of the Post-Purge cycle, the louver contacts will open.
13.
The control verifies that the blower stops running and the flap valve closes.
14.
At the end of the Pump Delay cycle(s), the pumps will be turned off.
10
Service Manual