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Operating information
(continued)
Sequence of operation
OPERATION
1. Upon a call for heat, the gas pressure switch(es) must be closed.
2. Once the gas pressure switch(es) are closed, the control turns on the appropriate pumps (system and boiler pumps for
space heating, DHW pump for DHW). The flow switch and/or LWCO must close.
3. The control turns on power to the louver relay. The louver proving switch, air pressure switch, and blocked drain switch
must close.
4. The control starts the prepurge cycle by initiating the blower.
5. The control starts the trial for ignition by firing the spark electrode and opening the gas valve.
6. 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. On the 500 and larger models, the control will lock out if this second attempt also fails.
On the 400 model, the control will perform a total of 4 attempts before locking out. Once the control has locked the unit
out, the RESET button will need to be pressed on the touch screen LCD.
7. If flame is detected, it holds the firing rate steady for a few seconds to let the flame stabilize, then it begins to modulate
the firing rate based on a set point or some other command (such as a 0-10V BMS signal).
8. If the space heating call for heat is active, and the tank thermostat or sensor starts a DHW call for heat, the boiler will
switch to the DHW mode. If programmed for normal DHW operation (not as a zone), the DHW pump will turn on first,
then the boiler pump will turn off (boiler and DHW pump operation briefly overlap to ensure flow is maintained through the
unit). This will divert the boiler’s outlet water from the heating system and send it to the tank coil instead. The control will
then modulate to maintain the outlet temperature to the DHW boiler set point.
9. If the boiler is not part of a Cascade, and both the space heating and DHW calls for heat remain active long enough, the
boiler will switch back and forth between the two heating modes until one of them is satisfied.
10. Once both calls for heat are satisfied, the control will turn off the burner. The blower will continue to run during the
postpurge period.
11. Any pumps that are running will continue to run for their respective pump delay times before turning off, unless
programmed to remain on continuously. A 60 second anti-cycle period will start, which will delay any new call for heat
until it times out.
12. In Standby, ready to start a new cycle.
Installation & Operation Manual
