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1 Service1 Service (continued)
Table 1-1 Sequence of Operation
OPERATION DISPLAY
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 a prepurge cycle by initiating the blower.
5. The control starts a 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 501 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.
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).
a
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Service Manual