Lochinvar FTxl 500 Service Manual page 12

1
Service
Table 1-1 (continued from previous page) Sequence of Operation
OPERATION
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.
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Service Manual
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