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3
THEORY OF OPERATION
3.1
ST Models
STARTING CIRCUIT (FIGURES 3.1.1 & 3.1.2)
When the two-stage thermostat signals the low-fire
circuit, the fan relay is energized. When the
operational static pressure is achieved the normally
open pressure switch closes initiating ignition
sequence. After a 7 second pre-purge, the spark
electrode and gas valve are energized simultaneously.
The trial for ignition is 15 seconds.
Stage two on the gas valve is powered directly from the
second stage of the thermostat. The gas valve will not
pass gas unless the first stage sequence of operation has
been completed. The thermostat will determine which
stage is required to maintain the desired comfort level.
RUNNING CIRCUIT
After ignition, the flame rod monitors the flame. As
long as a flame is present, the valve is held open. If
the flame is lost, the control acts to close the valve with-
in one second, and a new trial sequence identical to
that at start-up is initiated. If proof of flame is not
established within the 15 second trial for ignition within
the three try cycle, the unit will lock out. If lockout occurs,
the control can be reset by briefly interrupting the power
source.
SHUTDOWN
When the thermostat is satisfied the fan will enter into a
120 second post-purge cycle.
Figure 3.1.1
Figure 3.1.2
27
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