Chapter 4 — Sequence of Operation
Part No. 750-184 4-5
closed at this time, or if they subsequently open, the program relay will go
into a safety shutdown.
At the completion of the high fire purge period, the program relay signals the
modulating damper motor (MDM) to drive the air damper to its low fire
position.
To assure that the system is in low fire position prior to ignition, the low fire
switch (LFS) must be closed to complete the “low fire proving circuit.” The
sequence will stop and hold until the modulating damper motor (MDM) has
returned to the low fire position and the contacts of the low fire switch (LFS)
are closed. Once the low fire switch is closed, the sequence is allowed to
continue.
Note: The ignition trial cannot be started if flame or a flame
simulating condition is sensed during the pre-purge period.
A safety shutdown will occur if flame is sensed at this time.
Ignition Cycle - The ignition transformer (IT) and gas pilot valve (GPV) are
energized from the appropriate pilot ignition terminal.
Note: An oil-fired burner may be equipped with an oil pilot
rather than a gas pilot. The ignition sequence of both is
identical.
The pilot flame must be established and proven by the flame detector (FD)
within a 10 second period in order for the ignition cycle to continue. If for
any reason this does not happen, the system will shut down and safety
lockout will occur.
Note: Depending upon the requirements of the regulatory
body, insurer or fuel being burned, either the 10 or 15
second pilot ignition terminal may be used. Both provide the
same function but differ in time interval allowed for proving
main flame ignition. Refer to the boiler wiring diagram.
With a proven pilot, the main fuel valve(s) (OV or MGV) is energized and the
main fuel valve light (FVL) in the panel is lighted. The main flame is ignited
and the trial period for proving the main flame begins. It lasts 10 seconds
for light oil and natural gas, and 15 seconds for heavy oil. At the end of the
proving period, if the flame detector still detects main flame, the ignition
transformer and pilot valve are deenergized and pilot flame is extinguished.
Note: If the main flame does not light, or stay lit, the fuel
valve will close. The safety switch will trip to lock out the
control. Refer to flame loss sequence (section D) for
description of action.
The cause for loss of flame or any other unusual condition should be
investigated and corrected before attempting to restart. Failure to
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