CleaverBrooks 4WI, Operation, Service And Parts Manual

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Burner Operation and Control
2-20 750-211 (revised 2009)
Promethean Boilers, Model 4WI Manual
2.15 — Modulating Firing
The modulating motor, through a linkage arrangement, controls the air damper and the butterfly gas valve, or the
oil metering valve, to maintain a constant air/fuel ratio throughout the firing range.
FIGURE 2-19.
Burner Drawer and Fuel Linkage Assembly
During burner operation, the motor is controlled by a modulating pressure control on a steam boiler, or by a mod-
ulating temperature control on a hot water boiler. A manually operated potentiometer is provided to permit posi-
tioning of the motor at the desired burner firing rate. The potentiometer is used primarily for initial or subsequent
checking and setting of fuel input. Normal operation should be with the manual-automatic switch in the “auto-
matic” position and under the control of the modulating control.
The modulating motor (commonly called a damper motor) is reversible. It has an internal limit switch that restricts
shaft rotation to 90°. During normal operation the motor will move in either direction or stop at any position
within the range.
The motor potentiometer is electrically connected to a matching potentiometer in the modulating control. Chang-
ing steam pressure or water temperature alters the electrical resistance of the modulating controller potentiometer.
The change in resistance compels an integral balancing relay to start, stop, or reverse the motor rotation. Rotation
in either direction continues until the resistance ratio of the two potentiometers is equal.
When the resistance ratio is equal, the motor stops in a position that allows the proper fuel and combustion air
flow to meet operating demands.
A feature designed into the circuitry maintains the modulating motor in the low-fire position during ignition and
keeps it there until the main flame is established. A low-fire switch, integral to the motor, is actuated by the rotation