Adjustment Procedures
6-6
750-211 (revised 2009)
Promethean Boilers, Model 4WI Manual
Separate and independent controls affect modulated firing and burner on-off cycling. The Firing Graph depicts a
typical setting relationship of the operating limit control, modulating control, and the high limit control.
The burner will be “on” whenever the pressure or temperature is less than point B and “off ” whenever pressure or
temperature is greater than point A. The distance between points A and B represents the “on-off ” differential of
the operating limit control.
FIGURE 6-6. Firing Graph
In normal operation, the burner will shut down whenever the pressure or temperature rises above setting A. At that
point the switch in the operating limit control will open. As the pressure or temperature drops back to B, the oper-
ating limit control closes and the burner will restart. The modulating control will signal the modulating motor to be
in a low-fire position. If the load demands exceed the low-fire input potential, the modulating control will increase
the firing rate proportionately as pressure or temperature falls toward point D. The modulating motor will stop at
any intermediate point between C and D whenever the fuel input balances the load requirement.
As the load requirement changes, the firing rate will change accordingly. This it is referred to as modulated firing.
Point D represents the maximum firing rate of the burner, or high-fire. In the event pressure or temperature drops
while the burner is firing at high-fire, it indicates that the load exceeds the capacity of the boiler.
The Firing Graph shows that point B and point C do not coincide. Extreme load conditions could require the
points be closely matched.
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