Adjustment Procedures Chapter 6
750-177 6-3
The Modulating Control senses changes in the hot water
temperature or steam pressure and signals the modulating
motor to control the flow of fuel and air to the burner. With
either steam or hot water boilers, the modulating control must
be set to ensure the burner is at its minimum low fire position
before the operating limit control either starts or stops the
burner.
When adjusting or setting controls, first be sure all control
devices are securely mounted and level. With the temperature
sensing control, make sure the sensing bulb is properly
bottomed in its well and is secured against movement. Be
sure the connecting tubing is not kinked.
The dial settings are generally accurate; although it is not
unusual to have a slight variation between a scale setting and
an actual pressure gauge or thermometer reading. Always
adjust control setting to agree with pressure gauge or
thermometer readings. Accurate instrument readings are
required. When necessary use auxiliary test equipment to set
controls.
Burner controls correctly set to match load demands will
provide operational advantages and achieve the following
desirable objectives:
• The burner will be operating in low fire position prior to
shut down.
• The burner will operate at low fire for a brief period on
each start during normal operation.
• Eliminates frequent burner on-off cycling.
Separate and independent controls affect modulated firing
and burner on-off cycling. Figure 6-4 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.
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
operating 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. Thus it is referred to as modulated firing.
Figure 6-3: Hot Water Operating Controls
Figure 6-2: Steam Controls
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