Adjustment Procedures
5-6
750-96 (revised 2010)
Model CB Packaged Boiler Manual
In normal operation, the burner will shut down whenever the pressure or temperature reaches setting A. The
switch in the operating limit control will open. As the pressure or temperature drops back to B, the switch makes
and the burner will restart. The modulating control will be calling for the modulating motor to be in low fire position
at this point. If the load exceeds this low fire input, the modulating control will respond to increase the firing rate
proportionately as pressure or temperature falls toward point D. The modulating motor will stop at any intermedi-
ate 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 is referred to as modulated firing.
Point D represents the maximum firing rate of the burner. In the event pressure or temperature drops while the
burner is firing at its maximum input, this is indicative that the load exceeds the generating rate of the boiler.
Although a gap is shown between B and C, these points may well coincide if required by load conditions. When set
as shown, the burner will be in a low fire position upon a restart and will fire at that rate for a short period of time
before falling pressure or temperature requires an increase in the firing rate. From this illustration it can be seen
that this desirable objective will not be attained if setting C overlapped point B. In that event, upon a restart, the
burner would drive to a higher firing position immediately after main flame was proven, and the brief period of low
heat input would not occur. Actual settings will depend greatly upon load conditions which will affect the amount of
differential permitted the operating limit control and to the gap, if any, between B and C.
When firing a cold boiler, it is recommended that the burner be kept under manual flame control until normal oper-
ating pressure or temperature is approached. The size of the flame may be manually and gradually increased to
build up pressure or temperature. If the burner is not under manual control on a cold start, it will immediately move
to high fire as soon as the program control releases the circuit that holds the burner in low fire during ignition. The
modulating control will be calling for higher fire and the burner will move to that position as rapidly as the damper
motor can complete its travel. This rapid heat input can subject the pressure vessel metal and refractory to unde-
sirable conditions.
Any control setting must not cause the boiler to operate at or in excess of the safety valve setting. Settings that
do not exceed 90% of the valve setting are recommended, with lower settings greatly desirable if load conditions
permit. Avoid having the operating pressure too near the valve set pressure, because the closer the operating
pressure is to the valve pressure, the greater the possibility of valve leakage. Continued leakage, however slight,
will cause erosion and necessitate early valve replacement. The control settings on a hot water boiler must be
within the temperature and pressure limits of the boiler.
Ideally, the burner operating controls should be set under actual load conditions. Often especially on new con-
struction, the boiler is initially started and set to operate under less than full load requirements. As soon as possi-
ble thereafter the controls should be reset to provide maximum utilization of the modulating firing system.
To accomplish this, and assuming that air/fuel combustion ratios have been set, make approximate adjustments
to the controls to bring the boiler pressure or temperature to meet the load requirements.
To properly set the modulating control, carefully adjust it under load conditions, until the load is maintained with
the burner firing at a steady rate. The firing rate at that point may be full high fire or slightly less, depending upon
the relationship of the boiler size to the load.
When the modulating control is set in this manner and if the burner is in full high fire, the scale setting of the mod-
ulating pressure control on a steam boiler will have a reading that indicates the low point of the modulating range.
This fixed differential range is described later in this section. The scale setting of the modulating temperature con-
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