750-91 (revised 2009)
Model CB-LE Packaged Boiler Manual
6-7
6.5 — Burner Operating Controls: General
When set as shown, with a time lag between B and C, 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 fir-
ing rate.
If points B and C overlap when restart occurs, the burner would drive to a higher firing position immediately after
the main flame was proven.
When firing a cold boiler, it is recommended that the burner be kept at low-fire, under manual flame control, until
normal operating pressure or temperature is reached. If the burner is not under manual control on a cold start, it
will immediately move toward high-fire as soon as the program control releases the circuit that holds the burner in
low-fire. The modulating control will be calling for high-fire and the burner will move to that position as rapidly as
the damper motor can complete its travel.
Do not operate the boiler in excess of 90% of the safety valve relief setting. The closer the operating pressure is to
the safety valve relief pressure, the greater the possibility of valve leakage. continued leakage, however slight, will
cause erosion and necessitate early safety valve replacement. The control settings on a hot water boiler must be
within the temperature limits of the boiler.
Ideally, the boiler operating controls should be set under actual load conditions. Especially under new construction
conditions, 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 maximum utilization, and assuming that air/fuel combustion ratios have been set, make the required
adjustments to the controls to bring the boiler pressure or temperature up 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 and the burner is in full high-fire, the scale setting of the modulating pressure
control on a steam boiler will indicate the low point of the modulating range. The scale setting of the modulating
temperature control on a hot water boiler will have a reading that indicates the midpoint of the modulating range.
The operating limit control should now be adjusted and the differential established. In an installation that does not
require a very close control of steam pressure or water temperature, the adjustable differential should be set as wide
as conditions permit, since a wide setting will provide less frequent burner cycling.
The high limit control provides a safety factor to shut the burner off in the event the operating limit control should
fail. The setting of the control should be sufficiently above the operating limit control to avoid nuisance shutdowns.
The setting, however, must be within the limits of the safety valve settings and should not exceed 90% of the valve
setting. The control requires manual resetting after it shuts off the burner.
NOTE: On-off cycling in excess of 8 cycles per hour will shorten the life of the combustion air motor and cause
excessive wear on switch gear and pilot electrodes.
NOTE: It is not recommended that the boiler controls be set so as to overlap the modulating control range and oper-
ating control range.
NOTE: Rapid heat input can subject the pressure vessel metal and refractory to undesirable conditions.
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