750-96 (revised 2010)
Model CB Packaged Boiler Manual
5-7
5.5 — Burner Operating Controls: General
trol on a hot water boiler will have a reading that indicates the midpoint of the modulating range. This is also
described late.
The operating limit control should now be adjusted and its differential established. See directions later in this sec-
tion for the mechanics of adjusting. In an installation that does not require a very close control of steam pressure
or water temperature, this adjustable differential should be set as widely as conditions permit, since this will pro-
vide 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 to do so. The setting of this 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 preferably not exceed
90% of the valve setting. The control requires manual resetting after tripping.
In the setting of these controls, consideration must be give to the time required for a burner restart. Upon each
start, there is a pre-purge period of some length, plus the fixed time required for the proving of the pilot and main
flame. This, plus approximately 1/2 minute required for damper motor travel from low to high fire, may allow pres-
sure or temperature to drop below desirable limits.
The mechanics of setting the controls:
Control Setting
1. Modulating Pressure Con-
trol: Steam, Honeywell L91A
Turn the adjusting screw until the indicator is opposite the low point of the desired
modulating range. Modulated firing will range between this point and a higher point
equal to the modulating range of the particular control In 0-15 psi controls, the range
is 1/2 lb.; in 5-150 psi controls, the range is 5 lbs.; in 10-300 psi controls, the range
is 12 lbs.
NOTE: To prevent burner shutdown at other than low fire setting, adjust the modulat-
ing pressure control to modulate to low fire BEFORE the operating limit pressure con-
trol shuts off the burner.
2. Operating Limit Pressure
Control: Steam, Honeywell
L404A
Set “cut-out” (burner off) pressure on the main scale using the large adjusting screw.
Set the differential on the short scale by turning the small adjusting screw until the
indicator points to the desired difference between cut-out and cut-in pressures. The
“cut-in” (burner on) pressure is the cut-out pressure MINUS the differential. The cut-out
pressure should not exceed 90% of the safety valve setting.
3. High Limit Pressure Con-
trol: Steam, Honeywell
L404C
Set “cut-out” (burner off) pressure on scale using the adjusting screw. The control will
break a circuit when pressure reaches this point. The setting should be sufficiently
above the operating limit pressure control to avoid shutdowns, and preferably not
exceed 90% of the safety valve setting. This control requires manual resetting after
tripping on a pressure increase. To reset, allow pressure to return to normal and then
press the reset button.
4. Modulating Temperature
Control: Hot Water, Honeywell
T991A
Turn the knob on front of the case until the pointer indicates desired setpoint temper-
ature. This is the center point of a proportional range. The control has a 3º to 30º dif-
ferential and my be adjusted to vary the temperature range within which modulating
action is desired. With the cover off, turn the adjustment wheel until the pointer indi-
cates the desired range.
NOTE: To prevent burner shutdown at other than low fire setting, adjust the modulat-
ing temperature control to modulate to low fire BEFORE the operating limit tempera-
ture control shuts off the burner.
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