Cleaver-Brooks CBH 40 - Switch and Cam Adjustment

Burner efficiency is measured by the amount or
percentage of C02 present in the flue gas. The
theoretical maximum C02 percentage for natural
gas is approximately 11.7%. As shown in Chart 5-5
this is attained when there is no excess oxygen (02)
or carbon monoxide (CO). A definite percentage of
excess air (oxygen) is required by most local gas
authorities and of course, the burner should never
be operated with an air-fuel ratio that indicates a
detectable percentage of carbon monoxide.
Subject to local regulations pertaining to specific
amounts of excess oxygen, it is generally recom-
mended that C02 readings of between 9-1/2 and
10-1/2% be attained with corresponding 02
readings of 2 to 4%.
H. SWITCH AND CAM ADJUSTMENT
LOW FIRE SWITCH - ALL BOILERS
The low fire switch (LFS), used on all boilers, must
be closed to complete programmer circuitry
assuring that damper is in low fire position before
ignition takes place. The switch opens when the
damper motor drives to open during pre-purge and
closes when the motor resumes its low fire position
upon completion of pre-purge. The switch there-
fore, must be actuated just prior to the damper
reaching its closed position.
The low fire switch circuit is connected to damper
motor terminals B and W. The switch makes B-W
during the closing stroke (motor shaft moves in the
direction of the CLOSE arrow on outside of case).
To adjust the switch:
1. Remove motor cover.
12
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-Per cent 02 111 Fit~ Gas
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FIGURE 5-5. FLUE GAS ANALYSIS
CHART FOR NATURAL GAS
Chapter &-Adjustment and Maintenance
Disconnect power supply before adjusting the
switch cam.
2. Insert a screwdriver in a slot in the switch
cam (white plastic) located near the center of
the motor. Each slot in the cam equals ap-
proximately 20 degrees of motor rotation.
3. Move cam the correct number of degrees by
moving cam in the direction of the OPEN
arrow on the outside of the motor case. The
switch should make at approximately 8 to 10
degrees on motor closing.
IMPORTANT!
The switch should not be adjusted to operate
closer than 5 degrees from ends of the motor
stroke.
4. Repower the motor and check the point at
which the switch makes. Readjust if
necessary.
HIGH FIRE OIL VALVE SWITCH
(CBH 25-40 and 50A)
or HIGH FIRE SWITCH (CBH 50-100)
A Q607 A switch (single microswitch) attached to
the damper motor, serves as the auxiliary switch
that actuates the high fire oil valve on the CBH 25-
40 and 50A oil fired boilers; or as the high fire
switch, when insurance requirements call for it, on
the CBH 50-100 boilers. See Figure 1-1.
The high fire switch (HFS) is used, when required,
to prove that the air damper opens during pre-
purge. Its terminals should make when the damper
is nearly open and just before the timing of the
programmer de-energizes the damper motor. See
wiring diagram for sequence timing of the control.
The following directions are based on adjusting
the switch prior to its attachment to the motor.
While particularly useful during replacement, they
also serve as a guide for final adjustment.
a) BEFORE attaching the switch to the moun-
ting bracket, depress the scale lock and rotate
the scale so that the zero mark on the black
scale is approximately at the index mark as
shown in Figure 5-6.
b) Release the scale lock and loosen thumb nut.
c) Loosen both cam locking screws. Align the
two cam lobes so that only the operational
cam (silver colored) will cause switch lever
movement. Tighten the differential cam
screw.
d) The switching action desired is for the nor-
mally open leads (red and blue) to close as
the motor reaches its open position. Using the
index mark as a reference point, push the
5-15