Cleaver-Brooks CBH 40 - Combustion Air Proving Switch; Low Gas Pressure Switch

Chapter 5-Adjustment and Maintenance
to restart the burner. The unit will fire at its low
rate unless temperature continues to drop to 16.5
degrees, at which time the high-low fire control
will close to move the burning rate to high fire.
The settings listed are typical and will vary ac-
cording to job requirements. However, setting the
controls with these relations to each other is
desirable, since this will prevent the burner from
shutting down while in high fire or from im-
mediately going to high fire upon restarting.
OPERATING LIMIT
TEMPERATURE CONTROL
(Hot Water) (Honeywell L4008A)
Set "cut-out" (burner off) temperature on scale by
inserting a screwdriver through the cover opening
to engage the slotted head adjusting screw.
The "cut-in" (burner on) temperature is the cut-
out temperature MINUS the differential. The dif-
ferential is adjustable from 5 to 30 degrees F. Dif-
ferential is adjusted by rotating the wheel on the
back of the snap switch.
HIGH LIMIT TEMPERATURE CONTROL
(Hot Water) (Honeywell L 4008E)
Set the "cut-out" (burner off) temperature on scale
using adjusting screw. This control will break the
circuit and lockout on a rise in water temperature
above the setting. The setting should be sufficiently
above the operating limit temperature to avoid un-
necessary shutdowns. On 30 lb. hot water genera-
tors the setting is not to exceed 240 degrees F. The
control requires manual resetting after tripping on
a temperature increase. To reset allow water tem-
perature to drop below the cut-out setting, less dif-
ferential, and then press the manual reset button.
HIGH-LOW FIRE CONTROL
(Hot Water) (Honeywell L4008A)
Set the "cut-out" temperature-the point at which
the burner will return to low fire-on the in-
dicating dial. This setting should be sufficiently
below the cut-out setting of the operating limit
temperature control so that the burner will return
to the low fire position prior to shutting off at the
operating limit.
The "cut-in" temperature-the point at which the
burner drives to high fire-is set on the differential
scale. This setting is equal to the cut-out tempera-
ture MINUS the amount of the differential. It
should be adjusted so that it is sufficiently below
the burner "on" temperature of the operating con-
trol so that the burner when starting can operate
briefly in the low fire position prior to advancing
to high fire.
COMBUSTION AIR PROVING SWITCH
Air pressure against the diaphragm actuates the
switch which, when made, completes a circuit to
prove the presence of combustion air. Since the
5-6
pressure of the combustion air is at its minimum
value when the damper is full open, the switch
should be adjusted under that situation. It should
be set to actuate under a condition of minimum
pressure, but not too close to that point to cause
nuisance shutdowns.
The test switch in the programmer will stop the
timer only during the low fire period of the pre-
purge or during the pilot ignition period. To stop
the timer during the high fire portion of prepurge,
it is necessary to do the following:
CB-20 - Turn off both the burner switch and the
main power. Remove the programmer from its
base. Check the instruction manual for the control
and locate the position of contact M3A. Carefully
work a narrow strip of paper between the contact
points. An alternate method is to insert a thin
wood wedge between the timer cam and the spring
leaf for this contact.
Use extreme care not to bend or damage the con-
tacts. CAUTION: Line voltage is present at most
contacts.
Reinstall the control and restore main power.
CB-40 - Remove the purge extender or the purge
extender jumper. See the instruction manual for
details.
Turn the burner switch on. The blower motor and
the timer will start (provided the low water control
and limit controls are made). The timer will stop
after a few seconds. The damper motor will drive
to its open position and stop there.
Attach a test lamp or volt meter from switch ter-
minal W to ground. Turn the adjustment screw un-
til this circuit is broken. Then add a half turn or so
to the adjusting switch to remake the circuit.
Turn the burner switch off. Remove the paper slip
or wood wedge from the CB-20 relay Replace the
purge extender or purge extender jumper on a CB-
40 relay.
With the programmer returned to operating con-
dition, turn the burner switch on and observe
whether unit cycles normally.
If the air switch is not adjusted to make within its
allotted time, the CB-20 control will keep
recycling. The CB-40 control will lock out and
require manual resetting to restart.
LOW GAS PRESSURE SWITCH
Turn adjusting screw until indicator on scale
moves to a pressure setting slightly below ·the
operating gas pressure. The control will break a
circuit if pressure is below this value. The control
should be finally adjusted to prevent operation
with low gas pressure, but should not be set at a
value close enough to normal operating pressure to
cause unnecessary shutdowns. When setting this