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21.2.2 Belt Tension
• Belt tension should be checked with a belt-tension
gauge when one is available. Follow the belt
tension gauge instructions.
• When a tension gauge is not available, measure
the belt span of the belts (See Page 123, Figure
75).
• Allow for 1/64" (0.40 cm) of deflection for each
inch of center distance length for the charted
pounds of force. Check Page 123, Table 34 for
proper deflection force.
• EXAMPLE: A 40" (101.6 cm) shaft center
distance would dictate 40"/64" or 5/8" (1.59
cm) of deflection. With a standard B-belt and
a motor sheave measuring between 5.8"
(14.7 cm) and 8.6" (21.84 cm), the belt will
have proper tension if a 5/8" deflection can be
achieved with a minimum of 6-3/8 lbs. and a
maximum of 8-3/4 lbs. of pressure as
measured with a belt tensioning gauge.
FIGURE 75: Belt Tension
Table 34: Deflection Force B and BX Belts (in lbs)
Belt
Motor Sheave
Cross-
Dimension Range
Section
in - cm
3.4-4.2
B
4.4-5.6
5.8-8.6
NOTE: If drive belts squeal during start-up, increase belt tension
to the highest allowed value. Re-check tension during each
inspection. See Page 129, Section 22 for additional information.
21.2.3 Air Temperature Sensing
The sensing probe is factory mounted in the blower
housing (Not included if customer orders BMS-
ready).
TYPE B-X
TYPE B
(High HP)
Min.
Max.
Min.
Max.
4
5 1/2
5 3/4
8
5 1/8
7 1/8
6 1/2
9 1/8
6 3/8
8 3/4
7 3/8
10 1/8
21.3 Electrical
1.
Check motor starter for proper overload set-
tings. The overload setting should meet full
load amps (FLA) of motor.
2. Measure the supply voltage with the air handler
off and then on. For a system that is powered
with three phase power, measure the voltage of
each phase.
3. Verify correct blower rotation.
4. While the blower is running and the burner is
off, measure the total system current draw with
an ammeter. Measure the system current draw
again after the burner adjustments are made
and with the burner and blower both on.
5. If applicable, compare all variable frequency
drive (VFD) programming parameters with
specifications provided on electrical drawing.
21.4 Airflow
The air flow switches are factory calibrated safety
devices for burner air flow. If an air flow switch does
not close, the problem may not be the air flow switch.
It could be an indication of an air flow problem
(incorrect blower rotation, duct restrictions, etc.).
21.4.1 Differential Pressure
In order to verify proper airflow across the burner, the
differential pressure across the profile plate needs to
be measured. Attach a manometer to the pressure
test ports where the pressure switch is attached and
measure the differential pressure with a manometer.
This reading must be 0.9 in wc +/- 0.1 in wc.
(2.24 mbar +/- 0.25 mbar)
To adjust the differential pressure that was measured
in the step above, use the adjustable sheave. To
decrease the speed of the blower and the differential
pressure, turn the adjustable half of the sheave
outward (decrease diameter). To increase the speed
of the blower and the differential pressure measured
at the burner, rotate the adjustable half of the sheave
inward (increase diameter). Both sides of the sheave
must be turned equal, multiple grooves must be
adjusted equal.
After any adjustment, it is necessary to re-check the
alignment, belt tensions and verify that all sheave set
screws are tightened to the specified torque value
before engaging the blower. See Page 124, Table 35
and Table 36. Typically, all sheaves have two set
screws to secure the sheave to the motor shaft.
Some sheaves may be press fit onto the motor shaft.
On the two belt sheaves, there are four setscrews
that hold the size adjustment. On the single belt
sheaves, there are two set screws to hold the size
SECTION 21: S
-
P
TART
UP
ROCEDURES
123 of 154
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