5-3
TROUBLESHOOTING
SL 20-115 MODULATING GAS BOILER
5.2.2 Fan/blower
operating power is provided by means of a separate 120 VaC connector at the
bottom of the fan (white/black/green). Control of the fan is provided via a four
lead connector at the top of the fan. this connector feeds a PWM control signal
(black wire) from the controller and provides a tachometer signal (white wire)
back from the fan. unplugging the control connector will cause the fan to go to
high speed and trigger a “Blocked Vent error” within 6 seconds if the boiler is
operating.
LEAD COLOR
FUNCTION
TROUbLESHOOTING
red
35 VdC Positive power
terminal
Fan will only operate at max.
speed if disconnected.
Blue
35 VdC negative power
terminal
Fan will only operate at max.
speed if disconnected.
Black
signal from controller
Fan will only operate at max.
speed if disconnected.
White
Fan tach.
2 pulses/rev (freq x 30=rpm)
Table 10: Fan Operation
5.2.3 Differential Air Pressure Sensor
go to the
Advanced Diagnostics
screen, and with the fan off (
“Fan Duty Cycle
=
0) look for a
Fan Pressure
reading of 102 +/-5. next check with fan in operation,
and look for an increased
Fan Pressure
value. in normal operation, the
Fan
Pressure
reading should move toward the
Required Pressure
value. a backward
movement in
Fan Pressure
indicates the clear vinyl air pressure line has been
connected to the wrong port on the sensor body.
Control wire plug
Unplug to run fan full speed
Remove terminal plugs, then circuit
board cover
Removing circuit board terminal
cover
Figure 29 (right): Test
points on circuit board
for air pressure sensor
voltage readings
Circuit board