SERVICING
188 Rev. 3
Failures such as opens, shorts across the device, shorts to
ground, shorts to power and leakage path to ground shall be
sensed. The thermistor (or wire(s)) should be replaced for
proper operation.
If the LED indicates 6 flashes (system error) then the setup
and configuration of the system should be checked. This er-
ror mode could indicate an abnormal operating condition such
as a restricted inlet, blocked outlet, or possibly a leak in the
unit or ductwork. The system should be checked for such a
condition.
If the LED is in a continuous on mode (control board mal-
function), then all field and factory connections should be
checked. If the error mode still occurs after a power reset,
then the control board should be replaced.
NOTE: After an error mode occurs, the system requires
a power reset for normal operation after the problem
has been corrected.
The control board is programmed with a certain range of ac-
ceptable values from the thermistor, depending on the mode
of operation. The control board “knows” the mode of opera-
tion based on the thermostat inputs, and thus “knows” the
acceptable range of resistance readings from the thermistor.
Temp
°F
Thermistor
Res.
Ω
Temp
°F
Thermistor
Res.
Ω
50
79600
90
29610
65
54720
95
26130
75
41800
100
23100
80
36660
105
20470
85
33640
120
14970
Thermistor Resistance Table
The BBC*A2A model blowers have an interface board incor-
porated into the control circuitry. This board serves as a se-
lector for the various tonnage selections available (see unit
wiring diagram). The unit wiring diagram indicates the appro-
priate pin-positioning for each available tonnage selection.
The board also contains an “adjust” tap, which allows for a ±
15 % variation in airflow. This feature allows for an increase /
decrease of the airflow over the entire operating range. See
the tables in the "Airflow" section for the available airflow
ranges.
There is a LED included on the interface board on the BBC
models, in addition to the one found on the main control. The
LED serves to indicate the airflow that the motor is supposed
to be delivering, depending upon the positioning of the pin
selectors on the interface board. The number of blinks multi-
plied by 100 yields the programmed CFM. The indicated CFM
may vary, depending on the mode of operation and the sig-
nals being sent to the control board at the time. The variable
speed motor is controlled via a PWM (Pulse Width Modu-
lated) signal from the control board. Nominal CFM is at an
80% PWM output.
Also included on the BBC model control boards is a two-pin
header that allows for either 50% or 100% of nominal airflow
during fan only mode. The board is shipped with the connec-
tion for 50% airflow during fan only mode.
S-50 CHECKING HEATER LIMIT CONTROL(S)
(OPTIONAL ELECTRIC HEATERS)
Each individual heater element of EHK-A heater kits is pro-
tected with an automatic reset limit control connected in se-
ries with each element to prevent overheating of components
in case of low airflow. This limit control will open its circuit at
approximately 150°F. and close at 110°F.
EHK-B heater kits do not have individual limits attached to
the heater. Instead, the thermistor controls the maximum
temperature of the electric heat.
WARNING
Disconnect Electrical Power Supply:
1. Remove the wiring from the control terminals.
2. Using an ohmmeter test for continuity across the nor-
mally closed contacts. No reading indicates the control
is open - replace if necessary.
IF FOUND OPEN - REPLACE - DO NOT WIRE AROUND.
S-51 CHECKING HEATER FUSE LINK
(OPTIONAL ELECTRIC HEATERS)
Each individual heater element is protected with a one time
fuse link which is connected in series with the element. The
fuse link will open at approximately 333°F (167°C) on EHK-A
heater kits and 250°F (121°F) on EHK-B heater kits..
WARNING
Disconnect Electrical Power Supply:
1. Remove heater element assembly so as to expose fuse
link.
2. Using an ohmmeter, test across the fuse link for continu-
ity - no reading indicates the link is open. Replace as
necessary.
NOTE: The fuse link is a safety device designed to open at
extreme temperatures, possibly due to little or no airflow.
DO NOT WIRE AROUND - determine reason for failure
and correct.
S-52 CHECKING HEATER ELEMENTS
WARNING
Disconnect Electrical Power Supply:
1. Disassemble and remove the heating element.
2. Visually inspect the heater assembly for any breaks in
the wire or broken insulators.
