T500 Series Non-Programmable Thermostats Product/Technical Bulletin
17
T
roubleshooting
Table 8: Troubleshooting
Symptom Possible Cause Corrective Action
No display/faint display ...
Supply voltage incorrect Use a voltmeter to check the voltage between the 24V and
24V(c) terminals. Voltage should be between 20-30 VAC. If
voltage is less than 20 VAC, disconnect the thermostat and
check the voltage between 24V and the other system wires;
see possible causes below. If voltage is greater than
30 VAC, troubleshoot the power source and replace the
thermostat.
System transformer weak
or overloaded
Check and/or replace with a suitable 24V transformer.
Thermostat damaged
because system voltage
was greater than 30 VAC
Replace with new thermostat, and ensure new thermostat is
isolated from the system using suitable relays and a
transformer of the proper rating.
Keyboard inoperative ...
Keyboard locked Switch the keyboard DIP switch to the unlocked position.
Thermostat will not call for
heat ...
Short cycle delay still in
progress
Wait 4 minutes, equipment short cycle protection in
progress.
Thermostat setpoint is
satisfied
Raise the heating setpoint using the
∧
∧∧
∧
button.
T500HPN-1 ONLY: If DIP
switch number 5 is in the
“on” position, the auxiliary
heat will not come on when
the setpoint is raised more
than two degrees above
room temperature.
(See Table 2.)
Move the switch to the “off” position to force on the auxiliary
heat when the setpoint is raised.
Thermostat will not call for
cooling ...
Short cycle delay still in
progress
Wait 4 minutes, equipment short cycle protection in
progress.
Thermostat setpoint is
satisfied
Lower the cooling setpoint using the
∨
∨∨
∨
button.
Fan does not turn on with
the system ...
Fan failure Place a jumper between Terminals R and G. Fan should
come on. If it does not, troubleshoot the fan system. If fan
does come on, replace the thermostat.
“AC” is displayed on the
LCD ...
20-30 VAC absent from
R and C terminals
Using a voltmeter, measure voltage between the 24V and
24V(c) terminals. If the reading is less than 20 VAC, check
system transformer. If the voltage is between 20-30 VAC,
replace the thermostat.
* The Non-Programmable T500 Thermostats do not have a
supercap back up. The “AC” on the LCD will therefore
never flash.
LCD shows missing or
extra segments ...
LCD failure Replace the unit.
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