A.O. Smith BTH 120, Service Handbook

Page: 30 / 56

29
Servicing should only be performed by a Qualified Service Technician
10. TEMPERATURE PROBE SHORT (UPPER OR LOWER)
CHANGE BACK
ADVANCED
Upper Temp Probe (Short)
Fault occurred 0 mins ago
Upper Temperature Probe may be
shorted. Check connector and
resistance of Probe.
Press the Advanced button for
(press [DOWN] for more....)
CAUSES:
• The Upper or Lower Temperature Probe appears
to be shorted (very low resistance). This fault code
often means the connector has been corroded by
cleaning chemicals or other substances.
TEMPERATURE PROBE
This section of the Service Handbook provides information
on the Temperature Probe. The Temperature Probe contains
the ECO (energy cut out) and a Temperature Sensor. The
Temperature Probe connects to the J9 Socket on the CCB,
see "CCB Circuit Board Layout" on Page 49 and "CCB
Connection Identification" on Page 50.
ECO HIGH TEMPERATURE LIMIT SWITCH
The ECO (energy cut out) is a high temperature limit switch
designed to protect against excessively high water temperatures inside the water heater's storage tank. The
ECO is a normally closed switch that opens on a rise in temperature. The ECO switch is located inside the
Temperature Probe (two red wires). The ECO temperature setting is non adjustable; the contacts open at
201°F (94°C) and will close at approximately 140°F (60°C).
The Control System constantly monitors the state of the ECO switch contacts. If the ECO activates (contacts
open) due to abnormally high water temperature the Control System will lock out and display Energy Cut
Out (ECO) on the UIM.
Should the ECO activate, the water temperature must drop below 140°F (60°C) before the Control System
can be reset. Once the water temperature has cooled below this point the electrical circuit breaker that
serves the water heater must be turned off and on again to reset the Control System.
TEMPERATURE SENSOR
The Temperature Sensor located inside the Temperature Probe is a “thermistor” (two black wires). Thermistors
are thermally sensitive resistors. As the water temperature rises the resistance (measured in ohms) of the
sensor will decrease; as the temperature falls the resistance will increase - see "Temperatures" Table on next
page.
The Control System interprets changes in the temperature sensor resistance as changes in water temperature
and uses this data to activate and deactivate heating cycles and displays this temperature in the Control
System menus, see "(UIM) User Interface Module Screen" Figure on Page 54.
The Control System constantly monitors the Temperature Sensor. The Control System is programmed to
declare a fault condition if the resistance of the temperature sensor drops below 390 ohms (shorted) or above
56,000 ohms (open).
CHECK/ REPAIR:
• Make sure connector is clean and tight.
• Check probe’s resistance.
• Replace the Temperature Probe if shorted (resistance is below 390 Ohms). See Temperature Probe section
below.