Temperature Sensors - A.O. Smith 100 Series Service Handbook

TEMPERATURE SENSORS

Temperature sensors are 3/4 inch male threaded immersion probes. Temperature probes have embedded temperature sensors
(thermistors). The boiler/water heater's control system monitors these sensors to determine water temperature at various points
in the system. Thermistors are thermally sensitive resistors; as the surrounding temperature rises the resistance of the thermistor
(measured in ohms) will decrease and as the surrounding temperature falls the resistance of the thermistor increases. The Boiler/
water heater's Control System monitors these sensors to determine water temperature at various points in the system.
INLET / REMOTE TEMPERATURE PROBE
OUTLET TEMPERATURE PROBE
FIGURE 12. TEMPERATURE SENSORS
Inlet and Outlet Temperature Sensors
All boiler/water heater models have two inlet and two outlet temperature sensors for each heat exchanger, factory installed to monitor
the water temperature entering and leaving the boiler/water heater. The Inlet Temperature Probe is a temperature sensor only and
has two leads. The Outlet Temperature Probe also contains the manual reset high temperature limit switch and has four leads. The
control system displays the Inlet and Outlet water temperatures sensed from these two sensors on the default Temperatures screen.
Remote Sensors
All boiler/water heater models are supplied from the factory with a remote sensor. The remote sensor is used to control system water
temperature for a single boiler/water heater in a domestic hot water storage tank or in the return line from a primary/ secondary
hydronic heating system.
The boiler/water heater will modulate its firing rate in response to the actual system temperature and load conditions. The control
system displays the temperature sensed from the remote sensor as the "Lead Lag" temperature on the default Temperatures screen.
In addition to these sensors the XB hydronic boilers receive an outdoor sensor for monitoring the outdoor temperatures. The outdoor
reset functionality is disabled by default and can easily be turned ON at start up.
20 Servicing should only be performed by a Qualified Service Agent