LAARS NeoTherm NTV (Sizes 150–285 MBTU/h) - Service Manual - Manual (Page 55)

45 Service Manual NeoTherm 80 - NeoTherm 285
4. The outlet water temperature sensor is mounted in the shoulder of the heat
exchanger assembly, just above the point where the water outlet pipe is at-
tached to the heat exchanger. (The outlet pipe is marked with a label on top of
the unit.) Remove the top panels on the unit so you can reach the temperature
sensor.
5. Unplug the wiring connector (four wires). The connector is locked in place by a
“squeeze tab.” Press on the tab to release the connector.
6. Unscrew the sensor using the 14 mm socket or a crescent wrench. Turn
counter-clockwise to unscrew the part.
7. To reassemble, reverse the process we have just described. Be sure to use
pipe dope or Teflon® tape on the threaded part of the sensor.
Removing and Replacing the Outlet Water Temperature Sensor
(continued)
Testing the Temperature Sensors
The temperature sensors used in this unit are devices called “thermis-
tors.” The electrical resistance across a thermistor drops as the tem-
perature rises. The table below shows some typical resistance readings
at different temperatures.
There are three temperature sensors mounted directly on the NT unit:
• Inlet water temperature sensor
• Combined outlet water temperature sensor and water temperature
high limit (duplex sensor)
• Stack sensor (duplex sensor)
There are up to three more sensors mounted at remote locations:
• System sensor
• DHW sensor (if installed)
• Sensor for Outdoor Reset function (if installed)
Each “duplex” sensor assembly actually uses two thermistors. The con-
trol module compares the readings of the two. If the difference becomes
greater than allowed, the controller declares a lockout and prevents the
NeoTherm unit from firing.
To check a thermistor, unplug the sensor wires and check the resistance
through the thermistor using a volt-ohmmeter. Note the temperature of
the part, and check the resistance against the table. If there is an open-
circuit or no resistance at all, replace the part.
The connector for a “duplex” sensor will include four pins. Two of these
run to one thermistor, and two go to the other. You should see the same
resistance reading on both of the thermistors in the assembly.
°F ±°F Ohms
-22 1.6 176.1 kΩ
-4 1.6 96.7 kΩ
14 1.6 55.2 kΩ
32 1.6 32.6 kΩ
50 1.6 19.9 kΩ
68 1.6 12.5 kΩ
77 1.6 10.0 kΩ
86 1.6 8.0 kΩ
104 1.4 5.3 kΩ
122 1.4 3.6 kΩ
140 1.4 2.4 kΩ
158 1.6 1.7 kΩ
176 1.8 1.2 kΩ
194 2.0 912 Ω
212 2.2 674 Ω
230 2.5 506 Ω
248 2.7 384 Ω
266 2.9 296 Ω
284 3.1 230 Ω
302 3.2 181 Ω
Table B28-1 - Thermistor Resistance Chart
B27
cont.
B28