LAARS NeoTherm NTV (Sizes 399-850 MBTU/h) - Service Manual - Manual (Page 57)

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Service Manual - NeoTherm 500
The stack temperature limit switch will shut down the
NT unit if the stack temperature rises above 195°F.
This is a “duplex” sensor - it actually includes two
separate thermistors. The control system constantly
compares the readings from the thermistors. If there
is a large difference between the readings, or if one
of the thermistors is “open” or “shorted,” the control
system will shut down the NT unit, and display a
“Hold 95” error message.
Tools and equipment required:
• Crescent wrench or 17 mm socket
Procedure:
1. Turn off power to the system. Use the main dis-
connect switch mounted above the front panel.
2. The stack temperature limit switch is mounted in
the “far” end of the heat exchanger assembly (the
end which is farthest from the operator control
panel.) You should be able to reach this switch
from the top of the unit. Remove the top panels
so you can see the switch.
3. Unplug the wiring connector (four wires). The
connector is locked in place by a “squeeze tab.”
Press on the tab to release the connector.
4. Unscrew the sensor using the 17 mm socket or
a crescent wrench. Turn counter-clockwise to
unscrew the part.
5. To reassemble, reverse the process we have just
described. There is no need to use pipe dope or
Teflon® tape on the threaded part of the sensor.
Fig. B25-1 - Stack temperature limit switch (typical)
B25
Removing and Replacing the Stack Temperature Limit Switch