Laars MASCOT LX, Service Manual

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Mascot LX Service Manual
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 Mascot
LX unit:
• Inlet water temperature sensor
• Combined outlet water temperature sensor and water temperature
high limit (duplex sensor)
• Combined flue temperature and flue safety sensor (duplex sensor)
On a “Combi” unit which produces Domestic Hot Water (DHW), another
sensor is mounted above the flat-plate water heater behind and below
the unit:
• DHW high temperature limit sensor
Other sensors may be 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
control module compares the readings of the two. If the difference (the
“drift”) becomes greater than allowed, the controller declares a lockout
and prevents the Mascot LX 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 B9-1 - Thermistor Resistance Chart
B9
Testing the Temperature Sensors