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TESTING THE THERMISTORS
10
There are two thermistors that are used in controlling the temperature of the water in the heat exchanger system. The T- in that sam-
ples the temperature of the water going into the heat exchanger and the T-Mid that samples the temperature halfway through the
heat exchanger. The Control Board constantly monitors the temperature at the T-Mid Thermistor and adjusts the burner pressure by
way of the Modulating Valve according to how far away that temperature is away from the set point temperature.
Thermistors are devices that change their resistance value with changes in temperature. By running a constant voltage through the
thermistor, one can read the change of voltage caused by the change of resistance thus determining the temperature of the thermis-
tor. There are test points on the control board (see above) that can be read with a multi-meter on the Direct Current Volt (DCV) scale
for each thermistor.
To test the temperature at the T-in Thermistor
There is a BLACK test point along the top edge of the control board on the right side of center. This is where the NEGATIVE
(Common/Black) probe of a multi-meter is inserted. There is an ORANGE test point along the top edge of the control board on the
left side of center. This is where the POSITIVE (Red) probe of a multi-meter is inserted. A DC voltage reading is then taken. Find
the voltage on the chart below and find the corresponding temperature.
To test the temperature at the T-Mid Thermistor
Using the same BLACK test point with the NEGATIVE probe of the multi-meters in testing the T-in thermistor above, also find the
YELLOW test point on the left side of the board just under the large plug (see above). Insert the positive probe of the multi-meter
and read DC Volts. Since the two thermistors have the same values, the same chart is used.
Gas Pack Thermister Conversion
0
0.5
1
1.5
2
2.5
3
3.5
4
50(10)
60(15.6)
70(21.1)
80(26.7)
90(32.2)
100(37.8)
110(43.3)
120(48.9)
130(54.4)
140(60)
150(65.6)
160(71.1)
170(76.7)
180(82.2)
190(87.8)
200(93.3)
210(98.9)
Temperature °F (°C)
Voltage DC