19
Faulty domestic hot water thermistors will allow the boiler to operate in central heating
mode. If a demand for hot water is made during the heating sequence then the boiler will
lock out. (Fault 10). Values of the thermistor can be checked by measuring voltage in
parallel at J2 (black /black) with the power on at the boiler and the plug in situ on the PCB.
To check values by resistance, remove the multi pin plug from the P.C.B and check values
on the (black/ black) using 20
Ω
range.
IMPORTANT.
The D.H.W. thermistor on the FEB-24E is the same type as the central
heating thermistor used on the FEB-20E. New thermistors are not supplied with the
sealing “O” ring. Please remove and re-use existing “O” ring.
22. CENTRAL HEATING THERMISTOR
The central heating thermistor is a clip on type, located either on the heating flow pipe
above the hydraulic unit or on later models on the top left hand side of the primary heat
exchanger. Temperature range for central heating is 60°C -85°C.
The thermistor detects the temperature of the heating circuit (primary circuit in D.H.W.
mode) and provides the initial over heat protection for the boiler at 105 °C with fault 6
displayed. A faulty thermistor affects both the central heating and hot water.
A thermistor that is out of range or open circuit will cause the boiler to lock out (fault 12)
As the thermistor cools and re-enters normal operating range reset will automatically take
place, if the thermistor is faulty or open circuit then it must be replaced.
The thermistor also provides frost protection for the boiler if a temperature of 6°c is
detected the pump will run until the temperature reaches 9°C if the temperature continues
to fall until the thermistor reads 3°C Then the boiler will ignite at minimum output until the
heating circuit temperature is 20°C. To check values of the thermistor use the same
procedure as for checking the D.H.W. thermistor, or gain access directly to the thermistor
via the boiler without the need for entering the PCB
TEMPERATURE
0
C
VOLTAGE (VOLTS)
RESISTANCE (K
OHMS)
85
0.92
1.44
80
1.01
1.65
75
1.15
1.91
70
1.28
2.21
65
1.41
2.57
60
1.54
2.99
55
1.71
3.51
50
1.93
4.13
45
2.13
4.87
40
2.34
5.78
35
2.55
6.89
Summary of Contents for FEB-24E UK
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