Fault Finding
64
12 Fault Finding
12.4 Boiler Overheat Thermostat
Note 1 - Is heating system and boiler
full of water and vented?
The central heating system must be
free of any air locks. Ensure auto air
vent on secondary heat exchanger (at
left rear of the boiler) is open and
vented. Vent the primary heat
exchanger on the heating flow pipe.
If installed on a sealed system: check
expansion vessel charge pressure and
correct as necessary. Check system
pressure and top-up as necessary.
Refer to Section 7 for sealed system
details.
Note 2 - Is the heating thermistor
correctly fitted?
Isolate electrical supply and remove
boiler control panel cover.
Check that the heating thermistor, and
the overheat thermostat sensor, are
correctly fitted into the pocket (inside
the boiler control panel). Refer to
Section 10.3.1. Ensure that both are
held in place with the retaining clip
supplied.
Also, check the condition of the
thermistor and the overheat thermostat
capillary for any damage, replace if
necessary.
Note 3 - Is there continuity between
the overheat thermostat terminals?
With electrical supply isolated, reset the
overheat thermostat and check
electrical continuity across the
terminals. If no continuity found, replace
the overheat thermostat.
Note that the thermostat continuity can
be checked by removing the 7-way
plug (plug No. 1) from the socket on the
left of the burner and testing between
terminals T1 and T2. Refer to Section 8
and Figure 8-5.
Note 4 - is the circulating pump
operational?
With the burner ON/STANDBY switch
set to ‘STANDBY’, reconnect the
power supply to the boiler and check
that the heating system controls are
‘calling’. Set the switch to ON and the
pump should be running.
If not check that:
• the pump is vented
• there is power to the pump
• that the pump is not seized/stuck
If pump has failed then replace it.
Note 5 - Is there any restriction to
flow from the boiler?
Check that all valves on the heating
circuit open and that there are no
blockages.
Rectify as necessary.
Primary Fault
Secondary follow on checks
BLANK DISPLAY SCREEN
Is the central
heating system
& boiler full of
water & vented of
any air locks?
Yes
No No No No
Is the heating
thermistor correctly
fitted in the pocket
(inside the boiler
control panel)?
Unclip & remove
lead from stat &
check
Is overheat stat
capillary in good
condition & not
damaged, kinked
or broken?
Press reset.
Is there now
continuity between
the overheat stat
terminals?
Is the system
circulating pump
operational?
Is there any
restriction to
water flow from
the boiler?
Remove any
restriction e.g.
open any valves,
or remove
blockage
preventing flow
Correctly fit
heating thermistor
in pocket &
reset overheat
stat
Repalce faulty
overheat stat
Repalce faulty
circulating pump
BOILER OVERHEAT STAT TRIPPED
Yes Yes
No
Note 1 Note 2 Note 3 Note 4 Note 5
Yes Yes Yes
Fill & vent
system
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