37
221469B
Diagram 3.7 continued
Isolate supply. Is there
continuity between
P.C.B. and modulator?
Reduce D.H.W. flow to
3.7 litres/minute. Does
burner pressure reduce?
Modulation
O.K.
Rectify as necessary.
Faulty modulator,
replace.
Measure D.H.W.
temperature. Is
it between 57
and 63
o
C?
Isolate supply. Remove block 4 from
P.C.B. As boiler cools does resistance
across 'D.H.W. thermistor' pins on
block 4 increase from approx
2.5K Ω to 10KΩ ?
Faulty thermistor, replace.
Faulty modulator, replace.
Faulty modulator,
replace.
Faulty modulator, replace.
Faulty thermistor, replace.
Check continuity from
P.C.B. to modulator.
Rectify if necessary.
Is there continuity across
terminals on modulator?
Faulty P.C.B., replace
Faulty P.C.B., replace.
Isolate supply. Remove
block 1 from P.C.B. Is
resistance across
modulator pins on
block 1 between 120
& 140 Ω?
Isolate supply. Remove block 4 from P.C.B. As boiler
cools does resistance across 'CH thermistor' pins
on block 4 increase from approx 2.5K to 10K Ω?
As heating system
becomes satisfied.
Does burner pressure
reduce?
Does fan and pump
continue to run for
approx. 30 seconds?
Refer to fault finding
Dia. 3.8.
Turn on roomstat or
external CH control.
Does main burner
relight?
With heating system
starting from cold. Is
burner pressure
constant at approx:
S.I.T. 8.8mbar?
Honeywell 9.0mbar?
Set selector switch
to"on".
Turn off D.H.W. flow.
Does main burner
extinguish?
Is there between 4 &
15V DC across
modulator pins on
block 1 on P.C.B.?
Check continuity from
P.C.B. to modulator.
Rectify if necessary.
Is there between 15 and 30V
DC across 'modulator' pins on
block 1 on P.C.B.?
Isolate supply. Remove
connectors from modulator. Is
resistance across modulator
between 120 and 140 Ω?
Rectify as necessary.
Faulty P.C.B., replace.
Faulty transformer, replace.
Faulty P.C.B. , replace. Does pump operate.
Isolate supply.
Disconnect block 4
from P.C.B. Is there
continuity across
'flowswitch' pins on
block 4?
Faulty flowswitch,
replace
Ensure that all external
CH controls are not
calling for heat. Rectify
if necessary.
With D.H.W. demand
of at least 3.7
litres/minute, is there
D.H.W. priority?
Does pump continue to
run for a short period?
Boiler working correctly.
Measure CH flow
temperature. As
heating system
becomes satisfied
is CH flow
temperature
between 78 and
82
o
C
Faulty gas valve,
replace.
Faulty D.H.W. cut
out stat. Replace.
Is there 24V AC across
terminals on main
solenoid on gas valve?
Isolate supply. Is there continuity
across 'D.H.W. cut out stat' pins
on block 1on P.C.B?
Remove block 1 from P.C.B. Is
there continuity across
secondary pins 1&2 on transformer.
Restore supply. Is there
24V AC across 'main
solenoid' pins on block
1 on P.C.B?
Isolate supply.
Disconnect block 4 from
P.C.B. With D.H.W. rate
of at least 3.7 litres/
minute. Is there
continuity across 'flow-
switch' pins on block 4.
Is there continuity
between P.C.B. and
flowswitch in line
connectors ?
Check continuity of cable between
P.C.B., transformer, gas valve &
pressure switch. Rectify if necessary.
YES
NO
NO
YES
YES
Continued
YES
Continued
YES
Continued
YES
NO
NO
YES
YES
NO
NO
YES
YES YES
NO
NOYES
NOYES
YES
NO
NO
NO
NO
NO
NO
YES
YES
NO
NO
YES
NO
NO
YES
YES
YES
YES
YES
YES
NO
NO
NO
YES
YES
YES
NO
NO YES
YES
NO
YES
4457 cont.
With heating system
starting from cold. Is
burner pressure
constant at approx:
S.I.T. 8.8mbar?
Honeywell 9.0mbar?
3 Fault Finding
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