12.2 Domestic Hot Water
Switch on power, set
selectors to CH off and
DHW on.
Is DHW pump working?
Is there continuity
across connection 20
and 21 on the PCB?
Check wiring, replace
tank thermostat.
Is there continuity
across 25 and 27?
Check wiring, replace
relay, replace PCB.
Check wiring, replace
pump.
Does burner fire?
Do radiators get hot?
Check plumbing,
replace non-return
valve.
Refer to burner fault
finding.
Is lockout lamp on
burner or pump lit?
Does tank get hot?
Vent all air from system
pumps, ensure all
valves are open.
Press reset button,
replace limit thermostat.
Is high limit thermostat
lamp lit.
Does boiler shut down
when tank reaches
temperature?
Replace tank stat,
replace relay.
Check wiring, replace
tank limit stat.
Is there continuinity
across 23 and 22?
With tap closed, switch
the CH switch to on.
Check wiring, replace
relay, replace PCB.
Is there continuinity
across 22 and 16?
Do radiators get hot?
Check CH fault finding.
Check power from
clock or switch, check
wiring, check fuses.
Does CH pump stop
when tap is opened?
Check flow switch,
check relay.
Is hot water produced
when tap is opened.
BOILER IS WORKING
NORMALLY
NO
NO
NO
YES
NO
NO
NO
YES
YES
YES
NO
NO
NO
NO
YES
YES
NO
YES
YES
YES
YES
YES
Check mixer valve,
check plate heat
exchanger for blockage,
ensure all air is vented
from boiler.
YES
YES
NO
NO
NO
YES
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