Worcester Bosch Boilers



Worcester Cdi Standard Efficiency

Worcester Bosch 24Cdi, 28Cdi, 35Cdi

Problem Solution
Common Lockout, (Radiator light flashing)

Moisture in 21 way low voltage connector on PCB

Faulty sensing electrode/lead

Sticking gas valve, or incorrectly set low gas

Air Pressure Problems (Tap light flashing)

Damaged air pressure tubes

Sticking or slow fan, sticking air pressure switch

If flue is over 1m check restrictor has been removed


Poor hot water

Plate heat exchangers can block with magnatite, also check hot water sensor for correct resistance.

Diverter valve can pass down heating pipe when in hot water, giving poor hot water, requires new valve or service kit inside valve

No heating

Central Heating Knob Spindle can snap on PCB


Earlier plate heat exchangers can pinhole internally, causing mains pressure to go into central heating resulting in 3bar pressure on gauge and discharge from PRV

Also check expansion vessel diaphragm has not perished


Worcester Si II, Si2 Range


Worcester 24 Si2, Worcester 24 SiII, Worceseter 28 Si2 SiII, British Gas RD628

Problem Solution
Lockout light flashing PCB's are prone to failing and causing lockout, once all other tests have been carried out, suspect PCB
PCB identification

There are two PCB's on these models, dependant to the gas valve fitted, the earlier model has the solenoids at the rear if the gas valve, the later model the solenoids are at the front.

Code Plug Remember to remove the small module when changing the PCB as it won't work without it!


Worcester 240 RSF


Problem Solution
Central heating stuck on high gas, overheating Check diverter valve microswitches, these affect heating if faulty
Boiler on no demand

Pump over run faulty

Poor water Diverter valve can pass to central heating when in hot water mode, feel central heating pipes to confirm passing when running hot water


Worcester 24i Junior


Problem Solution
No gas but sparking Check low water pressure switch for continuity, aswell as overheat. These are wired in series and cut power to gas valve.
Intermittent faults

Poor flame/lockout, PCB's can fail meaning fan stops suddenly when in operation, flames then baffle momentarily and appliance locks out

Condensate in APS tubes Deflector kit available to deflect cold air away from AP tubes


Worcester RSF

Worcester 24i RSF, Worcester 28i RSF

Problem Solution
Common lockout

Low gas/ignition pressure not checked

Faulty ignition/flame sensing electrodes

Sticking Gas valve

Air pressure problems

Older appliances had a black air pressure switch which was problematic, new replacement is improved switch

Check for moisture in tubes, auto air vent can leak and drip water into tubes

No central heating

User has snapped control knob, check stalk onto PCB

Boiler dead Check overheat stat has not operated on top left hand corner of appliance


Worcester 280 RSF


Problem Solution
Intermittent Faults CH/primary sensor can error when hot, please check/change before suspecting driver PCB
No heating

Synchron faulty in diverter valve, it is possible to change just the motor rather than the complete diverter valve, check to see if the valve is motoring from A to B, and then operate the manual lever at the bottom, if the valve holds open once manually operated, suspect the motor



Worcester 350

Problem Solution
Central Heating fails to relight Time delay of 90 seconds on driver board before heating reignites, may be faulty
Heating coming on with water

Non return valves can stick on pumps

Hot water ok, heating problems Fan resistor may be faulty, should restrict fan to slower speed by reducing voltage to 170Volts, may be going open circuit


Worcester Greenstar i Junior

Worcester Greenstar 24i Junior, Worcester Greenstar 28i Junior

Problem Solution
Unable to reset lockout Water on gas valve due to frozen/blocked condensate, dry out wires and then try to reset again

clunk/noisy after demand

Internal bypass can be noisy after demand

Ignition issues after new PCB fitted Confirm the correct PCB has been fitted, there are several versions, mainly one to fit those boilers with on/off switch, and one without the on/off switch. If the incorrect PCB has been fitted, there may be a weak spark, or no gas
Overheat lockout unable to clear Early models had an air pressure switch wired in series with overheat stats, check flue for blockages or suspect air flow problem
No blue fascia light LED but appliance operating Press spanner button for approx 11 seconds, blue LED should light up, press eco button, press spanner button until light goes out, blue led should now re illuminate
Random water leaks underneath Water meters can cause excess back pressure at appliance, due to the non return valves at the meter, remedy by installing a mini remote expansion vessel


Worcester Greenstar Cdi


Worcester Greenstar 25Cdi, Worcester Greenstar 27 Cdi, Worcester Greenstar 30 Cdi, Worcester Greenstar 31 Cdi,

Worcester Greenstar 35 Cdi, Worcester Greenstar 37 Cdi, Worcester Greenstar 40 Cdi, Worcester Greenstar 42 Cdi

Problem Solution
FO fault code Check HT leads, electrode leads are not trapped or shorting to earth
SE code in display

Service Engineer visit reminder, may not have been reset on last service, not a fault, just a reminder.

Press/hold spanner button for 10 seconds, press/let go eco button until SA (5A) displayed, press/let go chimney sweep button,press/let go eco button, press/hold chimney sweep until brackets appear, press/hold spanner button to exit. Done!

Pressure Loss Heat cell can leak internally, central heating water leaks down condensate, disconnect condensate and observe for drips to test


Worcester Greenstar HE Plus ZWB


Problem Solution
C1 fault code Check for 230 VAC at blue white fan connector, if present, change fan
Pressure Loss Heat cell can leak internally, central heating water leaks down condensate, disconnect condensate and observe for drips to test


Worcester Highflow 400


Problem Solution
poor hot water/noisy on hot water

plate heat exchanger blocked

there is a filter on some models, in the top manifold, behind a brass plug held in by a plate and screw. these can block easily


as per manufacturers, there is a dip tube kit to fit into the return

make sure overheat phial is fully pushed home, too much capillary tube left in the combustion chamber can result in nuisance overheating