Boiler repair

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Boiler-specific faults

Keston C25 ignition failures
one common reason for this is the electrodes moving apart. Do not try and bend them back when cold. Someone here said that you can straighten an electrode when hot (hold in gas hob flame) but I've not tried this myself.
The biggest downside to Kestons seems to be parts availability

Ideal (Stelrad) Classic buzzing
fan, pcb (Stelrad 25b) or gas valve
* fans tend to buzz when the bearings have failed and it is no longer turning
* the gas valve tends to buzz because the relay on the pcb is buzzing because the drive circuit is faulty - not the most common problem with this pcb
* or you have an internal timer which has a faulty clutch - if so, is it losing time?
Ideal Response 120 losing water pressure
  1. "Is the heat exchanger leaking yet?"
  2. "How badly is it leaking now?"
Potterton Suprima locking out
PCB fault: replacement from CET Ltd
Note if Suprima has a red HT lead, it's a common reason for flame sense failure - it's carbon fibre, not wire and shouldn't be trusted

Potterton Combi 100 dead
Symptoms may include supply fuse/trip blown, burning smell and possibly burn marks or even hole in PCB
Modulation PCB fault: possibly replacement from CET Ltd
Baxi Barcelona
Ravenheat RS84F
  • short cycling: cause unknown Screwfix forum
  • thermistors "notorious" for unreliability
Ravenheat LS80
unreliable thermistors - can replace wet-pocket sensor p/n 0007TER05005/0 with clip-on p/n 0007son11010/0 (both cost over £20 + VAT!)
Icos / Isar / Istor boiler fault
fault not uncommon with this type of boiler gas valve.
l..f flashing
voltage at the gas valve on the conector (still connected) fine
negative vacuum on the venturi fine
connector is faulty: plug tight on gas valve pins but not making contact. Aparently this is common.
Icos / Isar / etc?
PCB fault which causes fuses to blow on supply cct (when DWH called for on Isar? also happens on Icos)
replace PCB module (complete with plastic shell).
Gloworm Fuelsaver F ... comes on and the flame kicks in for about 5-10 seconds and then goes out.
Fuelsaver - some have a problem - the fans have windings which are prone to overheating and a resettable thermal fuse which will cut out when the windings get hot (a matter of seconds to minutes) and close again when the windings cool down
Vokera Excell
Baxi Barcelona
Discussion: Intermittent Ignition Lockout on uk.d-i-y

Generic parts in boilers

Could do with info on what parts of what boilers are replaceable by standard or generic parts.

  • Diverter valve diaphragm repair kit from Banico (BES stock code 17115) is claimed to fit Giannoni and also Mutmacanica(?) valves. Checked OK for Worcester CDi.
  • Many boilers use standard Grundfos (or other) pumps and one can just replace the heads (though on some - such as Potterton Performas - you can't, even though they look from the outside like standard Grundfos heads).

Must-also-replace parts

Some repairs involve replacing various O-rings, fibre washers etc. Would be good to know which, and if generics are OK.

Info, manuals etc on-line



Worcester / Worcester-Bosch


See also

How boilers work