You have probably just done some maintenance work on a Worcester Bosch CDi, Ci, Si boiler, Greenstar or Junior Boiler (System, conventional and combination boilers affected) or associated heating system. Wondering what has happened to the boiler, answer nothing.

This cannot be found in the the manufacturers instructions as it is not a fault code. (You can stop looking for a accidentally unplugged sensor).

It will last about 20 minutes to half an hour and then the boiler will return to normal. Combination boilers may have operational hot water during this period but normal heating modes will not be accessible.

The reset button will not by pass this mode, it can re set this mode so you have to wait even longer for the boiler to return to normal.

The symbol something like a hh or HH with missing digits indicates the boiler is in a form of commissioning mode. When some Worcester boilers are first turned on they will fill the siphon and display this symbol. Interruptions in power supply to the boiler can result in a reentering of this mode. This mode cannot be skipped or opted out of. You will just have to wait for it to complete to continue tests or use your boiler etc.

Bit annoying if you've just fixed a boiler want to get to the next job but need to test and re assure your client. Also an inconvenient time waster when your waiting to get on with heat testing balancing or flushing etc. Unfortunately there is no way round this on most (or possibly all) models other than to wait it out. In hindsight perhaps Worcester should have allowed heating engineer code to skip this, never mind.

Jeff Culkin

Culkin Plumbing and Heating

North west London