Any idea why my Worcester Bosch 42CDi Combi boiler has stopped producing hot water? Normally we get an error code, but this time the big digital display just shows 125 (I think normally this is about 60 or so). Leaving it for a while it gradually ticks down in numbers to around 60.

The boiler has power, pressure etc, pressing reset and turning it on and off do nothing, pressing the spanner button doesnt show any other codes.

Next available engineer appointment isn’t until Thursday, so any help appreciated!

Bump

Does the central heating still work?

Nope – no heating either

Stuck diverter valve
Check gas is on in house
Check flue
Check 1 bar in system

Toggle the hw and ch switching a few times, or run tap ten try switching between ch and hw.
See if it comes alive

On the face of it it sounds the boiler is heating up the heating “water” but it’s not circulating and therefore overheating and cutting out. If it’s not circulating then I’d guess either a blockage in the DHW heat exchanger or the pump’s gone. Having said that, the boiler should cut out at 95°C regardless of any other faults so if you’re seeing 125°C then there’s something seriously amiss. More likely to be a DHW temperature sensor fault.

Does the central heating still work?

Ah, just seen the CH not working. Has the pump gone? Try the old screwdriver tip on the pump and handle to your ear to see if you can hear it running.

Not sure how I can check gas – no other gas appliances. Boiler feels warm (but not overly so) to the touch. When running the taps, boiler doesn’t make the usual firing up noise, so don’t think it is actually firing

Hmmmm. You’d normally get an indication on the panel lights if it wasn’t firing properly. Doesn’t sound like a simple fix. Best bet is have a phone around the engineers in your area listed on the GasSafe website cos I can’t imagine waiting until Thursday is floating your boat.

FLicking it back and forth to heating fired it up for a few seconds, so gas is obviously working. It then rapidly increased the temp to 105 or so, and flashed the siphon filling sign a couple of times, before flashing a sign Ive not seen before, but google tells me is temperature blocking mode.

Siphon filling symbol:

_| |_
| |

Temp blocking symbol:

|-
-|

Blocked pipe to expansion vessel ?

Try unscrewing the front off the pump, could be air locked. Then see if its a 3 speed pump, flick through settings to see if it whirrs into life.

Only readom for overheat isvpump fail or blockage in feed piping. Open up all trvs and the towel rail and try again

As said above, probably a pump failure.
Not a difficult job, if the large nuts can be undone. If they are seized, it’s a real pain.
£100-150 for a new pump.

So does it fire on demand or not that is the first question?

Can you feel any heat coming down the hot pipe or heating pipe?

Can you hear the pump run?

And pumps in boilers tend not to have valves either side and can be awkward to change.

I can get it to fire for a few seconds by flicking back and forth between heat and water. It wont fire when water only. Boiler itself is warm, pipes not.

Hopefully found an engineer who can have a look today rather than waiting until Thursday – sons and wife were rather grumpy (although my older sons screams from a very cold shower were amusing)

Diverter valve motor failing, works then cuts out
Do you get as far as the fire up sequence of..
Fan on. Whirrrrr
Clicketty click gas train opening up
Bzzzzzz spark plug ignition
Whoomph… fire in the hole

If not then i think the diverter valva gives a go / no go to the ecu via a switch contact.
Hence cycling between hw and ch with a tap running can coax it along

Is there a secondary manual reset up top on that model, orange button up high l h s

Thanks for all the help. Appears that the pump has gone, so engineer fitting a new one tomorrow.

@singletrackmind – no there is no whirring or anything (unless I flick it to C/H and back a few times, whereupon it will fire). No other reset button on this model.