Worcester 24cdi combi stuck on... Any ideas?

[Itsatoughjob]

So this has happened a few times now. My 24cdi decides to fire up in ch mode, despite the time clock being off and the stat being wound right down.
The boiler fires continually with no control over water temperature, it's like it is stuck in commisioning mode. All the rads get extremely hot and the pressure is above normal levels. I'm all for swapping out the pcb, but wondered if anyone had any suggestions or experience of this particular fault?

 

[kidgreen61]

Do you have a frost stat?
Has anybody interfered with it?
Could it be faulty?

 

[Tibbot]

I'm all for swapping out the pcb,

So your first thought is to spend over £200 on a part before carrying out basic FOC electrical fault finding checks. You don't repair these for a living do you?

 

[stardanny]

Switch off at the main supply to boiler and switch back on. Any different?

Daniel.

 

[Itsatoughjob]

Do you have a frost stat?
Has anybody interfered with it?
Could it be faulty?

No external frost stat.
The boiler has frost protection which I understand cuts in below 5c (based on CH sensor). i measured the resistance of this sensor, it is currently about 6.2kilo ohms. I don't know if this is about right, the pipes are still warmish since I shut it all off.

The manual suggests that this condition is either caused by a frost demand, or a faulty pcb. I hope its the former (due to faulty sensor reading) but having doubts now. Anyone know how to equate sensor resistance to measured temperature value?

 

[Itsatoughjob]

I'm all for swapping out the pcb,

So your first thought is to spend over £200 on a part before carrying out basic FOC electrical fault finding checks. You don't repair these for a living do you?

Sorry, I was going by the likely outcome based on the faulting diagram in the manual. I will definitely be pursuing all alternatives before committing any dosh, hopefully someone will have seen this problem before!

 

[Itsatoughjob]

Do you have a frost stat?
Has anybody interfered with it?
Could it be faulty?

No external frost stat.
The boiler has frost protection which I understand cuts in below 5c (based on CH sensor). i measured the resistance of this sensor, it is currently about 6.2kilo ohms. I don't know if this is about right, the pipes are still warmish since I shut it all off.

The manual suggests that this condition is either caused by a frost demand, or a faulty pcb. I hope its the former (due to faulty sensor reading) but having doubts now. Anyone know how to equate sensor resistance to measured temperature value?

Probably bad form to answer my own question, but having looked at http://www.gasboilerforums.com/thermistoreadingsR-W.html It appears that the sensor is ntc, and a 6.2k measurement is well in excess of 20c (like I said, the pipes are still warm) so its unlikely that its giving a false frost condition.

 

[Itsatoughjob]

I'm all for swapping out the pcb,

So your first thought is to spend over £200 on a part before carrying out basic FOC electrical fault finding checks. You don't repair these for a living do you?

Sorry, I was going by the likely outcome based on the faulting diagram in the manual. I will definitely be pursuing all alternatives before committing any dosh, hopefully someone will have seen this problem before!

It appears that the fault lies with the gas valve mode switch on the back of the pcb. Just moving this slightly with a screwdriver causes the boiler to stop firing. Now to work out whether the switch can be desoldered and replaced, rather than getting a new board.