Potterton Suprima short cycles for 5s, repeats, no fault led

[Sequel]

Hi - perhaps someone can help.

Another Suprima thread...

This suprima 100L ignites, fires up and runs for almost exactly 5 seconds - the burner is visibly fully on. It then shuts down, as if it had reached temperature and then cycles again, and again...

I replaced the Pcb and it ran correctly (at least running for some minutes) the first time, and then reverted to the same fault. I presume something has caused the PCB to fail, although the original faulty pcb did work once whilst we had the fualt so this leaves it possible that it is something else, and not the pcb. The boiler will cycle like this for ever - there is never a red fault light. It runs for such a short period that nothing gets warm.
Checks made:
- thermistor tested ok, but replaced anyway
- gas valve coil receives 200vdc and resistance is 4kohm, all of which I believe is correct.
- the pump runs
- the pressure / vac switch stays switched whilst the fan runs.

Has anyone seen this fault before, or have any advice on where to go from here?

Thanks,

 

[Tibbot]

Another Suprima thread...

Another one tampering with gas boilers? This is a DIY site but not for gas work. If you are an RGI, join the CC section, you'll get advice there.

 

[steve32]

It's not a boiler fault it's a pump fault or blockage in pipe work

 

[roro]

I have the same issue as Sequel but with a Potterton Suprima 80. If Sequel is still out there, did you get a resolution to the problem?

The suprima ignites, fires up and runs for almost exactly 5 seconds - the burner is visibly fully on. It then shuts down, as if it had reached temperature and then cycles again, and again....

The pcb and the thermostats on the boiler have been replaced and also the pump,but to no avail.

Any advice would be gratefully received.

Thanks.

 

[Agile]

As previously advised here, we don't give advice on DIY gas work.

Nor do I give advice to anyone who has opened the combustion chamber which is not advisable for DIYers.

The obvious advice is to get a competent gas reg engineer!

Tony

 

[bazdaman]

Why do people not just get a gas safe guy in who can find the fault quickly and repair. You guys then waste 10 times more money banging pcbs and other parts on boilers when not required, it may just need a slight adjustment that most guys would be charging a few quid for