Nasty boiler Playing up again!! Help please.

Hi guys

I am having a problem with my Alpha CB28X. On start up from cold, the boiler fires, runs for a few minutes then goes off, keeps cycling like this for around 20 minutes. Eventually it gets going and functions as normal. The rads have not reached temperature and the flow and return pipes are not that hot.
At one point during the cycling stage the B (bottom) light was going off and on every second which the book states is a faulty temperature sensor. I just doing understand how it starts working after about 20 minutes.

Could an intermittent sensor fault cause this to happen?

I don't know if this is doing this all the time at start up as I have only been putting the boiler on when cold but I suspect it has been doing this as house has taken alot of time to heat up.

A couple of year ago the diverted valve got clogged up. Could this be causing this problem again? The difference this time is that the flow pipe was not very hot and it is now operating normally?

Any thoughts. Thanks fluff

You have a faulty primary thermistor not difficult to replace and relatively cheap to buy

Fluff, how are you doing.

Sensors do fail. Combination boilers do have a lot of components. the sensor could be borderline so sometimes it works, other times not.

Almost certain will not be defective like the last time

What is the hot water temperature like?

Last time you did not have the flashing light I think.

Hi DP

How are you keeping? Good to know you are still keeping your hand in on the forum.

The water temperature is fine. The problem seems to be when starting from cold. After the boiler has been on for a while and pressure has increased it click on and off as normal.

The boiler fires and the B light illuminates to indicate that it is on. It then clicks off after around 10-15 seconds. Then after around 5 seconds it will fire up again and can continue like this sometimes for a couple of minutes or 20 minutes. Sometimes after clicking off during this cycle the B light will flash quickly. The manual states that this is a temperature sensor fault.

What I don't understand is that sometimes the B light flashes quickly and sometimes it doesn't - no logic in it.

Can a faulty temperature sensor act in this way? Our previous boiler had a faulty temp sensor but the boiler cycled continually and never heated up.

I am just hoping that the carbon hasn't built up again in the diverter valve.

Thanks for any advice you can offer.

Fluff