Worcester combi 28cdi - knocking and hissing - needs reset.

[machofairy]

I am a first time poster here and hoping for some advice.
We have an 8 year old 28CDi boiler.
Occassionally, when it is turned on, either for CH or HW, I hear loud knocking noises.
Then the lights would flash and the little valve on top of the boiler would emit some hissing smelly steam.

I would then press and hold the reset button. Sometimes, the flashing lights would stop and it would return to normal. Other times, I have to turn it off and on several times on the power dial before it would work normal again.

When the central heating is running, I can turn on the hot water and it will be fine. The problem only seems to occur when the system is being turned on, but not necessary from cold. It could be on auto, and the CH had just gone 5 minutes ago, then if you turn on the HW, the problem occurs.

The pressure is fine and not even near the minnimul (1) level.
It only had a service last month and they replaced the expansion vessel and heat exchanger because I had to repressurise it daily.

Can anyone suggest what may be wrong?
Thanks in advance.

 

[superdupergasman]

Sounds like an itermittantly sticking pump or the central heating sensor has packed up allowing the boiler to overheat.

 

[machofairy]

superdupergasman, you are probably right. When I came home tonight and turned it on, I heard all the noises and the pump was not vibrating at all. Now the system has packed up completely. However, does a dead pump also stop the hot water from working? Its when I am turning on the hot water that the boiler makes the most amount of noise.

 

[clf-gas]

A dead pump on your boiler will stop DHW and CH from working.

 

[petit_pablo]

I had this on a cdi a few weeks ago, the pump was, i assume, running slow and causing the water to boil up but the AAV was blocked so eventually it completely airlocked and the pump destroyed itself.

 

[bengasman]

From the sound of it your pump packed up due to small particles of debris in the system.
If it is not too badly soiled, you can get rid of it with chemical flush.
To check, run the heating with all the rads open and feel each radiator along the bottom. If the middle is colder than the sides, even a little, you have dirt in the system.