Vokera Mynute 28SE - fan and noise problem

[Emlin]

The last couple of days when my heating switches off the fan is still running for approx 30 mins and then a horrible banging noise starts (only lasts for a few seconds) somewhere in the region of the flue. It has only started doing this and not every time the heating goes off. I put the heating on again, I hear water rushing, then it fires up. I then switch the heating off again and then the fan switches off.
The fan blades have been cleaned recently and the boiler is well looked after and is aprox. 7/8 years old. Have had a few bits and pieces replaced, i.e. air pressure switch, heat exchanger, motorised valve.
Any ideas.

 

[Emlin]

Sorry I think its the pump that is still running for approx. 30 mins after the boiler is switched off and not the fan.

 

[DP]

Sorry I think its the pump that is still running for approx. 30 mins after the boiler is switched off and not the fan.

I would be looking at external controls and bypass as (I suspect) demand in place to boiler after timer has switched off,

Get a multimeter and check flowchart in manual.

 

[Emlin]

Thanks for that DP, but would that not happen every time the heating goes off then via the timer? The pump has kept running only twice in the last few days and only after the heating has been on at night for approx. 4-5 hours.

 

[DP]

I suspect sticky motorised valve/ auxillary switch

 

[Emlin]

Can I ask DP again about my Vokera boiler. My motorised valve on the central heating part of the system is working ok (or appears to be) it was replaced last April. The other motorised valve is on the hot water side of the system and also seems fine. If the valve or the aux switch was sticky - can I fix that and how?
Also I have checked and the only part still running after heating switched off is the pump which makes a terrible grinding noise for a few seconds if left to run. I then have to switch the heating back on at the timer switch and back off, then the pump switches off. However if the boiler is firing up when it's due to go off, then it switches off no problem. We wondered if the pump was on its way out!
We know the system quite well having replaced the air switch, had the fan out to clean, replaced a rubber diaphram, etc so hopefully this new problem won;t be too difficult to fix!