Vaillant Ecotec plus 637 Pump OverRun

[RK_London]

I have a Vaillant Ecotec plus 637. The pump overrun is set to 5 mins (default setting)

When the boiler cycles at flow temperatures around 50C the pump overrun drops flow temperatures to about 40C the radiators around the house start making a banging noise almost as if the radiators are cooling down too fast. Doesn't seem to be an issue at higher flow temperatures.

Does anyone have any ideas what is going on?

 

[MeldrewsMate]

It's unclear what you mean, please clarify.
Do you mean that when you operate your boiler with a temperature setting of 50C you get this noise, but not when you operate it at higher (say 60C) temperatures?
Does this symptom occur regularly during the evening heating period?
Does it occur only when the heating switches off on the timer or via the room 'stat?
What do you mean by 'boiler cycling'? Does the flame go out during 'boiler cycling? Does the flame stay lit at the higher temperature setting?

Can you see where I'm going here?....

 

[RK_London]

Apologies for not being clearer - I'm trying to figure this out as I go along

By boiler cycling I mean the boiler goes through the usual checks before the burner goes on then the temperature reading on the boiler (flow temperature) goes up. After a several minutes the burner switches off and the boiler goes into pump overun. Then the cycle starts starts again and as the boiler goes into burner mode I see the temperature reading on the boiler drop quite quickly ( about 10-12C in a minute or two). I'm assuming this is normal operation.

What I'm seeing is when the temperatire reading on the boiler is around 50C after the heating mode has gone off and into pump over run the boiler goes into burner mode as normal and begins to cool. The temperature drops to about 40C and the radiators make lots of banging noises. This happens all day and in the evening. When the reading on the boiler is 60-70c at other times of the day and evening I never seem to have any issues.

Not sure how to check if the flame goes out or stays on.