Dripping Overflow Pipe

[JMDS]

Hi
The overflow pipe,from the expansion tank,keeps dripping,but only when the central heating system is running. I know it might seem fairly obvious,but is it just a simple fault with the ballcock? Can it be adjusted, or does it need replacing?
All help appreciated.

 

[Softus]

One or more of the following:

1. System wrongly piped such that there's too much pressure at the bottom of the vent.

2. System dirty and becoming blocked - water is pumped round via the vent and cold feed.

3. Pump running at a crazy speed.

It's unlikely to be number three on its own, so it's unlikely that there is a quick/simple fix.