Hi

I have a Glow Worm Combi Boiler, model 23C.
Since last night I have had water leaking from underneath the boiler.
On closer inspection, and using the diagrams in the manual, I have
discovered that the water is coming out of the Reduced Pressure Zone (RPZ)
valve.
Can somebody explain to me the purpose of this valve? Also, does this fault
indicate a problem with the valve itself or another problem elsewhere in the
boiler?

If I open the filling tap of the boiler and refill the system this leak
stops. The boiler and therefore the central heating and/or the hot water
will work fine. However, after a while (I think it may be once the system
starts cooling down again), the water starts dripping out of the bottom
again.

Thanks
Paul