Combination boiler losing pressure

Hi,

My problem is as follows:

I have a Worcester 24CDi boiler, which seems to lose almost all of its pressure when the central heating is off. When the heating clicks back on (timer) the pressure doesn't increase and the rads don't get warm.

If i ramp up the pressure manually to about 1.5 bar and then turn on the central heating, everything functions happily (no drop in pressure at all), until the heating is turned off. Then the pressure gradually reduces until it hits zero and again requires manual intervention.

I'm pretty sure if there was a leak in the system the pressure would drop dramatically when the heating was on, and as this doesn't happen, i'm assuming I don't have a leak.

Anyone have ideas what the problem might be and how I might go about curing it?

Regards,

CS

Have you checked the PRV that terminates outside?

Are you sure the pressure don,t rise whae the heating is on?

If you are pressuring the system, ie putting more water in and then the pressure drops and you put more water in, I can only see one conclusion. Water is escaping somewhere, you just need to find where.

Perhaps wherever it is, closes up when it's hot due to natural expansion and then starts to leak again when its cool, but to loose all pressure it's not just the odd drip or two.

Nothing coming out of the external pipe.

I had wondered about water leaking when cold and not when hot due to expansion, but if this was the case, surely when I first ramp up the pressure (when the system is cold), the system would instantly start to lose pressure (especially if the leak is big enought to almost completely drain the system in a matterof hours (which is what would have to be happening each time the system cools down).