I'll give you the full story. 2001 Worcester Bosch Heatslave 15/19 oil combi boiler; the pressure gauge showed repeated pressure loss over a period of a week, from 1 bar down to 0. I topped up the system each day back up to 1 bar. No obvious internal or external leak is apparent. Then we ran a bath and had no hot water, and radiators were extremely hot. I checked the pressure gauge and it was at 3-bar with the pressure-relief-valve venting. We switched the boiler off, the PRV continued to drip until the pressure reached 0. I topped the system back upto 1-bar, switched the boiler back on, the pressure rose back up to 2.5 bar and held reasonably steady but once switched off the pressure dropped to 0. I had similar symptoms in Nov-09, we cured the problem then by replacing the expansion vessel. I have now fitted another new expansion vessel but this time the fault remains. I've noticed the PRV will often allow water to be drip released even when the pressure is below 3-bar. And when hot water is taken from a tap the pressure gauge will drop by about 0.25bar. I'm thinking the cause is either the PRV or heat exchanger. Any advice would be great.

Dave, January 2010

The boiler man from Ramtech came round today and fixed it. Two faults. The over pressure cause was a blocked hose to the expansion vessel - he found it straight away by switching the heating on and checking to see if the hose got hot at the expansion vessel end - it didn't get hot so that meant blocked hose. Simple, and simple to change.
The continuing loss of pressure was diagnosed as the pressure relief valve, so this was changed, not so simple but completed. I'll monitor the pressure drop now over the next few days / weeks.

Dave, January 2010

when you fitted the pressure vessel you did charge/inflate it ?
with NO water PRESSURE in the boiler, test the air pressure in the expansion vessel with a tyre pressure gauge, inflate using a car tyre pump to 1 bar cold pressure,
now repressurise the boiler on the water side allow up to 1.5 bar pressure cold using the filler loop.

wirecutterman, January 2010