over pressure on viessmann boiler

[Huckersmd]

Seeking ideas.... System is a Viessmann Vitodens 100-W WB1B regularly serviced. Boiler, pressurised to 1bar when cold, filling loop is always disconnected. This provides hot water (no problems) and heating is via a single output in 22mm and T's into two two port valves, one serves downstairs UFH via a Watts manifold and actuators....all working correctly. Both 2 port valves checked and working correctly. When the UFH is calling for heat boiler provides and pressure increases between 1bar to around 1.5bar. If the CH is called for independently pressure increases over 2.5 bar as system heats up and I turn off prior to 3bar. If UFH calls for heat system is fine as described but if CH then calls for heat same thing happens, turn off thermostat for CH and pressure returns to Norman after a period of time. Radiators balanced as best I know how......any ideas gents and ladies
System has a Fernox FI filter which remains very clear and system has been flushed a few months ago

 

[Huckersmd]

if its relevant the boiler is a system boiler NOT a combi

 

[terryplumb]

Sounds like an expansion vessel issue. When was the system last serviced ?

 

[Huckersmd]

Hi Terry thank you for the response it was serviced several months ago...... update, I have removed the expansion vessel cleaned it out to remove any sludge (bit of black water only in it, used the car tyre inflator to .85 bar (says .75 on the vessel) and have it back on the boiler now just fired it up in the last two minutes. Ive set the temperature to 60 degrees for now and the pressure is holding steady at 1.25 bar at the moment.
Thankyou for the response and looks like you're right....also checked the valve for leaks and seems ok..... i'll check again in a few hours