You either have a leak on the external to the boiler system or the pressure relief valve is passing or your expansion vessel is flat (making the prv leak) and needs recharging or replacing. Check at every radiator, are all the valves clean or is any of them ‘scaled up’? Is all visible pipework solid with no signs of leaks? Once it’s clear that you have no external leaks..
From cold re-pressurise the boiler to 1bar-1.5bar check the pressure relief valve is not dripping outside and then select heating on and turn the heating thermostat on the boiler to MAXIMUM, if the pressure gauge rises up and keeps going upto 2bar-2.5bar and further to 3bar, the pressure relief valve will open and allow the depressurisation then the expansion vessel is at fault. Saunier duval aren’t known for ‘cheap parts’ so if the expansion vessel is at fault it may be cheaper to fit an external expansion vessel underneath the appliance on the heating return pipe, these vessel kits can come with a secondary pressure relief valve so if the old one on the boiler is passing and strained or dirty from system debris and passing then fit the optional prv that comes with the kit.
A drain down may be required from the radiator drain cock (don’t use the prv) so obviously the necessary inhibitor chemicals will need to be replaced to the system water aswell. An external expansion vessel kit costs £70-£100, chemicals are £15 allow 1.5-2 hours labour to fit.
Phill Dee, March 2018
