Work on a gas appliance should be performed by someone who is Gas Safe certified.
https://www.acwilgar.co.uk/Boiler-Manual-PDF/Potterton/Potterton_Performa_System_12-18-24-28e_Install_Guide.pdf
https://forums.moneysavingexpert.com/showthread.php?t=4499941 (Long-ish thread, answer at the end)
sdg
November 2019
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