https://www.freeboilermanuals.com/assets/pdf/potterton/Potterton-PowermaxHE.pdf
See Page 25, Heading "Sealed System" item 5.
You should have been handed the manual when the system was handed over.
Download the manual, pop it into a couple of memory sticks and your phone/laptop..
Read through.
Eeeysy, January 2022