Step 1: tell us what it is you are talking about. I assume it is a central heating boiler, but you NEED TO SAY.
What I don't know is the make and model. Again, YOU NEED TO SAY.

Actually, there is an earlier step you could take: find the manual and read it. You should be able to find it online if you've lost or ignored the one that came with the boiler (or whatever it is)

Not a clairvoyant
October 2020