A boiler start-up follows a fixed order. You are not doing the technical work. You are watching it, checking it against the installer's paperwork, and keeping the record. Ask the installer to walk the sequence with you, and stay until the burner has run a full cycle.
Before any water goes in
Confirm the boiler stays isolated from gas and power until the water side is ready. Check that the expansion vessel is in place and that its charge has been set for the height of the system. The installer should write that charge on the commissioning sheet. Look at the relief valve and confirm its discharge pipe runs to a safe, visible spot, not into a closed cabinet.
Filling and bleeding
Fill the system slowly through the fill valve until the pressure gauge reaches the cold level the installer has noted. Then go to the highest radiator or the high point of the piping and bleed the air. Top the pressure back up after bleeding, because air release often lowers it. Run a dry paper towel over the joints and fittings. Any wet spot gets marked and reviewed before the boiler is powered.
The first fire
Restore power and set the thermostat to call for heat. Watch the circulator start. Watch the burner light, run for a while, and shut down when the call ends. Note any unusual sound, smell, or change in the flame, and ask the installer to explain it on the spot. Confirm the carbon monoxide alarm is in place and working before the boiler runs for the first time.
What to keep
Ask for the commissioning sheet in writing. It should show the fill level, the expansion charge, the bleed points, the test results, and the controls checked. File it with your owner's record, along with the permit paper and the installer's name and contact details. Your planning date for later major checks comes from the install year, which you can compare against the boiler service life and planning page on this site.