Bleeding a radiator means opening a small valve on the radiator so trapped air can leave. On a hot water heating system, air collects at the high points and leaves cold spots in the room. The job itself is small. The decision is whether you should do it or book the boiler visit.
Confirm the system before you open anything
Shut the boiler off at its controls and let the radiators cool. Hot water can scald you, so wait until the metal is no longer hot to the touch. Confirm the system is hot water. Steam radiators work differently and this walk-through does not cover them.
Open the valve the right way
Find the bleed valve at one end near the top of the radiator. Have a cloth, a small cup, and a bleed key or flathead screwdriver ready. Open the valve slowly. Expect a hiss first, then a few drops of water. Close the valve when water runs steadily. Note which radiators were cold, so you can tell later whether the change holds.
Read the boiler gauge before and after
Bleeding releases a small amount of water, and that can lower the system pressure. Read the boiler gauge before you start and again when you finish. A pressure that falls and keeps falling is a question for the service visit, not a reason to keep bleeding and topping up.
Book the visit instead when
Book the visit if the valve is seized, if it leaks around the threads, if paint has sealed it shut, or if the same radiators need bleeding again and again. Ask the technician to check the boiler pressure, the expansion tank, and whether air is entering through a fitting or a pipe rather than through the radiator itself. Ask whether the repair is a valve, a fitting, or a question about the whole system.
When you plan the job, check your boiler against the boiler service life and when to plan page on this site. Age does not decide the bleed, but it shapes how much you want the visit to tell you.