To fix the problem of trapped air in a central heating system, bleed the radiators and the circulating pump and then top up the water in the heating system. Repeat the bleeding process after a day or two to release any air left in the system.
To bleed the radiators, switch off the boiler and shut off any upstairs radiators. Starting with the first radiator on the heating system circuit, close the return valve, but leave the flow valve open, then open the bleed valve at the top until it fills with water. Shut off all the valves, then repeat the steps with each radiator. Once this step has been completed with each radiator, open all the return and flow valves, starting with the first radiator.
To bleed the circulating pump, loosen the bleed screw if present; if not, loosen the head screw on the main pump body, then close it once it starts to trickle water. Top up the water in the heating system, turn the heating back on, and check that all the radiators heat up.