A question that was asked recently, we thought we would share our answer to you all. Hope it helps.
Turn off the drain valve and remove the hose used for draining down. With a vented system, switch the water supply back on at the feed and expansion tank in the attic to allow water to flow back into the system and begin to fill it up.
With a sealed or unvented system you will need to connect the filling loop – usually a braided hose – which is usually located just beneath the boiler, and open the valves at both ends. Open the filling loop and re pressurise the system to the manufacturer’s recommended pressure – normally around 1 bar.
Go round the radiators, one by one, opening the bleed valve and releasing any air. Do the downstairs radiators first and then the upstairs ones. This will minimise the chance of trapping air in the system. On a sealed system you should then re check the pressure and top up if necessary.