Air can become trapped in a central heating system causing an air lock, resulting in no heating. The primary causes of air becoming trapped are following the wrong sequence when refilling a system or, air being drawn in at some point. If you recently refilled the system without initially closing the upstairs radiators, while the downstairs ones were being filled, you may be better off draining down and starting again.
Bleeding the Radiators
To remove the heating air lock, the first step is to bleed the radiators in the correct order. Switch off the boiler and isolate the electrics. It is important that the pump is not running when carrying this out. When turning off the lockshield valves, make a note of the number of turns required so that you can open by the same amount when you’ve finished. This will allow you to keep the system balanced. Turn off all the upstairs radiators and work on the downstairs ones first. Start at the first radiator on the circuit.
With the return valve closed and the flow valve open, open the bleed valve at the top of the radiator to allow any air to escape. As this air is released, water will replace it via the flow pipe. If you have an open system, water will be topped up by the feed pipe from the feed and expansion cistern in the attic. On a sealed (unvented system) you will need to top up the pressure via the filling loop usually situated beneath the boiler.
Once air has been cleared from the first radiator, close both radiator valves as well as the bleed valve. Move on to the second radiator on the circuit and repeat the process. Once all downstairs radiators have been bled, move on to the upstairs radiators and bleed those one at a time as before. By the time you have worked on all the radiators you will have eliminated all air from them.
Making sure all bleed valves are closed, open the radiator valves one at a time starting with the first radiator on the system – remembering to open the lockshield valves by the same number of turns used to close them
Bleeding the Circulating Pump
Locate the central heating circulating pump and bleed this as well. This will either be via the centrally located slotted head screw on the main pump body or, a separate bleed screw. Only slacken the screw, do not completely undo it. You will hear air being released if there is any and then see a trickle of water. Close the bleed valve or central screw head.
Topping Up the Central Heating
Once the heating air lock has been released, you will need to top up the water in the system. If you have an open vented system, ensure that the feed and expansion cistern has been working and topping up the system. The water level in the cistern should be at its normal point.
If you have an unvented system you will need to top up the water via the filling loop located beneath the boiler. Do this gradually so that you don’t over pressurise. Normally the pressure will be around 1 bar, but check your boiler manufacturer’s details and set accordingly.
Switch the heating back on and make sure all radiators are heating up. If you need to balance the system again once you are finished, see Balancing a Central Heating System
Automatic Air Release Valves
Depending on your system, there may also be automatic air release valves which should take care of releasing air in other parts of the system
Balancing the Radiators
Once the air lock has been resolved you may need to go round balancing the radiators. The purpose of this is to ensure even distribution of heat around the circuit. Without balancing, you would find that those radiators nearest the beginning of the system will receive more than their fair share and the furthest radiators may not heat up properly. By restricting the flow to the nearer ones, you can get an even distribution to all radiators. Follow the Balancing a Central Heating System article for this.
Repeat Bleeding Process
After the heating has been running for a day or two, go round bleeding all the radiators again to remove any further air that has worked its way round.
Draining and Refilling the System
If after following the sequence of steps above, there is still an air lock and you have no heating, you might be better off draining the whole system and refilling, making sure it is done in the correct order – downstairs radiators first followed by upstairs radiators. By filling the downstairs ones only first, you will hopefully push any air upwards in the circuit so that it doesn’t become trapped. Filling the whole lot at once can cause air to get stuck and be troublesome to release.