How air enters a water central heating system.
When new water is added to a CH system, a certain amount of air also enters the system. The impeller action of the pump also ‘creates’ a certain amount of air from freshly filled water. As air will rise in water, it will collect at high points in the system, such as radiators and pipework.
Once all the air dissolved in the CH water comes out of solution, you can bleed it from the radiators. After that initial period (which can be from a day or two, to a few weeks) you should never have to bleed any “air” out again.
If you keep finding “air” it’s either being sucked in, or it isn’t air at all, but gas which is a corrosion by-product and you should check there is sufficient inhibitor in your system.
If you can feel that the top of the rad is noticeably cooler than the bottom,you have air in the radiator which needs bleeding out.
How to bleed a Radiator.
Always bleed radiators with the central heating pump OFF as there is a risk that air might be drawn into the system making your problems worse.
1. Switch off the Central Heating system.
2. Fit a bleed key ( cost you £1 from a plumbers merchant) onto the bleed valve.
3. Wrap a piece of old cloth around the key to catch any expelled water.
4. Open the bleed valve by turning the bleed a 1/2 turn anti clockwise - hissing of air being expelled will normally be heard.
5. When water starts to dribble out of the valve, close the valve by turning the key a 1/2 turn clockwise. DO NOT OVER TIGHTEN.
6. If the Central Heating system is a sealed system, check the pressure and, if necessary, top up the pressure as required
(often 1 bar but check the instructions).
Bleed point shown above but can also be found at the back (top) of the rad covered with a plastic cap. To locate you should be able to feel a bump with your finger.