You should bleed your radiators as often as once a year at minimum, especially before the start of the heating season and before the weather turns cold.
Although sometimes you might need to bleed the radiators more often, especially if you have cold spots at the top of the radiators or you hear any unusual noises or have slow heating.
Air can accumulate in the radiators and prevent the hot water from circulating through the radiators and system efficiently and reduce the heating's effectiveness.
Hearing any gurgling, hissing or banging sounds from your radiators also indicates the presence of air and if your radiator are warm at the bottom and cold at the top then it's a clear sign of air being trapped and the radiators need bled.
Bleeding the radiators of the air can improve the heating effectiveness and efficiency of the radiators.
To bleed your radiators start by checking the systems pressure and ensure that the boiler pressure is within the recommended pressure range.
Then locate the bleed valve which is often a small valve at the top of the radiator.
And then use a flathead screwdriver or a radiator key and turn the valve to release the trapped air.
Place a container or a rag under the valve to catch any water that escapes during the bleeding of the radiators.
Once water begins flowing consistently without air, then you can close the valve.
Repeat the procedure for any other radiators that need to be.
And after bleeding the radiators, ensure that the boil pressure is still within the correct pressure range.