Cold spots in your radiator can be caused by a build-up of debris, or if air is trapped in the radiator.
If you find there’s a cold spot at the bottom of your radiator, it’s likely that debris is obstructing the flow of water. To fix this, your radiator will need to be flushed out and potentially even replaced.
If you’ve noticed a cold spot at the top of your radiator, there may be trapped air inside the radiator which is an easier fix. You will need to bleed the radiator to release the trapped air.
Use a radiator key which fits into the top of your radiator. Turn it anti-clockwise to open the valve. Close it by turning the key clockwise when all the air has been released and water starts to appear.