Question

turn your heating off and then, using a radiator key, open the valve and wait for water to spurt out. "

Answer 1

Cold spots are an indicator of trapped air, and so you'll need to bleed any radiators on which you do find them. Before you bleed your radiators, ensure your heating is turned off and your radiators have completely cooled. ... Holding the key with a cloth, slowly turn anti-clockwise to open your radiator's valve.