As fall sets in, households that don't have central air are turning their heating back on for the first time in months, only to find that one or two radiators aren't warming up properly.

Reaching for a radiator key the moment a panel feels chilly, however, could be a completely wasted effort. Heating expert at U.K. Radiators Rob Nezard warns that a cold radiator doesn't automatically mean you have trapped air, and performing the wrong fix won't solve the real underlying issue.

"Early fall is a sensible time to test your heating rather than waiting until the first really cold week," says Nezard. "Turn the heating on and give the system enough time to warm up. Then carefully check each radiator for uneven temperatures and listen for any unusual gurgling."

Here is how to diagnose your cold radiator in seconds using a simple touch test, plus six common bleeding mistakes to avoid this season.

Before touching any valves, turn your heating on and gently run your hands over the surface of the problem radiator. Where the cold air is trapped reveals everything.

Cold at the top? Bleeding will help. "The classic sign that a radiator needs bleeding is when the bottom is getting warm but the top remains noticeably colder," explains Nezard. "You might also hear gurgling, clunking or groaning noises when the heating comes on.

These can be signs that air has become trapped inside the radiator.

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