I regularly had the same problem.
I fitted a 22mm Brass pipe fitting under the flap and the base to prevent it closing fully.
I unscrewed the union nut till it was exactly the right height.
The nut is secured with PTFE tape on the threads to stop it vibrating loose.
That way the flap does not stick ever.
The flap is to stop the passage of cooling air through the boiler when it is stopped.
A tiny gap by this method makes no difference.
The hair dryer works but the fault comes back to bite you at a later date.
Bob
February 2013