My boiler is constantly dropping pressure and coming up with the fault code F.22, yet i can't find a single leak on the pipe work or the radiators. I've bled the system, and topped it up to the running pressure of 1 - 1.5 bar but the problem still occurs. Is there any other reason the boiler could drop pressure apart from a leak? (boiler fault?)

YAN, February 2010

on condensing boilers the main heat exchanger can leak internally and the water will flow out with the condensate pipe and make it difficult to see where the leak is

Paul the Gasman, February 2010

does the system contain the appropriate level of inhibitor. if not corrosion may cause hydrogen formation inducing a drop in system pressure
what make and type of boiler do you have.
the checking of the pressure relief valve is a good idea.
the pressure gauge may also be faulty allowing over charging together with high temperature operation may cause the prv to operate causing low pressure when the system cools down

daver, February 2010

look outside at the PRV valve outlet,the copper elbow on the outside wall when the boiler is working wall maybe wet or rust stained . tie a plastic bag on the elbow leave it for an hr see if the bag now contains WATER .
(it should NOT)
check the boilers expansion vessel air pressure, may be flat or diaphram gone

wirecutterman, February 2010