constant pressure on combi boiler

[tressa]

can anyone please advise my boiler is constantly going to high prssure and water overflowing from overflow . it does this even whn off .i have had a plumber out who said it was the refill loop and i paid £70 to replace it but the problem still exisists and im now told the loop did need replacing but i have a secondary problem he doesnt seem to know and said he will contact worcester. pressure only stops rising if i turn my mains water off . reflill loop has been replaced .he seemed to thing i had crossed pipes and hot was going back in but since turning off then on i have discovered it is the hot flowing out of the boiler into cold fill pipe slightly on turning water and power back on . pressure high when bolier off and on unless mains water stopped
any advise greatfull :

 

[Whitespirit66]

any advise greatfull :

Tip no. 1 - When posting re. boiler problems, don't keep the model of boiler a secret.

 

[tressa]

Lol thanks for tip it's a worcester 24i juniour .

 

[Terry lambert]

Could be a pin holed heat exchanger on the older junior or plat to plate on greenstar model. If your sure the filling loop isn't passing

 

[gigz]

Is the loop disconnected ? It needs to be as they often pass, even new ones. If you've been getting hot water your pipes are not crossed (unless you have just had work done). Easy way to check if its an external filling loop, turn the cold water service valve off under the boiler, the water out of your hot taps should stop.

 

[tressa]

Believe it's pinhole in heat exchanger got Worcester service coming out later another 300but hay ho .x