system pressure increasing

I have a Worcester Bosch 240 Combi Boiler (Sealed System). Everything was fine untill a few months ago. There was a water leak coming from the Automatic air vent inside the boiler. I drained the system and replaced the Auto air vent. Pressured the system the 1.5 bar and bled the rads. pressured system again to 1.2 bar. This seemed to do the trick and the CH worked ok. Then after a week or so< I noticed that I had lost some system pressure, it had gone down to 0.5 bar. I pressured the system again to 1.5 and bled rads ( No air in system this time). Pressure kept dropping every week or so to 0.3 bar. I kept topping up the system again through the loop. The last time I topped up system to obtain 1.5 bar, I noticed that when I turned the CH on then the pressure kept increasing to 3 bar. I have had to depressurise the system to 0.1 bar, then when the CH is turned on, the pressure gradually increases to 1.5 bar and when CH is turned of then the pressure gradually drops back to 0.1 bar. Also the HW takes at least 60 secs to warm up and is not very hot, the only time it gets very hot is when the CH has been on for a while, turn tap on and the water is very hot for 30 secs or so and then goes warm again. I can get the water to go hot by reducing the water flow on the tap. Any help would be very much appreciated, and if I can give any more info then please tell me.

You need a new secondary a.k.a. plate heat exchanger.

Its a generally a Worcester thing. Doesnt happen often but when it does its usually a Worcester

Its odd that ! I have changed two on Worcesters and not on any other type of boiler!

Tony

Thanks for the reply. Just to clarify--is this the "Gas to Water" heat exchanger? or the "Water to Water" heat exchanger?