I HAVE A BAXI COMPACT HATER AND IT IS CONTINUALLY LOSING PRESSURE AND I HAVE TO USE THE WATER VALVE TO BUILD THE PRESSURE BACK UP. THE OVERFLOW IS BUILT DIRECTLY INT THE TOILET STACK SO THAT I CANNOT SEE IF IT IS OVERFLOWING OR NOT.
Sounds like you expansion vessel has gone. You must stop adding untreated water to your system otherwise it will rust it up. Unfortunately this a job for a plumber and will cost about £150.
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