The pressure gauge keeps rising slowly,quicker if the heating is on.

Steve, May 2008

One of two things
1 The filling loop has been left turned on
2 there is an internal leak in your heat exchanger.

AND

You could have a faulty pressure vessel.

The 'and' could be an 'or' but you didn't give much detail in your one line question

Slugbabydotcom, May 2008