There is another and more costly failure which can cause pressure rise - a
pinhole leak between mains and heating circuit in the boiler secondary heat
exchanger. You can check this after you have eliminated the filling loop as
above by turning off the cold mains inlet to the boiler and open a hot tap
which will depressurise the mains water circuit through the boiler. If the
heat exchanger is the culprit the pressure rise will cease. You then need to
replace the secondary heat exchanger.

What model of boiler is it anyway?



It's a Worcester Bosch 24 Si II combi, fitted summer 2003