depending on waht model it could be may things
1. air pressure vessel at the back of boiler is empty of air, so when your heating warms up water expands and pressure increase.
2.if you have a secoundary water to water heat exchanger mains pressure water could be passing through this filling up your c/heating system slowly, this could also apply if you have a dual main heat exchanger. (see manefactures booklet for which)
liam, February 2009
