Water is getting past the overpressure valve, it may need replacing or more likely your pressure is going right up when the central heating is hot because your expansion vessel needs pumping up (and then the pressure release valve is letting by). Google it. If the pressure rises gradually with time turn off the water inlet tap, open a hot water bath tap to release any pressure and see if that stops the increase; if it does there is a path from the mains water to the radiator water loop, possibly via heat exchanger.
c, February 2010
