Alpha Combi boiler. Working fine, or so I thought. A bit chilly this evening, so put the heating on for the first time in 3 weeks or so. Pressure was a little low, so fill up to the recommended 1 bar. Tap is shut off. Heating comes on fine. 20 minutes later, I'm outside and hear water running - the pressure relief/overflow from the boiler is spewing out a bit more than a trickle, but less than a hosepipe, of hot water. Back inside, turn off heating. Notice pressure guage now at 4 bar, hence the pressure relief. First thought, air in system, though bleeding all the rads doesnt produce much air - typical of what I'd expect after not being done for 6 months. Pressure now down to 2 bar when it is cool.
So, why has this happened? The pressure relief has never opened, to my knowledge, in the 8 years we've had the boiler. All rads were warm, though not hot. Thanks Alan.