the expansion tank has lost its air bubble. it will either need refilling with air or may have burst internally and need to be replaced. Water gets bigger as it heats up (by about 4%) the expansion tank takes up that expansion and keeps the pressure steady.
I think the tank is on the back of a 105he accessed from the top via a metal panel. its the same type of valve as on a car tyre. Take the cap off the valve press needle in untill any air left in is gone. If water pours out it needs a new tank. If not pump up to 1bar ish while letting water out of the system. when water stops coming out the drain keep pumping it up till the gauge on the pump says 1 bar. then fill by the filling loop to 1 bar ish
geoff shuttle, February 2012
