you can check the expansion vessel pressure. on the vessel is a valve same as a car tyre , get a tyre pressure gauge and check pressure it should be about 1 bar. also push the valve to see if it releases any water. if the pressure is low you use a car foot pump to get back up to 1 bar, but the system must be empty of water. most cases if the vessel is faulty the pressure rises quite high the warmer the system gets.
hi-spec plumbing, December 2006