can anyone give advice,the boiler on my wall starts up with the taps running,wallah hot water after 30 seconds the boiler shuts down,we noticed the pressure on the water side was low,fitted loop and topped up,tried again ran a little bit longer,same again boiler stops,its the same on the heating side,starts up and the rads get too hot,then the system shuts doen,I have bled the air out of the rasds,again starts up and then after a minute shuts down.any advice welcome.thanks