First off I'm not a gas engineer but a fire engineer. I've just had a bit of a problem on a job where the main incoming solenoid valve had been left off but the heaters on the site where then turned on or well where tried to be. They called in a Gas company who found the problem. But they are now saying that the hole system needs purging with nitrogen as there will be air in the system. Is this correct as I cant understand how air has got into the system? Surely the heaters/burners would just shut of if the pressure was to low and wouldn't let air into the system. Any advice would be appreciated thanks.