Hi, I think you will find that the end marked boiler tubes gose to the pressure side of the boiler and this pressure acts against the diaphram on the fuel side. So when your boiler reaches its operating pressure it will move the diaphram to reduce or shut off the fuel to the burner, which of course is what is heating the water and generating the steam. If this does't operate the safety valves on the boiler will lift. The safety valves are there so you can't over pressure the boiler and have it explode. Hope this helps. Cheers Ian. 😊

"To boiler tubes basiclly means connected to boiler pressure, the pressure acts upon the attenuater valve to regulate the gas burner. Here is the instruction leaflet for a UK manufactured example...