Hi. I was taking a qualification for FGAs - CPA1 - and something happened I'm thinking through... The assessor caught me checking the burner-pressure of an Open Flued boiler while the boiler was running and, even though I'd been mega-quick, he told me that since its a CF boiler I could've caused the gas to 'light back' by doing this. Did he mean that the gas could light all the way back from the burner into the gas valve? How come this would be more likely to happen on an open-flued appliance than one where the burner is sealed to the air around the test nipple I took off? Maybe the assessor meant that the danger was in gas from the valve being drawn by convection into the burner and light-back happening outside the pipe-line - is this possible? I read an article in Gas Installer about whether its safe to purge to an open flame and it said that gas rarely lights back down any line since it has a very slow rate of catching on. Doesn't this apply to the above scenario then... any comments?