I was recently at a neighbour's house when the central heating man came for his annual visit to check the Gas Boiler. Being of a curious nature I accompanied him to the boiler room. After the usual clean up he got out a digital gas analyser , put the sensor up the flue , and started to adjust the gas pressure/flow until he was happy with the results. They were O : 12% CO: 75 ppm Temp: 228 C (442 F) The gentleman said that you no longer looked at the flame : that was not good enough. You need digital equipment to tell you if all is well. Now on this forum you read < look at the flame> very frequently. Do you think in a few years time ,we also, will all be putting sensors up our chimneys ?