First, I know nothing about HVAC and can barely work a screwdriver (not a DIYer). I have 2 Lennox furnaces in my house that is 8 years old. Last winter one of them had a problem with the burners igniting, running for about 10 seconds are shutting down. I had an HVAC contractor come out and he said the burners needed to be replaced, which he did (or said he did). Recently the other furnace had the exact same problem. This time I did some online searching and saw a suggestion to clean the flame sensor, which I did (without so much as loosening a screw)...and the furnace started working normally again. So...now I wonder if the burners really needed to be replaced last year...and if they even were. How do I know the guy didn't just clean the sensor? Is it typical for burners to go out after just 7 years...or did I probably get ripped off?