The boiler shut down recently and when the technician came to look at it, he decided that although it started, it shut down within 15 sec because the cad cell wasn't reading the light because the holes were too small. He filed one hole just a bit and the problem was solved. What I want to understand is how this can have happened when no one had touched the boiler and it had run successfully for 3 years. Why couldn't the cad cell see the light? Please send a simple explanation because I don't have much background in this area. Many thanks, Janet Webster