From a consumer perspective that link is invaluable thank you. I could call 100 hvac people on the phone and the people answering the phone wouldn't be able to spell combustion. By having a website like that it makes it possible for me to find someone. As much as I have been troubleshooting here with the techs, it isn't what I want to do. It isn't really my engineering interest. You nailed it when you talked about guessing. I want to pay someone who understands fundamentals and isn't only guessing. I just haven't been able to find them. I haven't been satisfied with the guesses from the onsite folks hence this thread. Measuring combustion is the key to all of it as pointed out by folks in this thread.