Recently a friend of mine whose opinions and recommendations I value — on anything from movies to marriage to motor oil — told me he had just had his home’s air ducts cleaned. In fact, he couldn’t stop raving about it. He claimed he’s never breathed easier, the air is fresher and he regrets not doing it years ago. I politely nodded and tried to change the subject. Working in the HVAC industry, I’m no stranger to the debate on duct cleaning. Debate? Yes. The fact is, empirical