Duct cleaning’s never really a normal thing to think about—until there’s a problem. How can a homeowner tell? You may find you’re dusting more often. Another tell-tale sign is finding a grimy film on the return air grilles; that’s a very visual indication that you’ve got buildup. Look at the condition of the filters when you change them, which should be done regularly. You or your guests may notice unusual odors, especially when you first walk in the door. Any of those things indicate that you should really consider having someone look at your duct system. To find quality duct cleaning, you can go to N-A-D-C-A--dot--com or breathing-clean--dot--com. There you can learn about the procedures, what you should be looking for in a contractor, what you should be looking for in the job specifically. After all, you’re better off not cleaning your ductwork at all than having someone come in, agitate the debris and not get it out entirely. Ultimately you’ll be breathing in their mistakes, so you definitely want to have a NAD-CA certified person on every single job.