Sometimes you have to go at something backwards or inside out or sideways. Sometimes you've got to invert the normal thinking. Sometimes you have to invert it once and then invert it again to get something completely different, as Frank N. Furter did in the Rocky Horror Picture Show:
I'll remove the cause
but not the symptom.
In the case of getting your air conditioner's ducts cleaned, though, we're dealing with a single step of inverted logic. I'll get to that in a minute, so stick with me here.
Let's say you're concerned about the indoor air quality in your home and are facing the question of whether or not to have that duct cleaning company come in and clean out all the ducts in your heating and air conditioning system. They've showed you the magnified images of the dust mites that breed in your ducts and described in great detail all the other creepy crawlies and biology experiments that your dirty ducts nurture. You're thoroughly horrified and are not only ready to hand them your first-born but to sacrifice goats and virgins if necessary, too.
But hold on a minute! Before you go forking over all your hard-earned cash or taking more money from your home equity line of credit, make sure the company you want to hire is going to ask all the right questions. In addition to, Are your air conditioner's ducts dirty?, they should be asking:
Why are your air conditioner's ducts dirty?
As long as they're asking this question and proposing to solve the problems it reveals, then duct cleaning may be a good investment. If not, they're treating a symptom and leaving the cause there, which means if they can hook you, they've got repeat business down the road.
But dirty ducts aren't like dirty windows or a dirty car. If your ducts have a serious problem of being too dirty, you need to figure out why and then stop the dirt from getting in the ducts if you can.
One reason your ducts may be getting dirty is that your return vents, which pull air from the house back to the air handler for another round of heating or cooling, are in the floor. This arrangement is great for keeping your floors clean, but unfortunately, all that dirt goes into your ducts. Short of retrofitting the duct system to get the returns out of the floor, you should be using a filter inside the grille to keep your ducts cleaner.
Another major reason is duct leakage, especially on the return side of the duct system. Remember, the return ducts are under negative pressure, so any leaks in them will pull air into the ducts. If that leak is in a dirty crawl space or attic, it's likely to be pulling in a lot of dirt, too. One of the worst types of return duct configurations is the panned joist return, as shown below.