Should You Get Your Air Conditioner’s Ducts Cleaned?

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.

In the panned joist return (or simply, panned return) the installer panned the joists with sheet metal and attached a duct to the bottom. In the house above, you may find a big floor grille, or they may have used a wall stack to get the vent into the wall.

Either way, panned returns are usually very leaky because the many joints and seams are often poorly sealed. In the photo above, they’ve used a little bit of asbestos tape (a whole nother issue!), but many of the places where crawl space air can leak in are unsealed. Even if they were sealed initially, expansion and contraction at the wood/metal junctions can cause the seal to break down over time.

The Lowdown

Duct cleaning isn’t a bad thing. Without treating the causes, though, it’s a short-term relief of symptoms at best and perhaps a nightmare for someone with allergies if they stir up and release into the house all that nasty stuff that’s inside the ducts. Duct cleaning should never be done without complete duct system analysis to find out where the dirt is coming from followed by the necessary repairs to stop the dirt from getting in. Of course, any duct system repairs should take into consideration the air flow in the duct system. If they do all that, you’ll get clean ducts that stay clean as well as a more energy efficient and comfortable home.

Photo of dust mites by Gilles San Martin from flickr.com, used under a Creative Commons license.