Richard Trethewey installs a media filter in a forced-air furnace
Photo: Keller & Keller
Q: Despite daily dusting and vacuuming, and frequent changes of the forced-air filter, the inside of my house is covered in dirt. Could the ducts running through the dirt-floor crawl space be to blame?
Rhonda Mensen, Atlanta

A: Richard Trethewey replies: Yes, they certainly are the prime suspect in this case. When air moves through the ducts, the pressure inside them drops, and if those ducts aren't properly sealed, that low pressure sucks in dust, insects, radon gas, or whatever else happens to be airborne nearby.

To prevent this from happening, every joint and connection in the system must be sealed either with aluminum-foil duct tape or a duct sealer, like RCD's #8 mastic (sold online at RCD Mastics). Never use plain gray duct tape—it's great for a lot of things but not for sealing ducts. Also, check all the connections to floor registers to make sure they're tight too. By the way, taking care of leaky ducts can also improve the efficiency of your system by up to 20 percent, so there's another good reason to seal them up.

Once your ducts are sealed, make sure the system has an effective filter. Those inexpensive see-through filters just won't do the job, no matter how many times you replace them. Instead, you need a high-efficiency pleated media filter with a minimal efficiency reporting value (MERV) of 9 to 12. Ask your HVAC company to recommend the best one for your system; you don't want to install one that makes the blower motor work too hard. Make sure the new filter fits snugly in its tray so that dust and dirt can't sneak past.

One last thing: Vacuuming can throw a lot of dust into the air. Unless you have a vacuum cleaner with a high-efficiency HEPA filter or a whole-house central vac, some of the dirt you pick up is just going to be spread around again.

Shown: Richard Trethewey removes a standard fiberglass furnace filter so that he can put in the more effective pleated media filter.

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