Treating your indoor air as it passes through the forced-air system ductwork is not a new idea. Most furnaces already have built-in dust filters and some are equipped with humidifiers. However, the latest filters are far more effective than standard models, and adding or upgrading a humidifier is a simple way to enhance comfort and health.
In addition to these two components, we’re including something new—a pair of ultraviolet light probes designed to kill molds and bacteria. Ultraviolet (UV) lights have been used for years to kill germs in hospitals and municipal water systems, so they should work in homes as well.
This project was originally published in the September 2001 issue of Popular Mechanics. You can find more great projects at Popular Mechanics DIY Central.
Step 1: Filters
The big guns in dust and allergen control are accordion-style paper (media) filters and electronic air cleaners. Either of these can be installed in the place where you would find a standard filter, but the support frames are larger—about 5 to
8 in. wide. In a retrofit like ours, that means reworking the sheetmetal that connects the vertical return-air down box to the furnace.
A media filter (about $250, uninstalled) is a popular choice for allergy sufferers. Pollen can be as small as 6
microns, and media filters trap 99 percent of 6-micron and larger particles, plus about 65 percent of 1-micron particles. Interestingly, paper filters grow more effective as they fill with dust. After six months, they’ll trap 82
percent of 1-micron particles. Maintenance consists of replacing the insert (about $28) each year.
Electronic air cleaners are a little more complicated because they require a 120-volt receptacle within 3 ft.
of the furnace to supply power. Then, instead of a yearly filter replacement, the air cleaner’s cells containing the collection plates need to be washed monthly. It’s easy work, and the cells are sized to fit into dishwashers, but it’s still a chore. These units produce a small amount of ozone, which is itself an irritant. The amount is well below the Environmental Protection Agency limit, however.
So why opt for an electronic air cleaner? Because they trap more of the tiniest particles—the ones that settle deepest in our lungs and are potentially the most harmful. Electronic air cleaners trap 70 percent of particles 0.3 microns in size. That’s four to five times more effective than a media filter for this particle size.
The electronic air cleaner’s cabinet contains two metal prefilters, for pet hair and large dust particles, and two electronic cells. The cells contain rows of high-voltage/low-amperage wires that negatively ionize all particles that pass near them. This charge causes the particles to stick to rows of positively charged metal plates behind the wires, where they remain until you wash them away. The unit we installed in our forced-air system is the Honeywell Enviracaire Elite, Model F300 (c.2001, about $520, uninstalled).