Picture of Furnace Add-Ons to Improve Home Air Quality
When thinking about indoor air quality, furnaces and central air conditioners should come to mind. After all, every cubic foot of air in a house is eventually drawn through these systems—and is altered by the process. Air is mixed, filtered and redirected, emerging hotter or colder, wetter or drier, cleaner or dirtier than it was before.

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.
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Step 1: Filters

Picture of Filters
Standard, disposable filters catch only the largest particles in the air and allow anything smaller than about 10 to 20 microns to pass through. (As a point of reference, a human hair measures about 100 microns across.) Fungi can be as small as 0.5 microns and bacteria 0.3 microns. Smoke particles are as small as 0.01 microns.

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).
KellyCraig3 months ago

UV light in forced air systems are usually used at just the coils, where mold and such builds up. All else is mostly hype and anyone pushing that as making their system better should be avoided for the hucksters they are.

If UV is used to purify the air, it would take a monster system, because the air would have to be exposed to UV long enough for the UV to do its job. As such, you could have to run UV the length of the ducting, or recirculate the air before it's released into a room to kill airborne bacterial and such.

Mindmapper19 months ago
I love the handling of sheet metal without protection, I wonder how many sticking plasters were needed before the job was finished? Interesting 'ible though.

I was thinking the same thing.

Pretty sad that they don't even suggest proper PPE.

neo716659 months ago
UV does kill some molds and can be helpful for that but yeah too clean is just as bad as too dirty.
carlos66ba9 months ago
Very nice. Could you explain why you think the germicidal UV lamps are necessary? I mean, we always live surrounded by germs, and too much cleaning is bad :)
ylexot029 months ago
I wonder if you could use UV LEDs instead. They'd take less power, last longer, and cost less.
NO. UV Led's are near UV. You need to get to ~5eV, only available with Hg bulbs.
astral_mage9 months ago
Environmental Protection Agency. is a easily bought out agengy.