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.

Step 1: 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).
<p>For kicks and giggles:</p><p>Submarines have used electronic air scrubbers for decades. They work.</p><p>Popular Electronics published an article telling how to build an electronic air scrubber, a transcripitor, back in the sixties. It used a flyback transformer out of a color television, some stove pipe, and a wood case with an interlock.</p><p>The contaminants in the air drawn through the stove pipe was given a negative charge and the stove pipe a positive charge. Since current flows negative to positive and the two attract, the contaminants adhered to the pipe, until power was cut.</p>
<p>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.<br><br>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.</p>
<p>Look up the Sanuvox approach at the link above. Your second paragraph is simply wrong. Coil irradiation is most commonly found in commercial HVAC systems. </p>
<p>Back from looking up the Sanuvox and I have to say the same rules apply, unless a VERY expensive and complex system is used.</p><p>Bulbs have to be changed in a timely manner, since their efficiency drops over time. Too, the air must be exposed for an adequate amount of time to kill bacteria, fungus, mold and so forth. As such, it takes more lighting than just a few bulbs in the system.<br><br>Is that not true?</p>
<p>I came in the residential systems I had installed in customer remodels. I wasn't aware it was uncommon. By the hype at the home shows, it would seem not to be.</p><p>Will look into the Sanuvox. It wasn't around or was not common before I retired a few years ago. <br><br>The Sanuvox system aside, until I have looked into it, I do know businesses were pushing UV for ducts, but for the reasons stated, it was wasted money.</p>
<p>I forgot to add that having UV may be incompatible with using reusable or 'lifetime' filters because the UV is not friendly to the plastic materials. One reusable filetr I recently saw speciafically said it was not UV compatible. </p>
<p>Some of this information will be very dated given the original article was done almost 15 years ago. I have a Sanuvox UV in-duct unit* in my forced air furnace and it looks quite different form the Honeywell unit shown. It is great for odour control and reduced dust in the house. In terms of ongoing operating costs, the special bulb lasts three years. I understand the Sanuvox units have been sold under other brand names. </p><p>I would recommend anyone interested in UV air treatment in a home to look into their technology and product line. </p><p>*similar to this current model: http://sanuvox.com/en/sanuvox-r+.php</p>
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.
<p>I was thinking the same thing.</p><p>Pretty sad that they don't even suggest proper PPE.</p>
<p>this is from 2001, before the sissification of the populace via Facebook and Twitter. It's pretty sad you failed to recognize that, and at you actually need someone to hold your hand and tell you something that is common sense. </p>
UV does kill some molds and can be helpful for that but yeah too clean is just as bad as too dirty.
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 :)
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.
Environmental Protection Agency. is a easily bought out agengy.

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