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.

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).

Humidity has a lot to do with indoor air quality and comfort. Too much humidity can rot windows and walls and cause molds and fungi to grow. Too much humidity is a health concern, but so is the opposite extreme. With too little humidity—below about 20 percent—our skin cracks and our respiratory system dries out, inviting infections. Humid air also feels warmer than dry air, so you can lower the thermostat and reduce energy costs.

A traditional furnace-mounted humidifier has a drum that’s covered with a foam rubber pad. The bottom of the drum rotates through a water reservoir and the air moving over the drum absorbs some of this water. These units do a good job, but the water standing in the reservoir can become stagnant so it needs to be treated periodically.

The type we chose is a bypass, flow-through humidifier (Honeywell Enviracaire Elite, Model HE225, about $225, uninstalled c.2001). Instead of a reservoir, it uses a textured aluminum sheet that’s coated with an absorbent clay. This coating is antimicrobial, so it prevents the growth of bacteria, molds and algae.

The air moving past this sheet absorbs some water, and the excess, about 3 gal. per hour, is sent into a nearby floor drain. This may seem an inefficient mechanism, but it is a good way to gain moisture while leaving the minerals and microbes behind. You will need to replace the aluminum/clay core about once a year.

These UV light units are really quite simple. They consist of a plastic base that mounts on the ductwork, wiring for the power supply from a 120-volt outlet, an indicator light and a UV tube. The back of the base has a flexible gasket to keep harmful UV light from leaking out.

The two light probes we used are similarly built, but they have different functions. The longer probe, installed in the return duct, is designed to kill airborne bacteria in the system. The other, installed in the plenum, focuses on the slimy molds that grow on air-conditioner evaporator coils. These molds can generate allergic reactions and they’re responsible for the “dirty sock” odor common to air conditioners.

While the lights last three to four years, they lose about 10 percent of their effectiveness per month, so you’ll need to replace the bulbs about once a year. A replacement tube runs about $100. The light in the return-air duct should be used continuously, but the plenum light needs to be on only during the air-conditioning season.

We chose Honeywell Enviracaire Elite UV lights (Models UV100E1001 and UV100E1027). We paid about $815 for both units c.2001. 

To install our wider electronic air-cleaner housing, we needed to replace the down-return ductwork. Factory-made, return-duct components are available at home centers. If you decide on a professional installation, costs can run between $200 and $300.

First, shut off power to the furnace. Look for a toggle switch on a nearby wall or a fused switch on
the side of the furnace cabinet.

Remove the furnace’s front panels and fiberglass filter. Then, examine the down-return sheetmetal. This sheetmetal box consists of three or four sections, joined by S-locks. An S-lock is made by bending 3⁄8-in. folds along the ends of mating boxes. The folds are then drawn together by a strip of bent sheetmetal called a drive. Bend the ends of the existing drives straight and use pliers to pull each from its lock. If you’re replacing the entire down return, start near the top and work down.
With the upper sections separated and removed, reach in and straighten the tabs on the flange of the return connection and pull the lower box away (Photo 1). If you need to replace only this lower box, leave the upper sections in place and adapt the new box to them.

Using tin snips, trim away enough of the new return box to accommodate the width of the air cleaner (Photo 1). Fit this new edge with premade, L-shaped flanges, called standing S-cleats. Slide the fold of each piece over the edge of the box so that the flange is oriented inward (Photo 2). Then, screw or blind rivet the S-cleats to the box.

Set the frame of the air cleaner on the new return box and screw the frame to the cleats using self-tapping screws (Photo 3). Then, set the air-cleaner frame against the side of the furnace cabinet and screw the frame to the cabinet (Photo 4). If the existing cabinet opening is substantially smaller than the air cleaner, enlarge the opening as needed.

With the air-cleaner frame installed, rebuild the return box, section by section. If necessary, start by enlarging the opening in the overhead horizontal return-air trunk. Hold a premade tap over the opening (Photo 5), and secure the flange to the duct with self-tapping screws. Finally, join the tap to the return box with premade sections of sheetmetal. You’ll need to cut the last
section to fit. Measure the opening and add 3⁄8 in. to allow for a new S-lock fold. Chamfer the corners slightly and use a hand seamer or folding tool to make the fold. Finish by drawing the sections together with drives. Bend each drive at one end and tap it onto the folds with a hammer. Install the air-cleaner components (Photo 6) and add the cover.

The UV lights come with paper templates to help you lay out the openings in the down return and plenum. Starting with the light for the return-air side, tape the template to the front of the down box, about 40 in. off the floor. Mark through the template and drive an old screwdriver into the sheetmetal to start the hole. Finish the opening with tin snips and bore the screwholes with a 1⁄8-in. bit (Photo 1). Screw the light base to the box (Photo 2). Then, slide the UV tube into the base and twist it clockwise to lock it in place (Photo 3).

Locating the plenum light is a little more challenging. Mold is much harder to kill than bacteria, so you’ll need the light to cover the entire air-conditioner coil. We removed an old patch from the front of the plenum to locate the top of the coil. If you can’t easily gain access, position the light as high as you can.

Flow-through humidifiers are usually installed in the return-air down box and are connected to the warm-air supply duct with 6-in. vent pipe.

Use the humidifier template to mark the rectangular opening, make a starter hole and cut the opening with tin snips (Photo 1). Position the housing over the opening, with its bottom tabs over the lower edge of the opening. Level the housing and screw it to the sheetmetal with self-tapping screws (Photo 2).

Next, cut a 6-in. opening in the supply-side duct. Insert a 6-in. collar and fold down the tabs to lock it in place. If your furnace has air conditioning, the collar needs to have a baffle that can be closed each summer. With the baffled collar in place, attach an adjustable elbow and aim it toward the humidifier (Photo 3). Run pipe to the humidifier, install a second elbow and secure all joints with screws (Photo 4).

With the sheetmetal connections made, connect the water-feed tube to the humidifier pad assembly (Photo 5) and press the assembly into the housing.

As for the water connection, you’ll need to decide whether to tap the hot or cold side of the water system. Hot water makes the humidifier about 10 percent more efficient, but it flushes away water you’ve paid to heat. In any case, install a saddle tap on a nearby water pipe and run 1⁄4-in. copper tubing between the tap and humidifier. Slide a compression nut and ferrule onto each end and bend the tube to meet the fittings. At the humidifier, coat the ferrule with pipe dope and thread the nut onto the unit’s solenoid valve (Photo 6). Make a similar connection at the saddle tap and tighten the nuts (Photo 7). Then install the humidifier cover (Photo 8).

Run low-voltage, two-wire cable from the humidifier to the furnace cabinet and from the furnace to the humidistat. The exact wiring method will vary with the furnace. Some systems have humidifier taps and some do not. If you don’t see a terminal board with humidifier taps labeled, you’ll need to splice into the blower fan’s wiring according to the manufacturer’s instructions. To power the unit, install a transformer on the side of the furnace and attach low-voltage wires to the two external terminals. Finish by running cable to a humidistat in an adjoining room and connect the wires to the leads with twist connectors (Photo 9).