Buy the coil light. Skip the air purifier. That is the short version of a question we get asked several times a week, and the two products look similar enough on a quote that most homeowners never learn there is a difference.
Ultraviolet light in the UV-C band does damage microorganisms. Nobody disputes the physics. What determines whether you get a benefit in your house is dose, and dose is just intensity multiplied by time. Change where you point the lamp and you change the time term by three or four orders of magnitude. One installation gets a useful dose. The other gets almost nothing, and is sold as though it got the same.
Coil-mounted UV: the version that works
A coil light sits inside the air handler and shines continuously on the evaporator coil and the drain pan below it. Those surfaces are the wettest place in your house. In Tampa Bay the system runs most of the year, so condensate is forming on that coil for months at a stretch, and the pan underneath holds standing water. Warm, dark, wet, and fed a constant supply of organic dust that slipped past the filter. If you were designing a mold incubator you would build roughly that.
What makes the lamp effective there is that the target does not move. Biofilm on a coil sits under the lamp twenty-four hours a day, week after week. Total exposure is enormous even though the lamp itself is modest. Growth that would otherwise mat the fin surfaces never establishes, the drain pan stays clean, and the drain line clogs less often because there is less slime to wash down it.
That last point is worth dwelling on, because it is the benefit nobody advertises. Most emergency calls we run for water dripping through a ceiling trace back to a condensate line plugged with biological gunk. Keeping the pan clean addresses that at the source.
There is also an efficiency argument, and it is a real one. A coil furred with biofilm transfers heat poorly and restricts airflow, and both of those make the compressor run longer. Keeping the coil clean keeps the capacity you paid for.
In-duct air sterilization: the version that is oversold
Now consider the same lamp mounted in a duct, aimed at moving air. Your blower pushes air past that lamp at somewhere between 500 and 1,000 feet per minute. A lamp with a twelve-inch effective zone gets contact with any given parcel of air for roughly a hundredth of a second, maybe two hundredths.
Compare that to the coil, which is under the light permanently. The dose difference is not a matter of degree.
Manufacturers publish kill-rate percentages that look impressive, and the numbers usually state the conditions they were produced in. Those conditions tend to be a sealed test chamber, a single organism selected for UV sensitivity, and a residence time nothing like your ductwork. Read the footnote and you will generally find the airflow assumption. It rarely resembles a residential system in cooling.
The effect in your ducts is not zero. It is small enough that it should not be what convinces you to spend the money, and it will not fix the problem you actually called about.
Some manufacturers try to close the gap with much higher-output lamps or with reflective duct sections that bounce light back through the airstream. Both help at the margin. Neither changes the underlying arithmetic, which is that air spends a hundredth of a second in the zone and a coil spends the whole summer there.
Telling the two apart on a quote
Ignore the product name. Look at where the lamp physically goes, because that determines everything.
A lamp installed inside the air handler cabinet, aimed at the face of the evaporator coil or down into the drain pan, is a coil light no matter what the brochure calls it. Mounted out in the supply plenum where nothing but moving air passes it, the same lamp is an air-treatment device and should be judged as one. Vendors blur this line constantly, sometimes because their own literature does.
Ask the technician to show you where it goes before the work starts. If the answer is the plenum and the pitch was about killing airborne viruses, you now know what you are buying.
Four things UV cannot do for you
- Remove particles. Dust, pollen, dander and smoke are physical matter, and light does not capture matter. Filtration does that job, and a properly sized filter does it well.
- Solve a humidity problem. Most mold complaints in Florida homes are humidity complaints wearing a costume. If your indoor relative humidity sits at 60% or above, mold will grow on drywall, in closets, behind furniture, and on window frames, none of which any duct-mounted lamp can reach. Fix the moisture and the mold problem mostly disappears on its own.
- Reach mold inside porous duct liner, or behind a wall, or under the pan. UV disinfects surfaces the light physically strikes, and nothing else.
- Compensate for a leaking return duct. If your return side is pulling 130-degree attic air through gaps at the plenum, you are importing insulation fibers, dust and whatever else lives up there faster than any lamp can address. Sealing the ductwork is the actual repair.
Ask one question before you buy anything
Ask whether the product generates ozone. Then get the answer in writing.
Certain UV wavelengths produce ozone as a byproduct, and a whole category of add-on devices produce it deliberately while calling it something friendlier: ionization, oxidation, activated oxygen, plasma. Ozone is a lung irritant. It is a bad purchase for the household that most wants an air quality product, which is the one with an asthmatic child in it. A device sold on the strength of a smell, or on producing "charged" air, deserves more skepticism than it usually gets.
Reputable coil lights are ozone-free and say so plainly on the specification sheet.
What ownership involves
UV-C lamps lose output long before they go dark. Output falls off through the first year of operation, and a lamp still glowing cheerfully at month twenty is producing a fraction of the germicidal intensity it started with. Annual replacement is not a manufacturer upsell, it is the whole basis of the thing working.
A lamp nobody ever replaces is an expensive night light.
Bulb changes are also not a comfortable DIY job for most people, since it means opening the air handler, and looking directly at an energized UV-C lamp will burn your eyes and skin. We handle replacement as part of routine maintenance so it actually happens on schedule.
The order we recommend doing things in
If mold, musty smells, or allergies are the real complaint, spending on UV first is spending in the wrong sequence. Work down this list instead, and stop when the problem goes away.
- Get indoor relative humidity into the 45 to 50% band and keep it there. This single step resolves more musty-house complaints than everything below it combined. Right-sized equipment that runs long cycles does most of the work, and where the house is oversized for its load, a whole-house dehumidifier finishes the job.
- Seal return-side duct leaks. Unfiltered attic air entering after the filter defeats every other measure you take.
- Fit the best filter the blower tolerates.
- Keep the coil and drain pan clean.
- Then, and only then, consider adding coil-mounted UV to hold that clean coil between service visits.
Coil-mounted UV earns its place in a Florida house, particularly near the coast where systems run long hours and the coil is wet almost year-round. It supplements a well-maintained system. It does not rescue a neglected one, and any salesman who tells you otherwise is selling you the wrong lamp. Our approach to indoor air quality starts with moisture and airflow, because that is where the results are.
