Anyone have one? Do they work? They’re not to “clean” the air per se, rather to keep molds and mildew from forming on the coils etc. A/C guys want about $400 to install. Is it worth the investment?

I had one installed when we replaced our HVAC 5 years ago or so. The lamp went out at least a year ago and I haven’t replaced it due to me and cramped “attics” not getting along.

In theory it sounds reasonable but I really don’t know anything about how UV kills bacteria. Who knows if the light is the correct type or if the air is exposed long enough.

Maybe I’ll muster up the motivation to switch the lamp this weekend.

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UV lights in A/C

Great timing, we're looking at adding a 4 inch MERV filter cabinet, getting the stock filter area out of the circulation since it has issues with location and filter size.

Our unit is in the garage, in FL, where it's humid so mold and mildew are issues that need to be addressed and with asthma I've been thinking a UV might be beneficial.

This is the reference article I've found that has plenty of links to different setups and information on the use of UV outside of AC to control mold, mildew, disinfect in food service and hospitals.

Seems the consensus is it has benefit, and there is a unit you can get on Amazon for $70 and install yourself, as there are others.

Be interested to see the comments from our resident HVAC people...

Link

Studies have shown that UV will kill bacteria provided it is strong enough, close enough, and long enough.

When I worked in Syracuse, our orthopedic surgeons demanded high power UV lights be placed in every room they used to reduce the incidence of post-operative infections. These lights covered every square inch of the room and were very powerful, but were placed up in the corners of the ceiling 10 feet off the floor. It had no effect on their rates and the only thing they did accomplish was giving the staff in the room really nice tans. When this side effect was noticed, the remedy was not to put a kibosh on the whole practice, no.... They're solution was to require everyone in the room to cover all exposed skin and crank up the power. We all hated working the ortho rooms. Other than that, it was a great place to work.

My thoughts are that throwing them in AC units is more a money making gimmick than anything else. Consider how many millions of people have used AC over how many decades, and how many of those had any health issues related to mold, mildew, or bacteria in the unit. In fact, provided you have an adequate immune system, exposure to these things is a good thing.

^^^^^^

I agree that it's BS.

I'm no HVAC expert, so the math might be off here:

The typical furnace fan can move 1000cfm. On a 2x2 filter area, with a 1' light chamber you'd have 4 cubic ft in the chamber at any given time, that's exchanging 250 times per minute. Exposure time simply isn't long enough.

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The typical furnace fan can move 1000cfm. On a 2x2 filter area, with a 1' light chamber you'd have 4 cubic ft in the chamber at any given time, that's exchanging 250 times per minute. Exposure time simply isn't long enough.

From what I've read, they aren't intended to cleanse the air moving through, they keep the internals of the AC handler free of mold, mildew, etc.

Since the internals are kept clean, the air passing through now moves less of these contaminants through the house..

My parents had a home a decade ago that had the UV light and the 6" filter installed. The former owner had some serious health issues and they felt this could help them.
Is it better, who knows. The 6" filter is scheduled for replacement every 6 months as I recall. The problem is that the UV light degrades the paper filter and it falls apart before that. So I'd guess the filter needs to be replaced about 3 months to be safe.
In my own system I just use a higher quality filter, change it often, and put a small amount of bleach into the condensate pan every year or so to kill of anything that might be there. Be careful to not overdo the bleach or your house will smell of Clorox.

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Originally posted by 220-9er:
My parents had a home a decade ago that had the UV light and the 6" filter installed. The former owner had some serious health issues and they felt this could help them.
Is it better, who knows. The 6" filter is scheduled for replacement every 6 months as I recall. The problem is that the UV light degrades the paper filter and it falls apart before that. So I'd guess the filter needs to be replaced about 3 months to be safe.
In my own system I just use a higher quality filter, change it often, and put a small amount of bleach into the condensate pan every year or so to kill of anything that might be there. Be careful to not overdo the bleach or your house will smell of Clorox.

You DON'T want to use bleach. The fumes (chlorine) are very toxic to aluminum and will shorten the life of your coil. Use hydrogen peroxide or white vinegar instead or A/C coil cleaner as per instructions instead.

I know a few things about germicidal UV from a project I worked on. Here are the basics: there is a fairly narrow band of UV wavelengths that are very good at breaking apart DNA molecules that peaks around 260-265 nanometers. The effectiveness of UV at breaking up DNA drops off on either side of that. So, you will find a lot of germicidal lamps around 254nm to about 280nm. Anything much more or less is wasting a lot of power. UV light ranges from 100nm up to 380nm so there is a whole bunch of UV that is not good at killing germs. In order of germicidal UV to do its job it needs two things: power and time. The light needs to be powerful enough to reach the DNA inside the cell or spore, and to shine on it for long enough to break down the bonds of the DNA. The more powerful the lamp, the less time is needed. How much power-time does it take? Well that is a topic of research and varies with the medium the germs are in (air absorbs less power than water for example) and what sort of materials the UV lamp is made of (there are glasses and plastics that are better or worse at passing UV.) To find out exactly how much takes either a lot of modeling and/or a lot of testing of the actual system.

So, how does any of that answer the OP’s question? Well, it gives us some things to look at that might help spot bullshit products. First, not all UV is created equal, so make sure you are getting the germicidal kind. If the UV lamp looks purple, it is the wrong wavelength and doing nothing. Germicidal UV should be more of a light blue. Second, the most common types of lamps to generate the right sort of UV are LEDs and mercury vapor lamps. So, you want to see little fluorescent tubes or a bunch of LEDs. If you see a normal incandescent bulb, it is likely a scam. Since the lamp doesn’t get to control how fast the air is moving through the system, all it can do is hope to put out enough power to do the job. In order to do that you need watts and you want the air to get as close to a light source as possible. So, one big tube screwed to the side of the case isn’t likely to be as effective as a grid of small tubes that cover the whole cross section of the duct. That doesn’t make a single tube worthless, but it will likely be cleaning the air within a couple inches of the tube, but not all the air going past. As far as power, more watts is generally better than less, as long as nothing overheats and burns up. Also, quartz glass pure enough to pass UV well is expensive. Cheap lamps don’t use it, so they may be the right color but the glass of the lamp will absorb a bunch of the wavelengths you want. So, the cheap lamp replacements are not a good sign. In general, I would say that a kit built for your specific AC system is more likely to work than a universal or generic system. Getting good coverage and getting enough power over time is critical, and the more a system is designed to work with exactly what you have, the more likely it will actually do something useful.

Lastly, there is a second effect from some UV lights that helps with killing germs. UV can evolve Ozone on the surface of the quartz glass of the lamp, and that ozone is also good at destroying organic molecules. A real UV lamp should put off a faint smell like an electric arc. If you can’t smell that near the lamp when it is running, that isn’t good. By the time the air makes it out of the ducts and into the room it shouldn’t smell, but right near the lamp should have that ozone smell when running. LEDs tend to produce less ozone than mercury vapor lamps, so take that into account as well. Because of this, I think the tube lamps are probably a bit more effective than LEDs, but I don’t have numbers to prove it.

I know none of that was a straight answer, but hopefully it was still helpful.

- Bret

I've seen people and companies installing in-line UV lights in the ducting... I'm not convinced of its effectiveness. It will certainly use electricity and cause wear and tear.

If you want to control mold and mildews, control humidity and keep it under 60%. 40-60% is comfortable for people. Higher and you'll start getting enough moisture to grow stuff, lower and it dries the people out.

I've seen UV based air purifiers, sold for medical/industrial uses - but they're expensive - and basically, are a bunch of UV lights, and a fan so that the dwell time works out with the lights.

Measles is a larger concern these days, than COVID, but high rise buildings really do need someway to sterilize the air.

TMK, they do not have one.