Mold Remediation Blog

Why The UV Light You Were Sold Isn’t Working

Incorrect Installation Renders 98% Of UV Lights
Useless For Killing Mold.


Installing ultraviolet (UV) lighting in HVAC systems is currently a big trend all over Northern VA, Central VA, Southern MD, and Washington DC. And UV light has become an even hotter commodity amid the COVID-19 pandemic. 

It’s easy to see why. Theoretically, UV light kills mold, bacteria, and viruses in your HVAC system. This results in cleaner, safer air in your home.

But as the area’s honest to a fault [core page: Open-Book Honesty] remediator and unofficial “burster of bubbles,” it’s my duty to tell you… 

These UV lights that local HVAC and air duct companies install almost never work. 

By “almost never,” I’m talking a 98% fail rate. Literally, 1 out of every 50 UV lights I come across in HVAC systems and air ducts aren’t doing what they’re supposed to: Kill mold.

So what gives? Why won’t a UV light in your HVAC system work? And, if it doesn’t work, why do companies even sell them?

I’ll explain…

Does UV Light Actually Kill Mold & Bacteria?


There is plenty of research to prove UV light kills mold and bacteria. Places like hospitals rely heavily on UV light in their HVAC systems to prevent the spread of airborne contaminants and lower the amount of infections. 

So the product itself isn’t the problem. It’s the installation.

To understand why, first you have to know that UV light needs what’s called dwell time to kill mold and bacteria. The specific UV dosage required to kill different types of mold and bacteria varies. This dosage is measured in microwatts a second per square centimeter (µW/cm2)

Your average residential UV light puts out 50-200 µW/cm2. This means it needs a dwell time of 1,650-6,600 seconds (a.k.a. 30 minutes to 2 hours!) to kill a mold spore. Bacteria and viruses, however, only need 50-200 seconds to die.

In other words, it essentially takes UV light 66 times longer to kill mold than bacteria.1

This equation is key to answering our question of why UV lights installed by HVAC and air duct companies almost never work.

How Companies Get UV-Light Installation Wrong

Two kinds of companies typically install UV lights: heating/cooling companies and air duct companies.

These companies often don’t know much about mold remediation. All they know is that a) UV light can kill mold, and b) UV-light installation is a highly profitable upsell. 

Due to these companies’ lack of mold knowledge, most of them unwittingly install the UV light in the wrong part of the HVAC system. Air duct companies typically install UV light in the air ducts; HVAC companies often install it above the evaporator coil.

Both of these methods are incorrect… and render that shiny new $800 UV light useless. 

If UV light is installed in your air ducts, it doesn’t have the necessary dwell time to kill mold. Everything zips by too quickly for UV light to work its magic. (Imagine trying to wash your car squeaky-clean as it speeds by at 100mph.) 

If UV light is installed above your HVAC system’s evaporator coil, it’s not hitting the spot where all the gunk builds up—the bottom of the evaporator coil. (Imagine having a rash on your arm and putting ointment on your leg.)

The Installation “Sweet Spot” For UV Light

The only way a UV light can kill mold in an HVAC system is when it’s installed under the evaporator coil and above the drip pan. 

The drip pan in your HVAC system sits directly beneath your evaporator coil to catch the moisture that drips from the coil. This constantly wet and humid area has the perfect conditions to grow colonies of mold, bacteria, and viruses. 

When UV light is installed under the evaporator coil, the light comes in direct and unbroken contact with the underside of the coil and the drip pan. This allows the UV light the necessary dwell time to kill anything that tries to grow.  This is where the effectiveness of UV light is—it prevents mold, bacteria, and viruses from gaining a foothold in the evaporator coil or drain pan and growing into a colony that could eventually make people in the house sick. 

Are there downsides to this location? Yes. If the drain pan is plastic, then the UV light will degrade it and possibly surrounding wiring. 

The downside to UV light itself is the bulb degrades and MUST be replaced every year religiously. If you don’t, then it produces increasingly more ozone—which is a known respiratory irritant per the EPA.

As I said, many HVAC and air duct companies install UV light in the wrong spot because they don’t know any better. Some, however, don’t install UV light under the coil because it requires a delicate touch. 

Installing a UV light under the evaporator coil requires drilling a hole in an area where a bunch of really expensive components are located. Accidentally drilling into the coil is a $2,000 repair, so most companies are afraid to install UV light in this area. So they drill a hole in the top of the HVAC unit or in the air duct instead to avoid the risk of damaging valuable equipment.

But as I mentioned a moment ago, that’s like putting ointment on your arm to treat a rash on your leg. It won’t work!

UV-Light Installation: Yay Or Nay?

Yay… if you can be certain your HVAC or air-duct company will install the light under the evaporator coil.

Nay… if your HVAC or air-duct company tries to install the light anywhere else.

Have more questions? Need a mold test? Want to remove the mold in your home in Northern VA, Central VA, Washington DC, or Southern MD?

Get in touch. Whatever your mold issue, we would be honored to help.


  • “Using Ultraviolet Light in HVAC Systems,” Ft Wayne ASHRAE Seminar (2006).


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