Does my HVAC unit have to be running in order for a germicidal light to be effective?

The effectiveness of a germicidal light in your HVAC system depends on its type and placement. Here’s a breakdown:

1. Coil-Sanitizing UV Lights (Most Common and Always Effective):

• Placement: These lights are installed directly on or near the evaporator coil and drain pan inside your indoor air handler.
• How they work: Their primary function is to continuously irradiate these surfaces with UV-C light, preventing the growth of mold, mildew, and bacteria that thrive in damp, dark environments.
• Does the HVAC unit need to be running? No, not necessarily. These lights are typically designed to run 24/7. They are always on, continuously killing microorganisms on the coil and in the drain pan, regardless of whether your furnace or AC fan is actively blowing. This constant exposure is what makes them highly effective for surface disinfection and preventing buildup.

2. Air-Sanitizing (In-Duct) UV Lights:

• Placement: These lights are installed in the ductwork, usually in the return or supply plenum.
• How they work: They aim to kill airborne pathogens (bacteria, viruses, mold spores) as the air passes through the UV light in the ductwork.
• Does the HVAC unit need to be running? Yes, for maximum effectiveness. For these lights to purify the air, the air needs to be moving past the UV lamp. If your fan isn’t running, air isn’t circulating through the ducts, and thus, isn’t being exposed to the UV-C light.
  • Some in-duct UV lights are wired to turn on and off with your HVAC system’s fan, ensuring they are only active when air is circulating.
  • If you have your fan set to “ON” (meaning it runs continuously even when there’s no heating or cooling call), the air-sanitizing UV light will be working continuously to treat the circulating air. If your fan is set to “AUTO,” it will only operate when the heating or cooling cycle is active.

Key Distinction:

• Coil-sanitizing lights primarily address surface growth and work continuously.
• Air-sanitizing lights primarily address airborne pathogens and are most effective when air is actively moving through the ducts.