Ultraviolet lamps are comprised of two electrode filaments (the spring looking wire at each end), a glass and quartz combination vacuum tube, several inert gases and approximately 3 mg of mercury (Hg).

These specialty lamps are designed to provide light energy in the germicidal spectrum of 254 nm (nanometer = one billionth of a meter). As they operate, they slowly lose their germicidal effectiveness, about 15% per year. If the UV system is installed for the sole purpose of protecting HVAC equipment such as coils and blowers—surfaces that stay consistently in the range of the UV lamps, then the UV lamps will be effective for 9,000 hours for one year bulbs, and 18,000 hours for two year bulbs.

If the primary operation of the UV Bulb system is to aid in the reduction of indoor air contamination, then the recommended lamp replacement interval should be one to two years, depending on the lamp model. This will ensure needed higher irradiation levels are maintained to aid in the reduction of moving (airborne) microorganisms circulating through the HVAC system.

UV lamps should then be exchanged after a maximum two year installation period for several reasons, including:

The disinfection effectiveness will be significantly compromised and the system will not be able to generate UV-C light as specified. Note that even though a visible blue glow can be spotted though the system viewport, the glow is not UV light (which is invisible) but actually a by-product of the operation and an indication that the light is still powered and operating.

Depends on several factors.. NADCA (National Air Duct Cleaners Assn) recommends Annual inspections. 

We say 5-8 years depending on: 

 •   How many people live there
 •   How many people live there
 •   Sensitivities /Allergies/ Medical conditions of the residents.
 •   How often do you change the filter.
 •   Pets, Smokers, or other Indoor Air Quality factors (stuff that produces dust or contaminates or indoor pollution)
 •   How you set the thermostat, More runtime = More frequent inspection or Cleaning
 •   The age of your Home & its HVAC system

We use the source removal vacuuming process, the duct system is under negative pressure with our negative air machine during the cleaning process; plus we use HEPA vacuums during our process. We also place drop cloths and floor coverings & clean up thoroughly after every job.

 •  Look at the dust load in the house;
 •  Look at particulate or Microbial Growth (MOLD) showing around your supply air vents in the ceiling. If the ceiling is dark or discolored it needs attention.