We had a new AC system put in last summer. The contractor included a UV bulb between the cooling area and the supply fan. It's supposed to kill bacteria and other nasties in the air.

(The bulb is a Honeywell UV2400XLAM1 - Details here in case you're interested: https://customer.honeywell.com/resources/Techlit/TechLitDocuments/03-00000s/03-00011.pdf)

My question is "when should I replace the bulb?" Other than the obvious answer "when it stops working" I'm wondering if there's a point where (for whatever reason) it becomes less effective. Is it possible that the bulb could still be putting out visible light after it stops putting out UV, which I obviously cannot see?

These things are not cheap (they're around $70) so I'd rather not replace it sooner than I have to.


  • 1
    UV bulbs will continue to function as long as the coating on the bulb is in tact and the bulb still operates. Over time, the coating will break down and you will start to see spots on the bulb where the coating has burned off and you will begin to start seeing specks of white light. Once this has happened, then the bulb has lost its effectiveness and should be replaced. Dusting the bulb off with a soft cloth periodically can also help improve the lifespan of it. Dust will cause it to heat up more and will shorten the lifespan. Jan 14, 2015 at 15:01

1 Answer 1


UV lights in both HVAC and water purification systems should be changed yearly.

This is also in the Honewell spec sheet for the UV2400:

Lamp should be changed 1/year or every 9000 hrs to maintain its effectiveness

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Many systems have a timer and will start beeping and/or flashing a light after 1 year to remind you to replace them.

It's not that they will just stop working after that time, but that their output is reduced over time and they're built so they have an effective output for a year.

Should also be noted that the runtime is cumulative: if you only use this for 6 months of the year (and turn it off the other 6), you can go up to two years. The Honeywell documentation states 9000 hours (1 year = 8760 hours), which is typical of all water treatment UV systems (of which I have installed/serviced hundreds).

  • I'm a little skeptical when a manufacturer recommends I replace something with something I'd presumably buy from them, unless they give me a good reason. Jason Hutchinson's comment above is much more useful than Honeywell's simple edict to change the bulb. I also like your point about cumulative runtime. I didn't think of it until recently, but I will turn off the bulb for the rest of the winter and early spring. We only run the AC here about 5-6 months a year, so it seems like I'd only need to replace it every ~2 years or so.
    – user249493
    Jan 16, 2015 at 2:53
  • Fair enough. As I said, my experience is with water systems, where UV working properly is a more concrete health concern: the acceptable level of E.coli, for example, is zero. I have personally serviced UVs where the "treated" water tested positive for bacteria, and the UV bulb was either coated with a film (due to ineffective pre-treatment) or when you ask the owner when the bulb was last changed they respond "Oh, probably when it was installed a few years ago, I suppose". With HVAC, this dire health risk isn't present (eg you will not immediately get sick from no/ineffective UV treatment).
    – gregmac
    Jan 16, 2015 at 4:24

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