How to Install a Honeywell Ultraviolet Light Treatment System – Part 3

The sheet metal mounting plate is fastened to the air handler rigid duct board plenum and the Honeywell UV100A1059 UV light installed. This project is continued from How to Install a Honeywell Ultraviolet Light Treatment System – Part 2.

Air Conditioner Evaporator Coil Inspection

I haven’t opened up the evaporator coil case since last summer and was surprised to find mold had started growing on the foil-faced insulation! (What will they say about me at the garden club?!) There was no odor. Mold appears in a wide range of colors, including white, yellow, brown, black and green so identifying the particular species is difficult. The A-frame evaporator coils looked clean with the usual rust on the coil end plates and some rust stains along the top edge of the coils.

Mold Inside the Air Conditioner Evaporator Coil Case

Mold Inside the Air Conditioner Evaporator Coil Case

Yuck! Looking closer inside the plenum I found mold growing on the end of the rigid duct board at the bottom of the plenum box. What I believe happened is the edge of the fiberglass duct board tends to filter dust particles from the air stream which provides a food source for the mold.

Mold Growing on Air Conditioner Duct Board

Mold Growing on Air Conditioner Duct Board

Below is the opposite side of the evaporator coils and plenum box. Some mold colonies are visible on the foil-faced insulation. The evaporator coils appear clean and the rigid duct board plenum has only a few spots. The 12 inch flexible duct and metal start collar looks fine. Note that UV light will deteriorate the plastic flexible duct liner, causing it to become brittle… more on this later.

A-Frame Evaporator Coils and Plenum Box

A-Frame Evaporator Coils and Plenum Box

I bought the Honeywell UV100A1059 UV Surface Treatment System before I opened the evaporator coil case thinking it would be a nice add-on for my air conditioner. Now I see that I really need it!

Install the UV Light Sheet Metal Mounting Plate

The 1 inch thick fiberglass rigid duct board plenum won’t hold the UV100A1059 mounting screws. The 6 inch x 14 inch 16 gauge sheet metal plate provides hard points for the mounting screws and structural support for the Honeywell UV light by distributing the load across the face of the duct board.

The sheet metal mounting plate is fastened to the duct board with 2 inch wide aluminum foil HVAC tape. Don’t use regular “duct tape” because it won’t last. I used a carpenter’s bubble level to level the plate horizontally and centered it on the side of the plenum. The plenum is 18 inches wide and the steel mounting plate is 14 inch long. I therefore have a 2 inch margin on each end of the plate for the HVAC tape.

Note that the 2 inch hole in the steel plate for the UV lamp is well clear of the evaporator coils; when installed, the UV lamp will be about 3 inches above the A-frame evaporator coils.

Honeywell UV Light: Steel Mounting Plate for Rigid Duct Board Plenum

Honeywell UV Light: Steel Mounting Plate for Rigid Duct Board Plenum

Two overlapping layers of aluminum foil HVAC tape are applied to the secure the sheet metal mounting plate to the rigid duct board plenum. Space the layers of HVAC tape as shown to distribute the load across the duct board.

A 2 inch hole is cut in the rigid duct board with a narrow serrated knife, using the sheet metal hole as a guide. Cut the insulation to make a cone-shaped plug (like cutting the top of a pumpkin to make a jack-o-lantern) that is wider on the inside of the plenum. The purpose of the conical shape is to make an air gap between the UV lamp base and the duct board to prevent excessive heat buildup near the UV lamp filaments. Reach inside the evaporator coil case and remove the duct board plug. Trim the duct board as needed so the conical air gap is even on all sides.

Honeywell UV Light Installation: 2 inch Hole Cut in the Rigid Duct Board

Honeywell UV Light Installation: 2 inch Hole Cut in the Rigid Duct Board

Mount the Honeywell UV100A UV Light

Use a screw driver to fasten the two mounting screws to the sheet metal plate. Be careful not to over tighten the screws because you can strip the threads cut in the sheet metal. I don’t recommend using cordless drill/driver on the screws because it doesn’t provide the “feel” and fine control of doing it by hand.

Honeywell UV Light Installation on the Plenum Duct Board

Honeywell UV Light Installation on the Plenum Duct Board

Try not to touch the glass surfaces of the UV lamp (use a clean cotton cloth to hold it) and wipe down the glass with the cloth to remove fingerprints. Hold the lamp with the lamp indicator light in the UP position (refer to the Honeywell UV100A1059 installation guide for details).

Install the Honeywell UV Lamp UV100A1059

Install the Honeywell UV Lamp UV100A1059

Orient the UV lamp at an 11 o’clock position and insert it into the base unit, then right twist to lock it in place at the 12 o’clock position:

Honeywell UV100A Lamp Installation

Honeywell UV100A Lamp Installation

Plug the UV100A power cord into a grounded 110 volt AC wall outlet and don’t look at the light because it will damage your vision. The UV light is invisible, what seen here is blue light that’s close to the UV range in the light spectrum. I took this photo with a camera tripod and timer, then turned off the UV lamp before closing up the evaporator coil case. The UV Lamp Light Indicator on the handle will illuminate to show the bulb is working.

Honeywell Ultraviolet Light Surface Treatment System UV100A1059 above AC Evaporator Coils

Honeywell Ultraviolet Light Surface Treatment System UV100A1059 above AC Evaporator Coils

The completed Honeywell UV100A1059 Surface Treatment System installation:

Honeywell UV Surface Treatment Light over the Evaporator Coils

Honeywell UV Surface Treatment Light over the Evaporator Coils

The UV light is connected by an extension cord plugged into a grounded outlet by the attic stairway:

Grounded Attic Outlet and Extension Cord for UV Light

Grounded Attic Outlet and Extension Cord for UV Light

UV Light and Flexible Air Ducts

As noted in Part 1 of this project, UV light will deteriorate the inner plastic liner of flexible air ducts causing it to become brittle, crack and peel. The options for protecting flexible ducts from UV light are:

  • If the flexible duct is at least 3 feet away from the UV lamp, the flex duct should be OK and no protection is necessary.
  • Choose a UV light which has metal shield so the light only shines in one direction, e.g. downward on the evaporator coils.
    Honeywell UV lights don’t offer this option and it wasn’t important to me because I want the UV light to shine in all directions to help prevent mold in the plenum box.
  • Attach a metal 90 degree elbow fitting to the plenum box start collar, then connect the elbow flex duct to the elbow.
    The 90 degree elbow should provide sufficient UV shielding for the flex duct. An elbow may not be feasible depending on how your ducts are routed.
  • Replace the standard flex duct with a UV light resistant flex duct product.
    ATCO Rubber Products makes a special purpose ULTRA-FLEX Duct Kit made for use with UV bio-treatment lamps. ATCO only sells to distributors and it may be challenging for a homeowner to find the product.

What is my plan to prevent damage my flexible air ducts? Replace the 12 inch diameter flexible ducts with galvanized steel duct pipe and fittings.

Duct Board Plenum and 12 inch Flexible Ducts

Duct Board Plenum and 12 inch Flexible Ducts

The reasons for replacing the plenum flex duct with round metal duct pipe are:

  • Flexible ducts are a poor choice for trunk ducts due to friction losses caused by the ribbed inner liner, which reduces the airflow compared to smooth metal duct pipe.
  • Flexible ducts if not properly supported will pinch and constrict the air flow (see the above photo at the support strap).
  • Metal ducts are not affected by UV light.
  • The material cost to replace the two runs of 12 inch flexible duct will be $200 to $250.
  • The job is not difficult.

Why are flexible ducts used? Because flex duct is easy and quick to install versus metal ducts. Flexible duct is OK for short runs of branch duct, but not nearly as efficient and durable metal duct. As the following photos illustrate, the low-bid HVAC installer hired by the home builder (who quickly went out of business after my home was built) did a sloppy job installing the flexible ducts.

Flexible Air Duct Installation Errors

This is the 12 inch flexible duct on the front of the supply air plenum. All kinds of installation problems here:

  • The metal start collar is not sealed with mastic (best) or HVAC tape.
    Cold air is blowing and UV light is shining through the start collar ribs.
  • The inner core of the flexible duct is not taped to the start collar.
  • The flex duct is pulling away from the start collar because it’s unsupported for over 6 feet.
Flexible Duct Improper Installation

Flexible Duct Improper Installation

Another view of the flexible duct with the long unsupported span and pinched flex duct at the hanger strap on the left:

Pinched and Unsupported Flexible Air Duct

Pinched and Unsupported Flexible Air Duct

I’ll replace the flexible duct with sheet metal.

AC Evaporator Coil UV Lamp Performance

I took this photo several months later while installing the sheet metal air ducts and got a good look inside the supply air plenum above the AC evaporator coils:

Honeywell UV Light Treatment and AC Evaporator Coils

Honeywell UV Light Treatment and AC Evaporator Coils

Key observations are:

  • The Honeywell UV lamp spans both sides of the evaporator coils for maximum irradiation.
  • The evaporator coil faces are shiny and clean.
  • The condensate drip pan is very clean with no signs of algae or mold.
  • There is no indication the UV light has deteriorated or cracked the plastic condensate drip pan.
  • The mold blotches on the foil faces of the evaporator coil case insulation are bleached and crusty due to a really bad case of “sunburn” by the UV light.
  • The yellow insulation of the supply air plenum ductboard is very clean, almost like new.
  • A portion of the yellow ductboard insulation at the lower left and upper right has lifted, exposing it to the air stream. This promoted air filtering which darkened the edges and provided a medium for the mold growth as was seen in the previous photo.

Replacement UV Light Bulb

The Philips 265850 36 watt TUV PL-L Ultraviolet Germicidal Lamp used in the Honeywell system has a rated life of about 9,000 hours and mine burned out as expected after 2+ summers of use. I bought a genuine Philips bulb for 50% less than the Honeywell branded part and installed it Honeywell UV Light Treatment System Replacement Bulb.

Hope this helps,

Bob Jackson

Copyright © 2016   Reproduction strictly prohibited.

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21 Responses to How to Install a Honeywell Ultraviolet Light Treatment System – Part 3

  1. Phil June 19, 2014 at 9:35 pm #

    Thanks for the write up – very useful!

    One question. The Honeywell installation instructions show the light installed in line with the apex of the A frame – you installed yours at 90 degrees. Is this important?

    • Bob Jackson June 20, 2014 at 8:01 am #

      Hi Phil,
      I’m not sure which Honeywell UV Light installation manual you’re looking at but the manual for my model UV100A illustrates the light installed perpendicular to the A frame as I’ve done in Figure 2 on Page 3.

      An better (and more expensive) installation would install two lights parallel to the faces of the A-frame evaporator coils. e.g. * A * where the asterisks are a UV lamp unit. Another approach are UV lamps outside and inside the A-frame coils, however to mount a UV lamp inside the A frame requires cutting sheet metal and running wires inside the air handler.

      The single lamp across the coils was appropriately sized for my coils and it’s done a great job the past 2 summers.


  2. justin August 15, 2014 at 10:22 am #

    just installed one myself. did you have a period of a funky metallic burning smell afterwards? I have checked and rechecked my install and i see no visible issues.

    • Bob Jackson August 15, 2014 at 4:08 pm #

      I’ve not noticed any hint of a burning odor from my UV light. The UV light bulb is very hot. Is the bulb too close to the plenum insulation or metal top of the evaporator coils (cooking the dust)? Notice the conical plug I cut in the plenum duct board for an air gap between the light base and insulation.

      You might try wiping down the UV bulbs with a clean cotton cloth as there might be just enough dust to make an odor.

      > I have checked and rechecked my install and i see no visible issues.
      If you can’t see an obvious problem, open the evaporator coil case and feel around for very hot surfaces. Leave the UV light on until the moment before you remove the access panel.

      Let me know if you find the cause.

      • justin August 18, 2014 at 7:48 am #

        Hi Bob

        Thanks for replying back. After leaving the light on for a few days the smell has dissipated. I have checked the system numerous times I see no issues caused by heat as I used foil tape to seal the holes cut in the insulation.
        I think it is the lights cooking the remaining moisture off the coils after the AC shuts down and early on it was cooking the bio film off at the same time…. that is my guess any way as it no longer smells like it did. I need to look at my air handler settings to see if I can increase the fan time after the compressor shuts down to try and lessen some of the excess moisture remaining on the coils.

        Great site by the way! Looking forward to reading your other articles.


        • JR August 22, 2014 at 12:14 am #

          I came here looking for information on the burning smell too.

          I just had a new unit put in and added UV lights yesterday. Put them in late afternoon and noticed the burning smell the next morning when I woke up (about 16 hours later). It was pretty strong at first in the master bedroom (the first room the air gets to) but seems to be lessening as the day goes on. Same smell as when you turn the heat on for the heat for the first time after being off for the spring/summer.

          I figured it would go away after day or two. Glad to hear yours did.

          • Justin August 22, 2014 at 12:46 pm #

            Hi JR

            I still sometimes get a slight smell (or i am imagining it at this point) if the AC has been running for a while then shuts down and comes on again some time later. I have chalked this up to my light burning the moisture off the coils. But it is nothing like it was during the first 48 hrs when i believe it was burning off the bio film.
            my process
            Set the HVAC temp and turned the fan to on for about 24 hours
            Turned the fan and the hvac off for about 6 and just let the uv cook it.
            went back to normal mode (set temp and fan to auto) with very little to no smell now.

            Keep us posted. good luck.

            • JR August 27, 2014 at 10:45 pm #

              Hey Justin,

              Looks like we are in the same boat. It’s been about a week and the smell isn’t as strong as the first two days but I do notice it about once or twice a day. Like you, after the AC kicks on after being off a while.

  3. Stephanie November 7, 2014 at 10:56 pm #

    Thanks so much for your comments! I was also checking because of the burning smell. I was worried it was bad and I would have to call someone in the middle of the night. That would have been costly. I’m hoping mine goes away but if not I don’t mind. It’s slight and it at least let’s me know the bulb is working.

  4. Travis November 26, 2014 at 12:54 pm #

    Great article…thanks for taking the time to put this together! I had a lot of mold inside of my air handler on the blower and all around the surrounding insulation. We paid a company to come in and clean it up, and it looked great- for about 6 months. I recently checked again and found that some of the mold has started to return. Fortunately it has not gotten onto the blower yet, but it is on the side walls and in the corners. I went ahead and ordered the Honeywell UV light online from a local company here in NC,, (it was only $150 – best price I could find).

    As far as installing – my unit looks a bit different than yours. The coils are directly below the blower, and then there is a solid sheet of metal where the top of the blower connects into the duct work above. Given where the mold is and the layout of my air handler – the only place to put it would be in between the blower and the coils – although there will only be a few inches of clearance above/below. I will also have to drill the hole directly in the side of the air handler rather than in duct work. Are there any concerns with doing this? I’d be happy to share photos if that would help. Thanks again!

    • Bob Jackson November 26, 2014 at 6:30 pm #

      I’d like to see photos of your air handler before giving advice about drilling into the side of the unit. Please remove the front access panel to show the arrangement of the blower motor and coils. My e-mail is bob [at]

    • Max February 4, 2016 at 8:17 pm #

      I am wondering if Bob suggested you put the UV light between the blower and the coils. I have the same coil-blower arrangement in my air handler. I am trying to find a place to install a UV light.

      Thank you in advance.


  5. Mike Xavier November 28, 2014 at 6:10 am #

    Thanks for sharing. Just installed my unit below the A Frame coils, the unit is probably 9 inches above the filter, is this going to cause issues with destroying my filter? Also the glow comes throw the filter and is slightly visible through the closet door, is this something I should worry about? The underside of the A coils were disgusting, which is why I wanted to place the unit to shine directly on them, but I am worried to see if it affects the filter, and if the glow is a safety issue, I might just stuff a strip of weatherstrip in the slight gaps where the light is slightly visible. You cannot directly look at the light, because it is contained in the A coils.

    I am also thinking of placing a second unit in the ductwork for air purification. Let me know if you have any thoughts on the filter and glow.

    • Bob Jackson November 28, 2014 at 2:46 pm #

      UV light can degrade some plastics and cause them to become brittle and crack, especially with long term exposure. Some reusable filters have plastic frames and you’d want to inspect the frame for signs of degradation – cracking, powdery surface and discoloration. If you’re using disposable air filters with cardboard frames and changing the filter monthly then it shouldn’t be a problem. I personally prefer high quality disposable filters for convenience.

      > Also the glow comes throw the filter and is slightly visible through the closet
      > door, is this something I should worry about?
      I wouldn’t be concerned about it unless you have pets that might be prone to staring at the glow. (You can tell visitors the glow is caused by the fusion reactor!) The weatherstrip is a good idea if the door bottom gap isn’t necessary for air intake. What is the make and model of your air handler? Most air filters fit inside a slot that isn’t exposed.

      UV germicidal lamps for air purification can help if rated for this application. At issue is the brief period of time the flowing air is exposed to the UV light for disinfection of entrained microorganisms.

      • Mike Xavier November 28, 2014 at 3:54 pm #

        Thanks for the timely response, no pets and I told my kids they would go totally blind if they stare at the glow and I have them sufficiently scared.The weatherstrip filled the gaps around the door and glow isn’t noticeable any more. The gaps are not needed for air intake since the door is louvered but fold, it can still get plenty of air. I am using replaceable filters made of inorganic material called Nordic Pure 20x20x1M12-6 MERV 12 Pleated Air Condition Furnace Filter. I will be sure to check them monthly. The unit is made by carrier and it says the model number is fa4anc048. The ACoil actually had a plastic plug which I removed to place the light through

        It has only been 24 hours but the slight musty smell which used to eminate when I changed filters it totally gone.

        Thanks again.

        • Mike Xavier December 18, 2014 at 9:09 pm #

          Update, three weeks later and the coils look tremendous. Nice and spotless, now granted it hasn’t been as warm and the AC has not worked as hard.

          One problem, the disposable filters I am using are not going to work. There was a powdery residue on the cardboard and when I picked at the filter, the material came apart rather easily which let me know the UV light is attacking my filter and breaking it down. As I see it my options are : relocate the UV light above the coils, but I am not sure if this will be very effective, the space is tight and I cannot put the light perpendicular to the coils, so it would have to go lengthwise or parallel to the coils. the second option is to use a filter which is uv resistant, although I have not seen any with a merv rating of 11 like I currently use. Another option would be to reconfigure the air intake so the filter is not directly opposite to the coils, I am thinking maybe I can build a tight box added to the air handler and then place the filter on that box. All the air would go through box first with the filter vertically placed on that box. Sounds like the most complicated solution, or is it? Not sure if I could build it to fit directly into the filter slot, but I’ll see.

          Any thoughts?

          • Bob Jackson December 18, 2014 at 10:03 pm #

            A fiberglass AC filter would be UV light resistant and the simplest solution.

            It appears the Glasfloss Z-Line Pleated Series Filters have fiberglass media – “made of 100% synthetic fibers… which do not absorb moisture and will not support microbial growth” and have a MERV 11 rating. You can contact Glasfloss to confirm if the filter media is fiberglass or other UV light resistant material.

   sells your filter size 20x20x1:

  6. Mike Xavier December 20, 2014 at 3:06 pm #

    Thanks Bob, that’s what I figured. I went with an electrostatic UV resistant filter from the MERV rating is only 8, and I would have preferred higher; but until I replace the unit later in 2015, that will work for now.

    Happy Holidays!

  7. David R September 11, 2015 at 10:29 pm #

    Bob, found your project blog. Thanks for posting.

    It appears that you have the unit plugged into an unswitched receptacle. Is your model designed to run continuously?

    Would there be big disadvantages of activating with a timer or only when the HVAC unit is running? i.e. Any idea how much time is required per day to sanitize the area once initial sterilization completes?

    If the load time can be reduced, it might prolong the bulb life (assuming the startup/shutdown ‘trauma’ is not a factor); minimize the UV damage to sensitive materials; etc.

    About the installation, my coil is mounted “>” so the lamp ideally would be mounted “T” downward. Will lamp life be shortened when mounted vertically?

    Look forward to feedback from Bob or anyone who has experience in these areas.

    • Bob Jackson September 12, 2015 at 2:05 pm #

      That’s correct – the UV light for my air handler is plugged into an unswitched outlet and runs continuously during the cooling months. I unplug it for the winter months when heating. The effectiveness of UV germicidal treatment lamps primarily depends on the intensity and duration of exposure. Leaving the lamp on 24×7 during the cooling season ensures maximum exposure. I’d rather buy new UV lamps every couple of years than have mold growing in the air handler. So far it’s worked well and there’s been no deterioration of the plastic condensate drain pan.

      To automatically turn the UV lamp On & Off, the Honeywell Enviracom™ Communications feature in certain models connects to the 24V thermostat wires:

      “Enviracom™ Communications Capabilities:
      • Communicates with homeowner through three-wire communication bus using
      24 Vac thermostat connections.
      — Hooked up to Surface Treatment System model: uses thermostat load
      information to operate during times evaporator coil may be exposed to
      moisture to extend lamp life beyond two years.
      • Sends messages to communicate reset and receives remote reset, when available.
      — Bulb change indication cannot be reset by cycling power.”

      The Enviracom feature is only available on SmartLamp™ models with the Enviracaire label and apparently not sold on Amazon.

      > About the installation, my coil is mounted “>” so the lamp ideally
      > would be mounted “T” downward. Will lamp life be shortened when mounted
      > vertically?
      The Honeywell installation manual illustrates horizontal installation as “typical” but doesn’t say anything that would prohibit a vertical installation. You can contact Honeywell Customer Support to verify.

  8. Ron in Cape Coral May 2, 2016 at 2:07 pm #

    Thanks for the info. I was about to use the same mounting technique as shown above and it was really nice to see that others have done it this way too. Onwards and upwards tomorrow.

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