UV-C Germicidal Furnace "Filter"

Having suffered from allergies most of my life, anything that promises to help with "allergens" tends to be of great interest!

Premise

Ultraviolet "C" band light is well known to be "toxic" to almost any living organism; such as mold, bacteria, viruses, & people.

https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation

I wanted to use this same "Kill EVERYTHING" approach toward any living contaminants that may have found their way into my home HVAC system. UV-C is already used for this exact purpose in commercial products and systems but the pricing can be prohibitive for an off the shelf unit. Comparing prices kinda stank; an under-powered consumer product started about $500, but went sharply up to ~$4000 for a cool fluorescent system that MUST be installed by approved HVAC tech to have any warranty.

Being a huge nerd, I asked myself the ultimate question: "How hard can it be?"

Design

This only qualifies as a design in the strictest sense, since even a table lamp has a switch!

2x - 50W+ E26 size UV-C COB LED bulbs
- Edison #26 is the most common light socket in the US
- https://en.wikipedia.org/wiki/Edison_screw
2x - Ceramic E26/E27 light outlets
- Un-switched type are cheapest
2x - 1" deep electrical light boxes
- Metal would be fine, but I used PETG-V0
6' - 18/2 home wire (18ga with black, white, ground)
- 3-wire service cord would work equally well
4' - 1/2" weather stripping
- Anything "UV resistant" and soft since the metal is fairly thin
Right Angle 3/4" clamping box fitting
- Points the cable the right direction and holds it tight
24"x12" 22ga steel sheet
- Gotta make a plate out of something sturdy and UV resistant

Build

We begin by inspecting the furnace system we want to modify; Looking for a flat place about 6-8 by 10-16 inches. A standard "off the shelf" vent size such as 8x16" would enable easy retrofitting if the home is ever sold. I believe it is better to install the lights on the return side of the furnace due to the lower air pressure, but I'm also not certain it even makes a difference. Cutting the hole can be done in many ways, but I used a small rotary tool with an abrasive cut-off wheel. Since abrasive wheels create a bunch of sparks and dust can be VERY FLAMMABLE, I removed the furnace filter and added a couple damp towels inside the furnace to prevent any sparks from igniting something and BURNING DOWN MY HOUSE!

Once we have a rectangular hole cut out of the furnace ducting, we want to smooth the edges of the hole before cutting and also smoothing the edges of our sheet metal plate to match the typical 1" on all sides overlap of our hole. 3mmx12mm screws with nuts were used to mount the electrical boxes onto the sheet metal plate leaving room for a second nut to mount the ground wire inside each box. Because I had it, and where is the fun in buying a couple $2 metal fixture boxes when we can print them out of a neat flame retardant and UV resistant plastic for many times the price!

Both light fixtures are connected in parallel with the grounds each being terminated to the metal sheet inside each box for extra safety. The incoming power for this new system can be acquired fairly easily by either installing a plug to be plugged into a regular wall outlet near the furnace, or it can be "hard wired" into the electrical system feeding the furnace. My furnace has an outlet for a "condensation sump pump" to send the condensation outside, so I used this outlet to hard wire mine.

Notes

Wiring was harder for me than it should have been since I used "scrap" materials wherever possible. This left me wiring up 130W of total lighting power with 6 feet of 12ga heavy wire instead of the 18ga or smaller that could/should have been used instead.

I ended up going the LAZY way and wired these in as "on all the time", since my HVAC system blower motor is Variable Frequency Drive, meaning there is no convenient place to hook up the lights to a switched 120VAC output like I hoped. 130W is less than the basement lights I keep forgetting to turn off... eh.