Above the coil, so it can prevent bacteria from growing.
+1 what Beenthere said :thumbsup:
UV Lights are one of my best selling items, I ship several each week, but I also have some very personal experience with them as well.
Four out of the past five years, I have suffered extreme bronchial issues. The last two years, it manifested into full blown pneumonia in my left lung. After the last bout, I decided to do something about it. This hits me each year about mid July and had me down for over six weeks each time.
I put a BlueTube UV light above my A-Coil in my air handler and also added a 5" Trion Air Bear to my system. Last year I made it through without a problem, and so far this year all is good (knocking on wood).
When I installed the light, there was significant mold growing on the coil and insulation of the AHU. When I changed my bulb last year, it was all nice and clean:thumbup::thumbup:
If you are planning in installing a 24 volt, just make sure that you are confident that you can locate the two 24V wires. If not, just order a 120V and wire to your home power.
Also, if you have a variable speed blower drive in your air handler or furnace, you should add an additional 24 volt transformer (usually under $12). If not you are open to power spikes from from your blower system that will prematurely roast your power supply or bulb.
Good Luck,