I have an AquaUltraviolet 15W classic sterilizer. I was thinking about trying to use the 8W bulb in the unit to reduce heat since it is a bit oversized for my tank and it is producing a lot of heat. The thought process is that since the 8W and 15W bulbs are the exact same physical size 11.5" w/ 4 pin connector, and the ballast is capable of driving up to 15W, that it would be able to safely drive the 8W bulb or the 15W bulb depending on my needs. From my research I think it is OK and it would not be the same as overdriving bulbs, but in this configuration with similar bulb size would just be less demand on the ballast. The 15W ballast means up to 15W, but it would be the load (the bulb) that determines the draw. Does anyone have any thoughts or experience with this? I have a question out to the the vendor as well but I wanted to get the community input also. I'm a degreed Electrical Engineer so I'm not too uncomfortable with the whole thing, but would love some feedback.
UV Bulb vs Ballast
- Thread starter JamesP
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If it were me, I would get the matching 8 watt ballast. Though you’re welcome to test out the 8 watt bulb on the 15 watt ballast and to us know how it goes! 
Using a higher wattage ballast on the lower wattage bulb will overdrive it which would drastically shorten its life. My understanding is that they do not produce output according to what bulb is installed. Ballast output is fixed regardless of the bulb used. So a ballast that outputs for a 15 watt UV bulb will still provide the same output to an 8 watt UV bulb, thus overdriving it. At least this is my understanding of how it works with T5 fixtures and the different rated ballasts available.
Using a higher wattage ballast on the lower wattage bulb will overdrive it which would drastically shorten its life. My understanding is that they do not produce output according to what bulb is installed. Ballast output is fixed regardless of the bulb used. So a ballast that outputs for a 15 watt UV bulb will still provide the same output to an 8 watt UV bulb, thus overdriving it. At least this is my understanding of how it works with T5 fixtures and the different rated ballasts available.
I guess I am hoping that the bulb would pull what it needed up the the max of marked wattage of the ballast. When I shop around for ballasts not included with bulbs as an assembly they usually give a range of wattages like below which works with 20W to 40W bulbs...
ballastshop.com
If I did try it I would check the current draw of the ballast and post back so as to learn something at least. If it didn't draw half the current with the the 8W bulb, I'd probably want to get an 8W ballast too.
UV-20 UltraViolet Control Box with Lampholder Attached
The Ultraviolet UV-20 (Wonder Light) Electronic UV Ballast for PC-6 PC-8 PC-12 UV Sterilizers
ballastshop.com
If I did try it I would check the current draw of the ballast and post back so as to learn something at least. If it didn't draw half the current with the the 8W bulb, I'd probably want to get an 8W ballast too.
