Question
I just installed an irrigation system and now I ran the system a couple of time to finally adjust the heads.  I went around to the backflow and noticed that there was a drip coming out of the backflow. (this drip is coming out of the hole where the water shoots out to release air, I guess)  Does the backflow always drip out water? I left a gallon jug over night (12 hours) and it was filled 1/4 of the way with water.  Is this natural?  If it is, do this effect my water bill?  Thanks,

Answer
Thanks for the question.
Backflow preventers are not made to drip. However they sometime do. Chances are you have a tiny piece of contaminant on one of the seat of the check valves. It could be a piece of solder used on the pipe.
You need to disassemble the backflow and clean it. Depending on the type and manufacturer of backflow this may not be a job you want to tackle. They can be complicated and/or dangerous because of the springs they have. A simple thing you can try is opening the test ports, which are the small 1/4" valves on the backflow. By doing so you may get the dump valves to open and flush the debris out.
As far as the water bill, I doubt it will register. I don't know of any meters which can read such a low flow of water.
If you can send the model number and manufacturer of the backflow, I should be able to give you better advice as to how to fix it.

Matt