Water Heaters - How to tell if baseboard heat is leaking?

We have water that is dripping down a vent pipe in the basement. We went up on the roof and caulked around the vent pipe to ensure it wasn't leaking. Checked in the attic, everything is dry. We've shut off sinks/toilets/tubs etc and it still is leaking. We've even logged when the PVC pipe is wet/dry and the pattern it seems to follow is dry during the day and leaking at night. The only thing we can think of is that the heat is on upstairs at night. Possibly the baseboard heat is leaking and somehow picks up that vent pipe and follows it to the basement. Does that even make sense and if so is there anyway to tell if the baseboard is leaking. We've felt the floor, looked at the copper pipes in the basement, there are no water stains on the ceiling on the 1st floor. It's not tons of water, maybe 1/8-1/4 cup at the most for a 24 period. I tried tests pouring colored water down different areas...the water that leaks in the basement is clear,clean smelling water. Just when we think we've figured out where the leak is coming from we are wrong. I know everyone says water leaks are tough to find but it seems to me that eventually by elimnation we would find it. :wall:

If it is a pinhole leak in the heating system, it could be squirting or a fine spray that is hitting a joist and then following the joist to the vent pipe. This is the same as a roof leak. The leak is not always where you see the water appear. Good luck.

our heat upstairs is a separate zone. is there anyway to put a dye into the water to see if it is the heat that is leaking. although i did a test today. our heat is always off during the day and the pipe in the basement is dry...today i left the heat on upstairs and the pipe is staying wet.

I am assuming that the water from the heating system is recirculated. I would not recommend putting a dye into the system. By what you have done today, it seems you have your answer. Now comes the hard part, finding where the leak is. Good luck.