Why is my water expansion tank leaking?

Question

I live in the northeastern US and we just turned the heat on (oil) and this morning I went down to the basement and saw this:

The water is coming from the plumping above something labeled "water expansion tank". In the picture below, you see the green patina forming over the copper pipes above the tank.

My questions:

1. What is a "water expansion tank"?
2. What does this tank have to do with my heating system? Is it just coincidence that it started leaking right after turning on the heat, or does the boiler engage the tank somehow?
3. What is the likely root culprit here, and how might I diagnose what is exactly wrong with it (and then, subsequently, fix it myself)?

Answer 1

1. A water expansion tank gives water a place to expand in a closed system (frequently found with boilers, or home water supplies with a check valve). Otherwise, heated water cannot expand and instead increases pressure until some component of the system leaks or explodes.

2. If the tank is on the hot water heating system, then it's there to relieve expansion from the water being heated. I can't tell from the photo if it's on the heating system or on the home water supply.

3. Leaks tend to travel down with gravity, if you see evidence of a leak above the tank, then it's likely at the top of that evidence, not the bottom. Shutoff the water, find the source of the leak, and replace the problem area. It may be as simple as needing to unscrew a threaded connection, clean the threads and apply pipe dope, then tighten everything back up. But with visible damage, you'll likely need to to replace the problem area with new pipes.