There is some kind of little small tank that sits on top of the hot water tank, that is for expansion or something. Whatever, it was leaking so I called Benjamin Franklin Plumbing of Tyler, Texas. This was maybe the second time I used them in the last few years. They came right out, and replaced the part, and now everything works great. No more leaks! They were right on time, and very friendly. And I like how they saw what the problem was right off the bat and got it fixed. Very professional! No problems, and no hassles. They had the right part on the truck, so there were no delays. I appreciate that kind of efficiency!