He tightened the nut. This did not stop the leak, but he said, "Well, I slowed it down." He didn't. I had another plumber come and look at it. He told me that, if anything, the gasket was crushed, and the part would have to be replaced. The second plumber agreed with the original plumber that the pumps on the barely 3 year old furnace needed to be replaced. However, in order to get to the pumps, either the furnace would have to be cut open, or all the pipes above it replaced since, when the furnace was installed, the Sheppard plumber slid it in under the existing pipes without leaving enough clearance to take the top off. So, a job that should have cost just the amount of the pumps has become a fairly large undertaking.