What the heck is a thermal expansion tank, and why would I ever need one? That's certainly what I was wondering... and didn't know anyone that has one in their house. After reading online, I figured we probably did need one.
We've got some nasty water hammer that we haven't been able to resolve. After testing the water pressure, we realized the pressure reducing valve wasn't working (probably 1950's original), so we were getting whatever 100+ psi the City was providing us. That could obviously cause some issues, so we added another PRV. Dialed down to 45psi, that still didn't seem to do the trick - the water hammer remained. We figured it was just poor pipe supports.
Finally, I wondered why, shortly after a shower (especially), the water pressure seemed SUPER strong and would lessen after just a few seconds. It seemed to be getting worse recently, so I read some more online. Apparently, with the working PRV, the heated water after, say, using the shower, has no where to go (heated water expands)... hence the increased pressure. A pressure tester on the system revealed we had as much as 180 psi in our system! Yikes!!! I am amazed that our whole water system didn't explode... and that the pressure release valve on the hot water heater didn't appear to be working (note to self, still check that).
Anyway, I installed the thermal expansion tank and pressurized it to 50psi. It works beautifully. The water hammer is still there, but much reduced. Now to install some more water hammer arrestors and hope that helps as well. I'm just trying to avoid having to deal with all the spiders and nastiness under the house to install better pipe supports which is probably the real problem.
Here's the new thermal expansion tank, sitting happy just past the PRV.