I need to replace my water heater and have noticed that in all of the new construction that they are placing a small (2 gal.) pressure tank on top of the heater. Why?

I'm assuming that it has something to do with backflow presenters and being on city water. I'm on a well and have a big tank out back with all of the rest of the Chernobyl looking junk.

Japox

I installed one recently after my pressure regulator started to leak periodically. Problem solved.

Apparently, after using hot water, the cold water that replaced it would expand as it was heated. The pressure had to go somewhere. We had previously noticed a little weeping from the pressure relief valve on the water heater and the toilet fill valve seemed a bit weak.

We replaced both and now the weakest link in the system was the gasket on the pressure regulator.

I verified the problem before installing the tank by putting a gauge on the washer feed. I pegged the needle on the gauge!

Installing a tank was a lot cheaper than replacing the regulator. I'll probably have to do that too, soon. But I got a little more time out of it.