Thermal expansion occurs in closed systems. What does that mean? Well, for starters, water heaters need to fire up occasionally to maintain temperature, especially when nobody is using hot water, such as in the middle of the night. So they turn on and heat up. The water expands. If you are on a well or a city main, and nothing is in the way, it expands into those places. No problem.

But some cities require backflow preventers. And some situations require pressure reducers. Some systems have check valves. These can result in the expanding water having nowhere to go. What happens? The pressure spikes. We've seen it jump to 150 psi!

That kind of pressure can wreck water heaters, appliances and plumbing. The usual solution is to add an expansion tank, although there are other solutions, such as the Watts Governor 80.