If you are on municipal water and there is no check valve between you and the water meter then the municipal water supply is acting as the expansion tank. It may not be to code in your area but that's how many homes were built in the past.

[–] rockSlayer@lemmy.world 1 points 1 year ago (1 children)

Honestly, I don't know, I'm not a plumber. There are a couple possibilities, including that it was built into the water heater or the relief valve did it's thing. Some water heaters don't need expansion tanks at all, either. It's generally a "just in case" thing, since the right failures in the wrong way could lead to a hot water bomb going off in your house.

[–] AnotherPerson@lemmy.world 1 points 1 year ago

Yeah but that was like the coolest mythbusters episode ever /s