Are expansion tanks required for small water heaters?

[Joe.B]

IMHO, the Myth Busters Video is not applicable to this OP's question. I was given to understand that water expansion tanks will NOT prevent a water heaters from exploding. That is the function of the TPR valve.

Hahaha no I wan not trying to state that a water heater would fail in this way, simply showing the pub works guy that pressure can build up in a tank. I have lived in houses that should have had an expansion tank (or some other approved device) and in the mornings it was annoying to the first one up, you open the water and it shoots out under high pressure, but quickly drops back down. Same thing when you're the first one to use the water after the house has been empty all day.

The real danger (relative to the OP) with the water heaters is if the TPR valve is the relief point it will start with an occasional drip, but as sediment gets into the seal it will become a constant drip, and that's when a "clever" homeowner will cap that off to "stop the leak". Apparently that's happened more than a few times and that's why it was on their show.