One of the most common questions we get from customers about electric water heaters is whether they need to buy an expansion vessel with it. The reason that some heaters need to cope with this expansion is that when water is heated, it expands. If this is not allowed for then heater tanks or pipes will fracture and split, causing a catastrophic failure of the heater.
If you have an unvented water heater, you may have noticed a small tank attached to it or nearby. This is called an expansion vessel, and it plays an important role in protecting your water heater and your plumbing system from excess pressure.