I just moved into my house, the hot water heater was 18 years old and blew out (the bottom was skinny and the the top got fat!) What actually causes this. Just wondering if I've got a problem or just an old hot water heater.

 

An 18-year-old water heater doesn't owe anyone anything. It is long past when it would normally fail.
Most water heaters have cast iron tanks, and those simply rust/corrode through at some point, and then they need to be replaced.
If you're lucky, they'll just spring a slow leak, and you can replace it as soon as you see it. If you're not, then the whole tank can rupture, causing a flood.
Some newer water heaters have "lifetime warranty" tanks made of some space-age material that won't rust, but they cost extra. I have one, but I have no idea what that material is. "You get what you pay for", I guess.
Good Luck!

 

There is a differance between blow out or blow up, I'm sure you ment to say blow out.

Now if a water heater blows up this would have been caused in a closed system, (meaning no place for water to expand to), this will happen when there is no pressure/tempature valve on the heater, (called a T&P Valve).


The T&P valve protects the heater from those two possibillities, the T&P valve will trip at the temp of 210 degrees, and will trip if the pressure reaches 150 lbs or more.

If you you ever walk into a place and see a water heater trying to bounce or is rocking, then you know there is a major problem. This heater is about to blow.

What does one do when this happens?

1. Shut the power down to the heater

2. Never turn on a hot water faucet, this will cause a surge in the system, and cause the heater to blow.

3. Run like hell.

If you ever see a water heater without a T&P valve on it, do something about it, they can blow up without this device.

Plumber2000