When a water heater explodes, why does it shoot straight up like a rocket?

Question

Is it because the bottom is hotter? Does that make the metal weaker? I would think it would be weaker at the top, because of the holes for the pipes.

Answer 1

There are two type of explosion you can have with a water heater.  (1) is a pressure explosion if the internal pressure gets to be too great ant the pressure relief valve fails.  (2) If it is a gas fired water heater, there is a potential for a gas explosion.  If it is a standard style gas fire tanked water heater, the gas supply is at the bottom.  The pilot light could go out allowing natural gas to accumulate.  All that is needed is an ignition source and then the explosion will exert force on the tank (at the bottom) and thereby pushing the tank upward.

Answer 2

It depends where they rupture, I've seen one go sideways.

Answer 3

See http://www.youtube.com/watch?v=JmJoyuUJj...

http://www.youtube.com/watch?v=pu3FwgIHs...

for video of Mythbuster episode on this.  The seam at the bottom of the tank are the weak point.

Answer 4

An over-pressurized vessel with domed heads will usually fail by a split along the side. The shape of the split can vary and the horizontal or vertical trajectory is unpredictable.

A vessel with flat heads will  fail with a split in the head and the trajectory can be expected to be vertical with a bottom failure. Again the trajectory is unpredictable, but not a much. The failure is not because of the pipe fittings. They actually provide localized reniforcement. The weakness problem is due to the flat shape of the end of the vessel.

In either case the odds are 100% that it will go somehow upward and not down.

The real danger is not the trajectory of the vessel. If within a house it probably is not going to go very far before it is empty. The real danger is the huge amount of suddenly released, superheated steam which has indeed exploded houses and killed occupants.    

Answer 5

Have you actually experienced this, or are you basing your question on movie depictions of this event?  It happens that way in movies because it makes for a much more spectacular visual effect.  In real life, water heater failures are more likely to involve a leak from a valve or pipe joint, as these are the weakest spots in the system and most likely to fail under pressure.