why does a hot water tank not explode?

It would seem that with all of the heat, and all of the steam, your average hot water tank would explode.  Or, the PSI in the pipes would become something like 500. 

Is there some sort of pressure bladder somewhere?

My brother says they have a pressure relief valve just for that purpose.

Sue

Usually they don't reach the flash point (boiling point) and convert to steam. It's steam that's typically associated with the overpressure and exploding tanks and such... Now boilers are a different story.

Plus, the water heater is typically better made than the piping in the house, with all it's solder joints and compression fittings, so it has a "reservoir" of sorts to bleed extra pressure off into.

And of course, as Susan said, it has a relief valve if everything else goes haywire.

After researching far and wide, I found several more tidbits of information.

1)  The relief valve is usually fitted with a something you can attach a hose to so you can direct that extra water somewhere.  Some folks have noticed that there is a puddle of water under that and decided to be helpful and cap that.  The mythbusters made a little house, did this very thing, and then shorted out the termostat.  The result was one of the biggest explosions they ever encountered on their show.  At least, that's what they thought up until the moment that the water heater body landed.  Then they changed their evaluation from "explosion" to "massive rocket launch".  http://www.youtube.com/watch?v=pu3FwgIHsQA

2) There is a big difference between steam and water vapor.  A hot shower has lots of water vapor, but no steam.

3)  Keep those thermostats set high!  Some people lower the temp on their water heater to save energy.  And while that does work, you might be creating an excellent breeding ground for all sorts of icky things.  Apparently, commercial buildings have a special mixing chamber that mixes cold water with the hot water as it leaves the hot water tank so the temp is never above 110.  Just turning the temp down to 100 isn't gonna cut it.

What kind of icky things?

Sue

Susan Monroe wrote:
What kind of icky things?

Sue

I had a bunch of sulphur producing microbes in my staging tank (after my geothermal furnace, which "makes" hot water).  Funny, I figured the smell would be from the well, but I was growing my own!  This staging tank doesn't do any heating, it just holds the furnace heated water and feeds it to the larger actual water heater.

I had to clean out the tanks and found that replacing the sacrificial anode with a zinc one solved my problem...

Susan Monroe wrote:
What kind of icky things?

Bacteria.  Algae. 

Jeremy_IL wrote:
I had a bunch of sulphur producing microbes in my staging tank (after my geothermal furnace, which "makes" hot water).  Funny, I figured the smell would be from the well, but I was growing my own!  This staging tank doesn't do any heating, it just holds the furnace heated water and feeds it to the larger actual water heater.

I had to clean out the tanks and found that replacing the sacrificial anode with a zinc one solved my problem...

But sulfur is an element ....  wouldn't it have to already be there to get some?  Maybe it was more of a sulfur smell than actual sulfur?

paul wheaton wrote:
But sulfur is an element ....  wouldn't it have to already be there to get some?  Maybe it was more of a sulfur smell than actual sulfur?

Correct, a sulphur smell.  Sometimes I type too quickly for my own good!