Is the "Exploding water Heater= Missile" an urban legend?

Factual Questions
1

I have heard that improperly installed gas/electric water heaters can turn into missiles-supposedly, one blew through two house floors and out through the roof of the house! This sounds impossible to me-while the pressure in a water heater could build up 9as the water boils into steam), i can’t see this scenario happening:
-the pressure relief valve would open, and vent the excess pressure

  • either the tank or the hot water lines would rupture. relieving the pressure
    In any case, even if somebody welded the pressure relief valve CLOSED, how would a water heater generate suffiecient pressure to blow through two floors and a roof?
    Sounds like one for the “MYTHBUSTERS”! :smack:
2

Moderator’s Note: Sounds like one for General Questions.

3

Steam-powered tank rockets out of Burien store and over 6 lanes of traffic

From this thread: Water heater problem … is death imminent?

4

I’ve seen movie clips on TV, maybe Discovery or History Channel, of water heaters exploding in tests. These things flew 50-60 feet into the air. They explained that explosions such as that are what prompted the development of the temperature/pressure relief valves that are now standard.

Ive been told that we are supposed to open the valve a little once in a while to ensure that the valve is not corroded closed. But few folks do this.

5

Sounds like a good Mythbusters episode…

6

A few things to keep in mind: a given volume of water when converted to steam takes up 1700 times as much space. Said another way, one cubic foot of water converted to steam would fill a 14 x 15 room (assuming an 8’ ceiling). Water will boil at 212F when not pressurized. By placing water under pressure, it can be heated above 212F without boiling (your car’s cooling system does this every day), yet if that pressure is lost, there is a huge amount of energy released in the form of water flashing to steam.

It was a number of years ago, but my department responded to a reported “explosion in a dwelling” and upon arrival found no fire, but significant structural damage. After investigation, it was determined that the homeowner had never had the oil-fired hydronic boiler professionally serviced, and instead performed his own wire and bubble-gum fixes here and there, including a pipe plug in the T&P outlet fitting. Over time, he successfully jumped out every safety device ever built into the boiler just to keep it running, such that when it blew, most of the first floor joists adjacent to the boiler location were disconnected from the ribbon/rim board, and the CMU wall not below grade was outwardly displaced ~8-12" with all mortar joints fractured. While the boiler didn’t physically escape the dwelling, the stored energy, once released imposed a beatdown on the house.

7

[QUOTE=ralph124c]
I have heard that improperly installed gas/electric water heaters can turn into missiles-supposedly, one blew through two house floors and out through the roof of the house! This sounds impossible to me-while the pressure in a water heater could build up 9as the water boils into steam), i can’t see this scenario happening:
-the pressure relief valve would open, and vent the excess pressure

  • either the tank or the hot water lines would rupture. relieving the pressure
    In any case, even if somebody welded the pressure relief valve CLOSED, how would a water heater generate suffiecient pressure to blow through two floors and a roof?
    Sounds like one for the “MYTHBUSTERS”! :smack:
    [/QUOTE]

My recollection is that this was the reason pressure relief valves were required. That is, these missile stories happened before pressure relief valves became standard items on water heaters.

Ed

8

IIRC the rocket Evel Knievel (sp?) used to jump over Hell’s Canyon was powered by steam.

9

[QUOTE=ralph124c]
I have heard that improperly installed gas/electric water heaters can turn into missiles-supposedly, one blew through two house floors and out through the roof of the house! This sounds impossible to me-while the pressure in a water heater could build up 9as the water boils into steam), i can’t see this scenario happening:
-the pressure relief valve would open, and vent the excess pressure

  • either the tank or the hot water lines would rupture. relieving the pressure
    In any case, even if somebody welded the pressure relief valve CLOSED, how would a water heater generate suffiecient pressure to blow through two floors and a roof?
    Sounds like one for the “MYTHBUSTERS”! :smack:
    [/QUOTE]
    I was one of the people who contributed to the thread referenced by Squink, and I’ve seen houses destroyed by exploding water heaters. With modern TP valves, it’s very rare, but it can still happen.

I spent 9 years working at a wholesale supply house that sold (among many other things) lots of heating, AC, and water heater controls, gas and electric, residential and commercial. We sold to virtually every contractor in the city, and so were privy to all the horror stories about bad installations, failures, and other malfunctions.

Most of the time when a water heater malfunctions, it just quits working. In those cases where it goes bad and stays on instead of off, usually the temp/pressure relief valve opens and it’s messy but manageable.

In those cases where the TP valve doesn’t open, usually a seam splits and again, you have a huge mess and sometimes lots of damage, but usually no catastrophe.

But every once in a while the tank holds just long enough, and then the bottom or side lets go in just the right way, and it’s a 24” by 60” missile heading out your roof or your wall, or just a big bomb that blows down your walls.

There was one in my old neighborhood about 25 years ago that blew out through roof and across the street and landed in an empty schoolyard. The house was not only badly damaged, but the entire structure had been blown off the foundation by a couple of feet. It was knocked down and rebuilt.

I heard about (and saw pictures of) a few others over the years.

People don’t appreciate the explosive power of steam at a couple of hundred psig. It is truly awe inspiring. danceswithcats gave the expansion figure above, and believe me, you can make a missile out of a water heater. Rare, but not non-existent.

10

This reminds me of why DIY repairs on liquid nitrogen tanks are a bad idea:

11

[QUOTE=MikeS]
This reminds me of why DIY repairs on liquid nitrogen tanks are a bad idea:
[/QUOTE]

Wow.

12

[QUOTE=MikeS]
This reminds me of why DIY repairs on liquid nitrogen tanks are a bad idea:
[/QUOTE]

:eek:
That is SO FREAKING AWESOME.
Somebody has GOT to try replicating that…except maybe using a tank of liquid oxygen instead of nitrogen…

13

This post from the other thread is one way the heater can blow up. Specifically, this part:

Without water in the tank, the elements will just sit there and get the tank very hot (eventually the elements will burn out). The same situation can happen with gas. If the water valve is turned on, the water will hit the hot surface in the tank and flash to steam. Remember that 1700:1 volume ratio?

Boom.

At that point, a 3/4" T&P valve will probably not vent steam fast enough to prevent the explosion.

That’s the reason why whenever I have to shut of the water to my house, I also turn the water heater off. While I can’t reallly think of a way that the water heater could drain without me noticing, I know that when I turn the water back on that the heater is not hot.