hypothetical question

[ally68]

If you have a major malfunction in your water heater like elements not stopping heating or such. And your T&P valve fails an expansion tank will not prevent your water heater from exploding right because the build up of pressure will overtake the expansion tank also?

Just wondering.

[Terry]

Yes.
That's why the T&P is important.

[jadnashua]

If it just keeps heating, you'll create steam. The pressure will be huge, and the weakest link will burst, potentially explosively. I think I read heating water (liquid state) about 70 degrees causes it to expand about 3%. Multiply that by over a thousand when changing from liquid to steam (don't quote me on that, it's been a long time since that class!).

[ally68]

And to check and maintain the T&P all I need to do is pull the handle let some water out and release? About how often should this be done?

Thanks

[jadnashua]

That will tell you if it is not plugged or frozen, but won't tell you if it will open at the stated pressure/temp. Now, usually, they start to open early when they get worn out, rather than above the rating, assuming that it is not frozen, which is why you need to test periodically to verify it is not frozen shut. My unprofessional understanding of these things.

[hj]

A water heater exploding has little to do with expansion, other than the expanding water could cause the rupture that triggers the explosion. The actual damage is caused by the potential energy that pressurized water accumulates when heated above 212 degrees F. and it is then released immediately when the pressure drops due to a break in the system.

[ally68]

So what you are saying is if there is excessive pressure but the water is not super heated the tank will just rupture then leak with no explosion but if the water is super heated then the pressure will rupture the tank and when the super heated water hits the air it will explode?

So pressure with out heat will destroy the tank but not make it explode?
So the high temp cut off will also help a tank not explode?
Thanks hj

[ally68]

Also how often should I check the T&P and is the procedure I mentioned above correct?

Thanks hj

[hj]

Heat without pressure and the tank breaks equals about a cup of water unless the water is turned on.
Pressure without heat and the tank breaks is the same thing.
Pressure and heat and the tank ruptures equals a flat house and a water heater about 600 feet away.

[jimbo]

If the tank ruptured and released the pressure, then that's that. However, there have been some terrible explosions occur on water heater which did not have a TP. As alluded to above: Water at atm boils at 212º. However, the pressure inside your water sytem is much higher: maybe 60 normal, and as the heater heats out of control, the pressure goes higher. Because of the pressure, the water does not boil. NOW, when something finally ruptures and the pressure goes towards zero, a lot of that water flashes to steam instantly. The energy contained therein is condiserable.


There are some photos on the internet of the aftermath of some of these accidents. I will try to find them.

[ally68]

So the high limit switch will shut off the elements at 170deg should prevent this from happening right?

So if the water is below 212 it will just leak out.

If the tank ruptures from presure and the water is on it will just run out of rupture untill water is turned off right?

Also about testing t&p valve once a year? And I should not have to worry about my tank exploding even if all else fails but the T&P right.