why does a hot water tank not explode?

Lots of electric heating elements are thermally protected, they get too hot and break contact.
When they cool down they make contact again.
But if they are doing it continuously, I would guess they (most larger heaters have two elements), are toast by now.

I turn off the water,drain down, and blow out all my water lines, tanks, antifreeze the traps, and heat tape the incoming water line, for the winter.

I have started draining the water tank, without turn off the electric first, and had the thermo saftey break contact.
After checking it, I found an open circuit, and as I didn't have a spare at the time, just shut it down and brought one the next trip out.
Before pulling out the old element, I checked it again, and found a closed circuit, so it had reset it self.
I turn on the water, has a bury pressure tank, and listen for leakd, before turning on the pump.
I have had the flush control on the toilet freeze up, as well as the sprayer on the kit sink, in the past,

Good luck, I know it's a pain to try to remember every thing when doing a shut down.

get a new tank gerogia! chances are the elements are blown, most new tanks are more energy efficient plus adding in a plumbers labor for changing the elements possibly the controls then too have the tank leak and need replacing anyways! friend installed a new tank recently and has very little electric usage with the newer unit

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Wood boilers are controlled with a thermostat. Not sure about all of them , but mine is not a pressurized system, but open above the tank outside to release any pressure. Mine also has a pressure relief valve on it.
If your pressure relief valve on your household heater doesn't work, then yes it can blow up. They can also catch fire as mine did due to a leak in the propane line on the tank. Not sure what would happen if you don't get there to put the fire out other than potentially burning down the house.
Solar water heaters don't use fuel, and are pretty safe , but you would still need a pressure relief valve, among other things.

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As mentioned the P&T valve releases pressure above about 150 PSI or if the temp gets to boiling.

A quick story. A farmer friend recently had issues with his tank clunking when heating, and the P&T valve dribbling water. Previously, he had been on a well system with a bladder tank, but now was on county water.

Why was this dribbling happening now when it didn't before? There were a few reasons. First, his well system was at the usual low pressure +-45 PSI and the county water was more like 65 PSI and had a check valve to prevent contamination of the water in the county line. When he built the house he did not install risers in the bathroom, and at the low pressure of the well, he didn't have any water hammer issues. His well did have a fair amount of lime in the water.

So... The lime built up on his heating elements in the water tank. Water would get between the lime and the electrode, become superheated because of the insulation of the lime coating, and explode into a bubble of steam. The bubble would collapse upon hitting the colder water, making the noises. Most folks forget that water has dissolved air in it. The air wouldn't all reabsorb into the water quickly enough, and would build up pressure on the water during any time the element was cooking. Add the slight expansion of the water mentioned before, and the pressure got pretty high.

Because the check valve prevented the water from flowing back into the county line, and he had no risers or bladder tank to act as air cushions until the next time he needed water, the pressure in the tank and pipes got up to the 150 PSI cutoff, and the P&T valve would open momentarily, release the excess pressure by dribbling a little water, and re-seat.

He cleaned off the lime from the elements, added a length of 3" vertical pipe as I suggested to use as a riser and anti-hammer, and the problem stopped. Under normal conditions, even 12" of 1/2" pipe filled with air would have been enough to minimize the problem. I suggested the larger size cause his luck with plumbing is about like mine with yard equipment.

Water boils at 212 Deg F at atmospheric pressure. At well or city water pressure it will not boil until around 240-260 deg F. At 2800 psi, it boils at 776 deg, F. It is called the pressure temperature relationship. Hot water does not explode because it is a liquid, steam pressure does because it is a gas.

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At our last house, the water heater was coal fired with no check valve. My son would build a fire and we had to remember to use some hot water or else. We would start to hear pipes pinging and making noise. We would run and open the hot water faucets. We would get steam filling the house.

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And NOBODY came close to the right answer. You release the pressure every time you open a hot water faucet. DUH!

Madfarmer

I have not read all the replies so maybe someone talked about this.

Years ago I was a instructor at a plumbing apprenticeship school. I also was a master plumber for many years.

In the first year of the school we used to show a film about this very subject. It showed homes leveled by water heater blast as well as some schools and other larger buildings with major damage from water heaters.

Now a water heater has 2 safeties that keep it from blowing up. One is the t&p valve or temperature and pressure valve.
How it works if the water reaches a set temperature or pressure it opens up and drains some of the hot water out at the same time it is letting cold water in, the combo lowers the temp very fast.
The other other safety is the thermostats they shut the power off when they water reaches a set temperature.

99% of the water heaters that blow up is caused by well lets call them morons. It could have been who installed the heater and instead of putting a t&p valve in they just plugged the hole instead. They could have been the guy next door that was helping out with that leak coming from the heater and capped that leaking thing coming from the heater. Or it could be someone that bypassed a thermostat because it was not working right. But the point is they mainly blow up because people who have no clue what they are doing mess with them.

I forget the figure now but a 40 gal water heater is something like 2 sticks of dynamite or something like that when they blow.
What you do not see from the outside of one is the heavy steal tank inside of it that holds the pressure.

eek.......

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Sold water heaters for while worked at a hardware an plumbing store .Most modern heaters have a high temp overload with a manual reset .Got to push the red button .Should it fail the t&p valve should release.Water heaters have a dip tube on the cold side that goes near the bottom an forces the hot up thus keeping a steady supply of hot water . The only improvement in water heaters other than t stats is the insulation. When you got 220 volts going to a 4500 watt element it an't efficient anyway you do it .During our ice storm i pulled my lower element an replaced it with a 1400 watt 110 volt so my generator would work it .One element in the bottom is all two people need .At one time we sold water heaters that the person installing it had to also install the t& p valve an this nut put a plug in the place of it .Wouldn't want to spend any time at his house . Folks came in wanting to buy a hot water heater told them if their water was hot no need to heat it Proper name water heater

There is a situation as mentioned where a check valve in the incoming water line can cause pressure to build inside the tank as it heats. In a city I lived in it was required to install a small pressure tank (1 gal or so) right next to the hot water tank to give this pressure somewhere to go. It was supposed to also extend the life of the tank by reducing pressure fluctuations.

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Yep it is a bladder tank has a charge in if off setting line pressure . Those double check valves are good though anti siphon water closet valves are good too .If a water line breaks and your ice cubes in your ice maker is the same color as the blue in the john don't use them

We are on a water system, and it is code that we have a one way valve on the incoming side of the water heater that will NOT allow back flow into the water system. So any potential expansion isn't going that way. However, the particular valve also has a release on it. It cost a pretty penny too.

I think there also might be something like this in the "pot" where the regulator and meter are. Since this is a rural water system, they do get concerned about contamination from backflowing water.

Cathy

Most water heaters are set well below boiling point so there is no steam pressure to deal with. In the event of a malfunction there is a blow off valve that will release the pressure if it gets very high at all. Most water lines will stand 80 to 100 lbs or so pressure, which would require a water temp of 350 degrees. Without the blow off valve, a water heater can and will explode if enough pressure were to build up. I saw a deal a while back on tv where this was tested, the water heater blew through the roof, and went out of sight straight up! Dont be there when this happens.

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