why does a hot water tank not explode?

Paul Wheaton · #1 04/01/09, 11:01 AM

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?

beowoulf90 · #2 04/01/09, 11:08 AM

Quote:

Originally Posted by Paul Wheaton

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?

They have a pressure release valve on the side of it. They have been required by law for some time..

Also you would need to apply constant heat to it to build up the pressure.. Which you are not doing.. Water heaters cycle through the heating then turn off and cool down then re heat the water..

Also it shouldn't be producing steam.. You water is under pressure from the well tank or city water. There shouldn't be any air in your water lines or water heater to allow steam to build..

If it is producing steam you have the temp turned to high. Water temps should not reach more the 110 degrees at the faucet... Commercial buildings have to regulate that with mixing valves, while residential buildings don't.... You water heater shouln't be turn up past 180 degrees at the very highest, and caution should be taken if you do that..

Paul Wheaton · #3 04/01/09, 11:12 AM

So where does the extra pressure go? To a drain?

beowoulf90 · #4 04/01/09, 11:17 AM

Quote:

Originally Posted by Paul Wheaton

So where does the extra pressure go? To a drain?

There shouldn't be any extra pressure, you are not heating the water to a boiling point.. and if the temp sensores go bad that regulate the cycle you have a pressure release valve on the side of it.. Some of them are piped to a drain but most just out the side of the tank..

Again there shouldn't be any steam because you are heating water and there shouldn't be any air in the lines or tank..

Paul Wheaton · #5 04/01/09, 11:23 AM

AT 140 degrees, the pressure doesn't get higher?

I never thought of steam that way. For there to be steam, there needs to be air .... Interesting.

I was at a workshop not long ago where there was a lot of tinkering with wood heat. One of the instructors said that most instructors in that field refuse to even mention heating water with wood because it is an area so riddled with danger.

So that must be because the water might boil. Whereas with electric or gas systems, they are controlled with a thermostat to keep the temp below, say, 140F. No boom.

DavidUnderwood · #6 04/01/09, 11:25 AM

It doesn't explode because its not
that hot. At anything less than 212
degrees, it won't make steam. The
popoff valve in your water heater
will never operate unless the heater
malfunctions. The temperature in
a household water heater is between
125 and 140. 180 would severely scald
you.

beowoulf90 · #7 04/01/09, 11:29 AM

Like I said commercial buildings have to install mixing valves in the hot water lines to keep the temp at 110 degrees or less by law..

When we install a line at 140 or 180 degrees it can only run to a commercial dishwasher
or such equipment that isn't for public use..

Paul Wheaton · #8 04/01/09, 11:38 AM

I'm getting my head wrapped around this ... thanks!

So, when I take a shower the temp is probably 100 to 105 .... and there is steam ... but in the water tank the temp is probably 120 - but since there is no .... nitrogen? ... then there is no steam?

I would think that the water would contain dissovled oxygen. So there would be some oxygen. And it seems that it would seep out of the water and head for "the surface". But maybe that just doesn't happen? Maybe hot water can hold more dissolved oxygen, so it doesn't .... un-dissolve.

Paul Wheaton · #9 04/01/09, 11:44 AM

And as long as you bring up "commercial dishwasher" .... now THERE is a fascinating contraption. Dishes are all clean in 90 seconds. I'm guessing because of the freaky hot water?

rambler · #10 04/01/09, 12:09 PM

Heating the water to 130 degrees or less, as a water heater does, does not make 'steam'. It takes 212 degrees for that. What you see in the air when you take a shower is 'water vapor', not steam. There is no steam.

Heating water from 55 degrees or so as it enters, to 120 or so in the tank does make the water expand a little. Only a little, but it is expanding.

The water can flow back in your pipes towards where-ever it comes from. Someplace back there, is an expansion tank - with a cushion of air. This tank absorbs the expansion. If you are on city water, the water still can flow backwards through the pipes to wherever their air cushion is. Now, only a few ounces are going to go backwards, this is not a big amount, but - you are correct this pressure does need to go somewhere. It flows backwards in the pipes.

If your water heater fails and the heater continues to heat non-stop, then it could heat the water to produce steam. This is very dangerous. The water heater has a little toggle valve on it, with a pipe leading doen to the ground. This is your pressure relief valve, and it is set to open up if the pressure in your heater gets too high. It will vent the hot water out of the tank.

In some rare cases, the vent doesn't work right, or isn't big enough, and a water heater will explode. They can blow through a few floors in the house, and through the roof. This is very rare, as extra pressure should be able to go backwards in the pipes, as well as out of that pressure relief valve. But, it does happen. Steam has tremendous power.

--->Paul

Truckinguy · #11 04/01/09, 12:27 PM

It's been recommended that the tempurature in your hot water heater be kept at least 120 degrees because some bacteria can survive at temperatures under 110 degrees. I think Legionnaires disease is one.

There was an episode of Myth Busters that dealt with hot water heaters exploding. They built a small house about the size of an outhouse but built to the specs of a house, with stud walls, rafters and a roof. They then put a water heater in it, took off and sealed all safety devices and turned up the heat. If I recall correctly, they got the pressure up to about 400psi before it actually exploded. It went straight up and they thought it had just destroyed itself. They were very surprised when it fell back to the ground after about 15 seconds.They never looked up because they thought it blew itself to pieces so they never did find out how high it went but to get that much air time it must have gone pretty high.

newfieannie · #12 04/01/09, 12:29 PM

this is good to hear. i have been pondering this myself for the past few days. i haven't been 'home' (that's what i still call my place in the country.) since december. i had visions of water coming to meet me. i turned the temp. down to 120 when i left.i turned off the furnace so i might still have to worry about it freezing but i hope not.i did disconnect all water pipes so hopefully all i'll have to do is connect up and everything will be fine. ~Georgia.

fin29 · #13 04/01/09, 12:31 PM

deleted

rambler · #14 04/01/09, 12:38 PM

Quote:

Originally Posted by newfieannie

this is good to hear. i have been pondering this myself for the past few days. i haven't been 'home' (that's what i still call my place in the country.) since december. i had visions of water coming to meet me. i turned the temp. down to 120 when i left.i turned off the furnace so i might still have to worry about it freezing but i hope not.i did disconnect all water pipes so hopefully all i'll have to do is connect up and everything will be fine. ~Georgia.

I'm not sure I understand what you did.

You left your house freeze up in a cold climate; you removed the pipes from your water heater but you left it plugged in?

I hope I'm not quite understanding.

--->Paul

Paul Wheaton · #15 04/01/09, 12:45 PM

Wow, this thread has given me far more excellent information than I was fishing for! Thanks everybody!

The mythbusters and the exploding hot water heater:

newfieannie · #16 04/01/09, 01:01 PM

Quote:

Originally Posted by rambler

I'm not sure I understand what you did.

You left your house freeze up in a cold climate; you removed the pipes from your water heater but you left it plugged in?

I hope I'm not quite understanding.

--->Paul

i probably did something wrong. i've been known to do stupid stuff. i disconnected the hoses from the water tank and drained it and filled toilets etc with antifreeze. left part of a hose on the hot water tank. since it is on electric and the electricity is still on(the furnace is oil and is off) i thought it would be okay. no? ~Georgia.

Drizler · #17 04/01/09, 01:13 PM

I had one blow in my rental trailer about 6 years back. I can't say why but I do know the thermostat was set pretty low and it was summer so ice had nothing to do with it. At any rate the safety relief never opened up and it blew open the side. It caused no damage as I had replaced the floor years before from a previous tanks eventual leakage. Everything got wet but it made no effort to go vertical at all and blew out 6" of seam near the top. With only well pressure to deal with I guess you can safely say the culprit was a faulty thermostat and relief valve and that both failing can happen. The big thing to watch out for with any tank mounted on a TRAILER, pressboard floor is the slow leak. That puppy will cause more damage than you can imagine. Thats why you have to have them sitting on a DRAINED PAN or you will really have a headache to contend with in a year or less.

ericjeeper · #18 04/01/09, 01:16 PM

Odds are you will need new elements in the water heater.Hopefully "Home" is still there..without incoming water to the water heater,I am afraid it might have evaporated below the safe level.Next time you winterize drain it and turn off the breaker.

Windy in Kansas · #19 04/01/09, 01:21 PM

Quote:

Originally Posted by newfieannie

i disconnected the hoses from the water tank and drained it

since it is on electric and the electricity is still on(the furnace is oil and is off) i thought it would be okay. no? ~Georgia.

Draining the water from the water heater would leave the electrical heating elements exposed. They are normally covered by water which helps to cool them. I'm not sure if they will survive being exposed like an oven element or if they will melt once the get well above their normal temperature.

With the water gone the thermostat will not reach the desired setting and will be calling for heat continually, thus the elements will operate continually until it (or they if two) reaches failure or until you have such an electric bill that it will stagger you upon receiving it. Not sure which would be worse as both could be costly.

From my viewpoint and understanding--no, things will not be okay. Flipping the breakers to the heater would have been warranted.

Sorry.

Sorry for basically posting the same information. While you were posting I was still writing.

newfieannie · #20 04/01/09, 01:42 PM

i probably didn't explain it properly. the tank i drained was the cold water tank and the hoses on it. the hot water tank was already disconnected by this time and the hose upright and hooked. i already have received 2 power bills and they are the same as all the other years i have been away. i just called my son and told him not to go in the back room if he checks the house and to throw the main breakers. you guys are scaring me.most likely i will have to get new elements. no shame for it. it is 38 years old. i just dont want the roof to blow off. ~Georgia.