# a plumbing question-if you turn off main water and you turn on the hot water faucet d



## steppenwolf (Apr 9, 2003)

if you turn off main water so theres no water to house,and you turn on the hot water faucet does the hot water tank drain out?is it like a siphon?

or does only the incoming water from cold force the tank full of water out?


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## Drabdr (Nov 26, 2007)

Good question. 

I would think that once you lost the water pressure in the lines due to turning off the water, and the hot water is at the same level as the faucet, it would not flow.


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## jiml8 (Jul 3, 2005)

No.

The hot water from the water tank will exit the top of the tank. So, no pressure means no flow.

If you open the tank drain valve, the hot water tank will drain with the water shut off...eventually, and presuming that the drain valve isn't blocked by mineral deposits. To drain the tank, you normally would open the hot water valves in the house, then open the hot water tank drain valve. The taps being opened will let air into the tank through the hot water lines and thus it will drain.


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## steppenwolf (Apr 9, 2003)

jiml8 said:


> No.
> 
> The hot water from the water tank will exit the top of the tank. So, no pressure means no flow.
> 
> If you open the tank drain valve, the hot water tank will drain with the water shut off...eventually, and presuming that the drain valve isn't blocked by mineral deposits. To drain the tank, you normally would open the hot water valves in the house, then open the hot water tank drain valve. The taps being opened will let air into the tank through the hot water lines and thus it will drain.


"The hot water from the water tank will exit the top of the tank"?

did you mean will NOT exit?

i know theres a drain at bottom to drain it, but thats not the issue

just if water was depleted for short time water was off


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## steppenwolf (Apr 9, 2003)

maybe the top of tank is drained?but heating element is below that line anyway?water has to stay in to cover the element area

does the water touch the element inside?


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## guzzz392 (Jul 26, 2008)

as far as ive ever known, you have to drain the water heater by opening the faucet on the bottom of the water heater and letting it drain into the floor drain or attached hose or whatever. pressurized water will empty it of hot water, but you'll still have a full water heater of new cold water. It must be drained by the faucet, or if there isn't one there should be a plug at the bottom of the tank to drain it from.


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## jiml8 (Jul 3, 2005)

The hot water exits the top of the tank. Like I said. And there are two types of electric water heaters. The most common type has the heating elements immersed in the water and, of course, in contact with the water.


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## guzzz392 (Jul 26, 2008)

you answered your own question"or does only the incoming water from cold force the tank full of water out?" yeah and then the tank is still full. What happened, did you accidentally drain your tank with the elements still on? If so, they probably won't last very long b4 they burn out. If it was for a very short time, they should still be good. Can't understand your question too clearly. I guess the answer is no, it would not drain out like a siphon. go to the lowest sink or water spigot/s and open them and it will drain down to that level. the rest below that will remain full. The water heater will remain full.


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## steppenwolf (Apr 9, 2003)

i dont want to drain it 

stop talking about bottom spigot


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## steppenwolf (Apr 9, 2003)

the main house water was off

i turned the water on-faucet

of course some is in pipes -left over

or does a small amount also leave the tank?

its not like a siphon?

only the force of the incoming cold forces out the main tank water?

i think i explained it the best i can didnt it?


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## steppenwolf (Apr 9, 2003)

you do know about how siphoning works?


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## jiml8 (Jul 3, 2005)

I've answered twice, and won't bother answering again. You have an attitude and you don't listen. Too much trouble for me.


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## hewee (Oct 26, 2001)

steppenwolf said:


> if you turn off main water so theres no water to house,and you turn on the hot water faucet does the hot water tank drain out?is it like a siphon?
> 
> or does only the incoming water from cold force the tank full of water out?


NO 
The water now in the tank is now in a vacuum because no water or air is coming in so now water can get out.

So if any water does come out it's because the main is not shut off 100%

If you're working on the water heater and want to make sure no water comes out at the water heater then when shut the main off find the lowest place in the house and that should be a outside faucet and that will drain the cold water lines.

If your working say at a sink and have a faucet under the sink to the cold side turn off because you want to hook up the ice maker does not mean you can turn on the faucet to get water from the hot side. 
The water from the hot side will feed up to the faucet where the hot and cold mix and back feed down the cold water hook up that you just took off to hook up the ice maker.

So a safe way it to shut the main off and drain water from lowest point to empty the line.


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## buffoon (Jul 16, 2008)

steppenwolf said:


> i dont want to drain it
> 
> *stop talking about bottom spigot*





steppenwolf said:


> you do know about how siphoning works?





jiml8 said:


> I've answered twice, and won't bother answering again. You have an attitude and you don't listen. Too much trouble for me.


Let's keep this cool, eh?

It's about a plumbing question and possible answers and not supposed to be about egos. Politeness of tone will be helpful.

Anyone displeased with manner or tone of any post reports it and doesn't squabble on here.

Thanks.


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## Noyb (May 25, 2005)

There is no pressure to force the water uphill ... 
But if the top of the tank is higher than the open faucet .. It can Flow downhill.

I must have a weird Clothes Washer .. It I shut off the water to the house ...
The shutoff valve in the washer releases and the water heater tries to fill the washer .. Thanks to Gravity.
Took me awhile to figure out where the water was coming from.


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## Elvandil (Aug 1, 2003)

You ought to shut off the heating elements anyway in case it goes below the water line. Water can siphon from the tank so long as there is a source of air to fill the resulting space in the tank, and air can come from sometimes unexpected sources when you are working on plumbing (like someone opening a tap and leaving it open when nothing emerges).

But you may also have other valves you could use to isolate the heater so nothing can leak out. In any case, shutting off the heater is a wise move.

Maybe if you elaborated on the problem and the reason for your concern....


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## steppenwolf (Apr 9, 2003)

main water was shut off

maybe i turned on tap

heard weird sound from water tank

wonder how much water left tank-wonder if element burnt

i never said i want to drain a tank -if i wanted to, id say that

i cant see how water can leave a tank and go to basement unless if pipe is below the bottom of tank

how much filled the washer? i bet it was only one gallon

thanks for the help but dont start an argument over a simple plumbing question


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## steppenwolf (Apr 9, 2003)

maybe the colder weather just made the tank extra heat more to warm the water is all 

do electric tanks ever blow up?

ive had that fear at times

or just gas tanks?


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## rameam (Mar 15, 2003)

Shut the power off to the unit. It is possible for the water to siphon out.


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## Paquadez (Jun 9, 2003)

Worrying amount of ignorance in this thread!

Firstly, it all depends whether the system is a Gravity Fed Open System (i.e., open to atmosphere), or fully closed and pressurised - by the water main.

If it is a Gravity Fed System, then there will two tanks: one in the loft (Or up high) which is simply a storage tank for cold water and fed from the mains through a ball valve with a float.

The Hot Water Tank is supplied by the upper storage tank.

If the water is shut off at the main, water will still flow to the hot taps, until the level of the water in the tank, reaches the same level of the tap. Thus if one has a two story house and the hot water tank is upstairs, the upper hot tap will cease flowing when the level of the water in the hot tank equals the level of the tap. Taps downstairs will flow until the tank is empty: by gravity and the pressure of atmosphere in the tank.

If a sealed and pressurised system, once the cold water main is shut off, the water ceases to flow in both Hot and Cold taps: except a small dribble.

Hot Water Tanks cannot "Blow Up", since in a gravity feed system it is vented to air: all that will happen if an electric Immersion heater is left on as the water level drops is the thermostat will shut off the immersion coil.

If the thermostat fails the immersion heater will burn out.

In a closed and pressurised system, the storage tank has a safety valve: as the water heats up, once again the thermostat will cut in: if its fails, then the residual water would turn to steam and blow the safety valve will will vent steam and whatever water is left.

Most modern electric immersion heaters (As with most equipment) enjoy an "Overheat" safety device: called a Thermal Fuse.

If the equipment passes too much current and heats up to above safe working temperature, the thermal fuse "Blows" cutting the current.

In the UK we have a gravity Feed System, for Central Heating and Domestic Hot Water. Smaller systems are now all pressurised (To mains pressure).

In France for Hot Water we have what is called *un Ballon* (this is a large storage tank), pressurised by the water mains. As hot water is drawn off, at the top (Hot air and water rise, OK! it is how Hot Air balloons work), cold water enters in compensation at the bottom. It is heated by cheap rate electricity at night.

All Central heating systems in France are sealed and pressurised; whether oil, gas or solid fuel (Coal, Wood, whatever).

Hope this assists in comprehension.


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## steppenwolf (Apr 9, 2003)

ok thanks all


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## brbpab94 (Nov 26, 2002)

you should be alright .why are you worrying about it?


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## steppenwolf (Apr 9, 2003)

because im a worrier


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