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

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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....
 

steppenwolf

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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
 

steppenwolf

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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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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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