Hot water - gate valves??


Postby stevehayes » Sat Feb 02, 2008 8:35 am

Is my house just weird, or is this supposed to be normal? When I turn the gate valve in the loft off, for the cold feed to the hot water cylinder, hot water stops coming out of my bathroom taps within seconds! This isn't supposed to happen is it?
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Postby thedoctor » Sat Feb 02, 2008 10:12 am

See our projects on domestic hot and cold water systems. The pressure of the cold water from the mains, coming into the bottom of the HW tank is the foce that pushes the hot water out of the top of the tank. =- No cold - no hot.
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Postby Kenj » Sat Feb 02, 2008 10:27 am

As it is a Gate Valve I assume that this is not mains water, but the feed from your Cold Water Storage tank. When you shut this Valve you create an air lock, water cannot flow down the pipe to the taps unless air can get in to replace the water that flows out.
So yes even downstairs taps will shut off very quickly despite the weight of water above the tap. Shutting the Gate Valve has created a vacuum.
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Postby stevehayes » Sat Feb 02, 2008 3:59 pm

I thought the cold feed from my cold water sotrage cistern which feeds my hot water cylinder in the bathroom would only close off the supply to that. Thus to drain the hot water in my cylinder I would need to drain it all through the hot taps!? I'm confused now!
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Postby dcc » Sat Feb 02, 2008 6:30 pm

sadly mate it doesnt work like that. as thedoctor said, its the head from the tank that forces the water out of your tap.

if your wanting to drain the cylinder, (if the original installer put one in)there should be a drain cock on the pipe that feeds the cylinder.

if not then.............well..........thats a trade secret ;)
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Postby Kenj » Sat Feb 02, 2008 6:55 pm

Sorry to dissapoint, but you cannot drain the Hot Water Cylinder via the hot taps. The reason for this is partly explained in the 2 earlier posts.

The main problem is that the hot water comes out of the top of the H/W cylinder, not the bottom, and water will not flow uphill unless forced to.
The cold water enters the H/W cylinder at the lowest point, and it is the pressure on this water which pushes the water to the top of the cylinder, and then to the taps.
As the two earlier posts explained, by shutting the gate valve in the loft, you have removed this pressure, and cteated a vacuum effect.

To drain the H/W cylinder you need to do the following.
1. Shut the gate valve feeding cold water from the Cold Water tank.
2. Open all hot taps, especially those below the level of the tank.
3. Attach a hosepipe to the drain-off cock, hopefully fitteded to the cold feed pipe at the bottom of the H/W cylinder. A jubilee clip will ensure a tighter fit. and hopefully less spillage.
4. Run hosepipe into a sink or drain at a lower level.
5. Open the drain-cock hex end nut, until the water flows at a good rate. Have an old towel and a bucket handy as some water will probably escape out of the end of the drain-cock.
6. Do not fully unscrew thie drain-cock nut or you will flood the house.

If you do not have some form of drain-cock fitted to the cold feed to the H/W cylinder, then you really do have a problem
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Postby stevehayes » Wed Feb 06, 2008 3:35 pm

no i don't want to drain my cylinder. i know how to do that. and without a drain cock the water has to be siphoned out from the top which i know about too! my query was how does a gate valve in the loft, stop hot water from coming out the bathroom taps!! Ithought my system was just weird-but it seems it is normal! amazing how it works really!!
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Postby nitro23456 » Wed Feb 06, 2008 8:54 pm

It all works on gravity and pressure mate.

Normally the cold from your CW tank in the loft goes into the bottom of the hot water cylinder. This pushs the hot water out of the top of the cylinder due to the gravity and pressure force down from the CW tank.

Shutting the gate valve stops this cold flow into the bottom of the tank and thus removes this pressure. Therefore your hot water cant be 'pushed' out the top of the cylinder, although the cylinder is in fact still full of water!

It is normal for the taps to run dry in seconds although there will still be water in the pipes that feed them - this however isnt a problem and this water wont flow if you cut the pipes or remove that taps..... it will just drain from above the cut you make (so you will get about 5ml spillage)
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Postby stevehayes » Fri Feb 08, 2008 12:35 pm

thanks a lot for your help here. i've learned something new (even though pretty basic really)!! So basically by turning off your gate valve in the loft which supplies the cold feed to the bottom of the hot water cylinder, you shut off the hot water supply! (well after the small amount already in the draw off pipework is drained). is this correct?!!
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