it critically depends where yoy turn the cold feed off. If it is at te tank, all well and good.
I turned off the whole house supply. My tank is in the loft.
I had drained the hot tap and was working on it when my ex wife decided to have a pee.
And turned on the cold tap as well.
The tank emptied all over me, the floor, and the room below
Turn off inlet rising main somewhere before it feeds into the cylinder. Run hot water until no more comes out of the tap that you want to change. There is now no nett pressure in the cylinder. Since the outlet from a cylinder is always at the highest point of it, there won't even be a tendency for water to siphon out of it - even supposing that air could enter to replace the water which was lost by siphoning.
No. Once you have turned off the cold water supply somewhere, the only water that will come out is in the expansion vessel and in the pipes above. Once the pipes/expansion vessel are empty, there is no way that any more water can come out of the cylinder because the outlet is at the highest point.
I thought most unvented DHW is direct from the mains? Our entire house is supplied direct from the mains, and we don't have a tank.
I'm not sure I get that. You would have had to turn the mains off then run all the water out of the cold water tank. If you didn't then I don't quite understand what you meant by 'drained the hot tap'.
How annoying.
Oh dear.
Most may be, if they use a combi.
I have a pressurised HW tank and its in te loft.
I dont have a cold water tank
What stopped the hot water running out of the hot water tank was the partial vaccuum formed by shutting off inconing mains.
When that seal was broken, more hot water ran out.
Well that was easier than expected. (provided it doesn't blow up in the next 24 hours.........)
That does rather depend on a number of factors. Normally the cold shut off to the cylinder is situated right before its inlet control valve. So once that is shut, its isolated from the rest of the cold water system.
However if you were relying on the main cold stop tap to the property, then there may be an issue.
Depends a bit on where the cylinder is. On the ground floor, there would not be an issue since you would need a head of water above it to get more hot out. So at most you could get a "pipe full". If its in the loft then potentially you might get a bit more - but the moment air is drawn into the cylinder, it will break any siphon action and stem the flow.
Yup, they run at (near to) mains pressure. In reality there is usually a pressure reduction valve on the inlet to limit the pressure to somewhere around 3 to 4 bar.
Hmm
There are 3 forms of unvented system where both the primary and secondary systems don't have a header tank.
1) Combi boiler.2) Unvented/pressurised cylinder.
3) Thermal store.With all 3, simply turning off the cold water feed stops any flow through the hot water tap. [1]
There may be an expansion vessel to stop back return onto the cold supply, that will then need a pressure relief valve, which of course is mandatory for the Unvented/pressurised cylinder.
If the OP has another system then all bets are off! Unvented can mean other types of system too.
[1] I have seen a standard tank collapse where the cold feed and expansion were frozen, causing a partial vacuum in the tank to collapse it. For the more informed, is there any mitigation of this for 2 and 3? Are tanks so designed to not 'implode'?
Still don't get it
So not the whole tank, just the water stored in the pipes.
A pressure cylinder is only called such because it withstands water at mains pressure unlike the traditional thin(ish) open DHW tank that was topped up from a low pressure header tank in the loft etc.
Therefore your hot water is supplied at the same pressure as your cold water rather than at a much lower pressure of an older vented tank where the pressure is dictated purely by the difference in height between the highest hot water outlet/tap and the header tank more commonly referred to as water "head"
Correct.
Wrong
The great thing about it being a *tap* is that you can just open it to check that you have managed to isolate it without causing a flood.
Exactly what you have to turn off depends on whether the cylinder is pressurised by the rising main, or by a header tank. But you
*definitely* should have some some way of isolating it, without having to drain down all that hot water. If not, then this is the time to add a service valve. Often the easiest way is with something like this:since you then don't have to do any "precision" plumbing.
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