Unvented system confusion.

OK I thought I had got my head round unvented systems but clearly not.
and having looked at the wiki and several web pages havn't cleared things up.
If I have an unvented system and I need to bleed a radiator the pressure in the system will drop as I bleed it. Having completed the bleed I now need to top up the system using the filling loop until a set pressure on the gauge. My assumption here is that incoming water is compressing the air in the expansion vessel as I top it up. Having reached the required cold pressure I isolate the system at the filling loop. Now as the radiators heat up the water in the system expands out into the expansion vessel and the pressure rises to the working pressure. All the water I have been talking about so far is the heat transfer fluid (ie that with inhibitor).
Now the real crux of the problem - I simply want to change a hot tap washer. The tap does not have one of those little quarter turn isolator valves like all the youtube videos seem to conveniently have.
The hot tap is pressurised. Do I simply turn off the rising main at the cold water inlet point and let the pressure out of the hot tap? If so how to I "repressurize" the system when the job is done?
Any help appreciated.
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Chris B (News)

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You’re only pressurising and depressurising the heating circuit when you top up with the fillip loop or bleed a radiator.
The hot water system is just a “through the boiler/heat exchanger” system so just turn off cold inlet to boiler, change washer, job done.
Tim
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On 16/08/2018 20:44, Tim+ wrote:

No, its the 185 lt of hot water in the pressurised cylinder I'm worried about......

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Surely there is a valve on the output of the pressurised cylinder that stop hot water coming out of that that can be used to change a hot water tap washer ?
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John Angus wrote:

If you turn off the mains pressure cold going into the cylinder, there will be no hot water coming out of the cylinder ...
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On 16/08/18 22:21, Andy Burns wrote:

That depends on where you turn it off.
Critically, on whether there is a possibility of unsealing it downstream of where you have switched it off.
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The Natural Philosopher wrote:

Such as the tap with the leaky washer ...
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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.
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On 17/08/18 11:17, NY wrote:

Oh dear.
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On 17/08/2018 11:17, NY wrote:

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'?
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On 17/08/18 15:25, Fredxx wrote:

Correct.

Wrong
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On 16/08/2018 20:48, Chris B wrote:

If you turn off the mains, does any water come out of the hot tap?
It maybe that to get water out of the tank you have to let air in by loosening a compression joint at the highest point in the pipe work between the cold water feed to the house and the hot water tank. Turning on a cold tap might also let the hot water out.
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On 16/08/2018 22:19, Michael Chare wrote:

With an unvented cylinder there is another pressure vessel that copes with the expansion of the DHW as it heats. Shown as item 10 here:
http://wiki.diyfaq.org.uk/index.php/Unvented_DHW#The_components_of_an_unvented_cylinder
Once the cold feed to the cylinder is shut off, you will still get several litres of hot water out. However let it run and that will reduce to nothing as the pressure vessel de-pressurises.

There should be no need to drain the cylinder itself. Once there is no mains pressure to push the hot water out of the top of it, no hot water will flow.
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On 17/08/18 00:28, John Rumm wrote:

Until someone flushes a loo...
then the loo will allow air into the cold side of the system and the hot water will all come out.

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On 17/08/2018 07:50, The Natural Philosopher wrote:

Not in my experience, but in theory the head of water within the pipes could push a small amount of hot water out of the cylinder. There's nothing wrong with TNP's theory, so I guess it's stymied by anti-syphon valves in the toilet cistern.
Just to be on the safe side and to avoid an argument with TNP, leave the hot tap open and flush all the toilets in the house before changing the washer.
My 2p-worth: turn the boiler off today and change the washer tomorrow!
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On 17/08/18 08:32, GB wrote:

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

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On 17/08/2018 09:34, The Natural Philosopher wrote:

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.

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On 17/08/18 11:18, GB wrote:

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.

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On 17/08/2018 11:22, The Natural Philosopher wrote:

Still don't get it

So not the whole tank, just the water stored in the pipes.

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On 17/08/2018 07:50, The Natural Philosopher wrote:

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.
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