Mains pressure HW cylinder

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I came across something in my travels yesterday which surprised me a bit, and I wondered if others might be able to satisfy my curiosity?
I opened the airing cupboard door, inside was a larger than average HW tank which came up to about my chest (maybe 5ft tall). On the HW tank was a big notice with words to the effect that the HW was supplied at mains pressure.
As it happened I had to shut off the HW supply to work on something in the bathroom, and sure enough this required the rising main to be shut off in the kitchen in order to kill the HW supply.
My experience is obviously limited in this domain, but I've always been used to HW cylinders being fed by gravity from the cold header tank in the loft so it was a bit of a surprise to find one that's being fed directly from the rising main. Usually the rising main goes straight up to the loft to fill the header tank and pretty much everything else (apart from a tap in the kitchen perhaps) is dropped from that tank.
The usual CH/HW piping with divertor valve was in place to the HW cylinder to indirectly heat the HW thru a coil, so I'm pretty sure this HW cylinder isn't being fed directly from a CH boiler.
Is this a usual arrangement which I've managed to miss all these years, or did I trip over a house that has had something special done to it? :)
Just very curious about the piping arrangement in this instance because it seemed odd - I'm not trying to fix a problem with the HW supply.
PoP
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It's either a pressurised HW system or a Heat Store.
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G&M wrote:

And indeed is a far better way to do t8ings IMHO. lots of high pressure hot water.
The 'continent' has been doing it this way for years....

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On Fri, 13 Feb 2004 22:02:08 +0000, The Natural Philosopher wrote:

Except where they use combi boilers. I think we are on a slow path to 65% combi boilers 35% unvented cylinders.
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On Fri, 13 Feb 2004 18:34:23 +0000, PoP wrote:

This sounds like an unvented mains HW cylinder, if it had a number of pressure releif valves on it. OTOH if it did not then it was a themal store which also produces water at mains pressure.
If you looked around you'd likely find a valve which cuts the supply of HW to it saving you having to cut of the all the water.
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On Fri, 13 Feb 2004 22:13:00 +0000, "Ed Sirett"

Thanks for that - I'm just surprised that I've never come across something like this in my travels! And I've been in many airing cupboards over the years!
I saw no evidence of pressure relief valves so I guess it must have been a thermal store.

There was indeed a valve on what appeared to be the incoming feed. Wound it right down tight, didn't seem to make any difference (so at a guess it couldn't have been the feed).
PoP
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PoP wrote:

In fact it could have been the feed. IIUC Most of these mains pressure direct water storage systems are designed so that you can extract water from them faster than the mains can replenish it - hence turning off the supply would still leave you able to draw off hot water until the tank is empty.
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Duh?
Unvented cylinders, being fed directly from the incoming mains, stop as soon as you take away the incoming mains pressure. Is yer traditional low-pressure tank-in-the-roof jobbies wot can supply water faster than the mains can supply it.
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soon
Not so, they have a rubber diaphragm inside to maintain a level of pressure even if the mains water is turned off. Mine will still run for a while even after I turn off the feed.
Mike
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In an earlier contribution to this discussion,

That's right! AIUI, they have a built-in expansion vessel (or equivalent thereof) which stops the pressure getting too high when the water is heated. Presumably they also have a non-return valve on the mains inlet to stop hot water flowing backwards into the mains.
When you close the mains feed and open a hot tap, the energy stored in the expansion vessel will drive a certain amount of water out through the tap.
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wrote:

I don't feel so virginal in having asked this original question after all these splendid answers! Thanks to all for contributing!
PoP
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Sounds very interesting.
What sort of life span do pressurised tanks have? Does the diaphram not die at an early age?
Would it be better in the long run for separate storage tank and expansion tank?
John

pressure
even
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die
The diaphragm will certainly lose pressure over time, I gerenerally had to re-charge the system 3 times over the 5 years I had it for. Not too much of a pain really, more than compensated by the brilliant level of pressure with the hot water. A shower that scraped your skin off!
Mike
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How easy is it?
Car tyre foot pump?
John

of
with
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On Sat, 14 Feb 2004 18:22:14 +0000, Mike Hibbert wrote:

Most makes have an external expansion vessel which does indeed contain a diaphragm. These however will likely not need either recharging or any maintenance in 5 years. Their life is however perhaps 10-20 years at best unlike the vessel to which they are attached which has essentially an indefinite life.
Megaflow and perhaps one other brand use a trapped pocket of air. This pocket of air however dissolves into the high pressure water over time. This leads to excessive pressure fluctuations during heating with the result that a) The air pocket has to be renewed. (essentially a partial draining of the cylinder). b) One of the relief valves (usully the 6 bar over-pressure unit), operates then becomes get limescaled and fails.
Typically the air pocket needs to be renewed annually.
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In an earlier contribution to this discussion,

So is this air pocket in direct contact with the water, with no intervening rubber diaphragm? If so, isn't the control of its volume somewhat hit or miss?
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Set Square
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On Sat, 14 Feb 2004 22:57:23 +0000, Set Square wrote:

Essentially the top outlet is a 'dip tube' going down through the top 30% of the cylinder. When the cylinder fills the air remaining in the top pocket becomes the expansion volume, the air is typically squeezed to about 1/4 of its original size (3 bar on guage is 4 bar absolute). I'm not sure but one or more of the makes may include some sort of floating baffle.
In non-Megaflow (and clones) the top outlet is just like on a vented cylinder, a hole in the top.
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Ed Sirett - Property maintainer and registered gas fitter.
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Is replacing a relief valve a DIY job?
John

to
of
with
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On Sun, 15 Feb 2004 16:49:09 -0000, "JohnB"

Practically speaking yes, it can be.
Bureaucratically, probably not. It may invalidate the warranty of the system and installation is a controlled activity of the Building Regulations )I'm not certain about maintenance.

.andy
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die
Heat banks are better. water stored at low pressure and high pressure mains hot water. Heat banks also last longer if the inhibitor is changed every 4 years.
Explanation: http://www.heatweb.com
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