Why are expansion vessels so expensive, more than the heater they're needed for?

We're considering a small under-sink unvented water heater for our kitchen as it's so far from the hot water tank it takes several minutes (and lots of wasted water) for hot water to reach it after the tap is turned on.
It's only for the odd non-dishwasher bit of washing up and for cleaning surfaces etc. so we think a Redring MS6, 1.5kW 6 litre one should be quite sufficient.
Our water pressure is within the MS6 specification so we don't need a pressure reducing valve.
However we will need an expansion vessel as the cold tap for the sink is (unsurprisingly) adjacent to the hot tap (well, it's a mixer). The specified expansion vessel:-
https://www.fastlec.co.uk/87-783101-redring-cold-water-control-pack-87783101-cwp.html
costs *more* than the water heater! That's really ridiculous.
I'm thinking of simply adding a loop of copper pipe to give the required 1.7 metre length that's required to avoid the need for an expansion vessel. 1.7 metres of copper pipe will cost a whole lot less than £75!
Is this a sensible idea?
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Chris Green
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Chris Green wrote:

I'd have thought that you could find a potable water EV for less than that? How is 1.7m of pipe supposed to replace this function? Is it a dead leg full of air perhaps.
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Possibly but I've not seen [m]any much cheaper.

No, it's apparently simply the ductility of that much copper that allows for expansion of the water in the unvented heater.
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Chris Green
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On 04/02/2018 12:57, Chris Green wrote:

Incorrect, see my other post.
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Without prejudice to whether it is true, I think you mean elasticity, not ductility, the latter being irreversible.
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Roger Hayter

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True, I knew it was the wrong word! :-)
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On 04/02/2018 12:44, Bob Minchin wrote:

Its there to stop the water in the tank reaching a tap when it expands. This is because the heated water may contain bacteria or other stuff that shouldn't get out of a cold water tap.
If the run it too short you put a one way valve to stop the water going back and then you need an expansion vessel to take up the expansion.
With a long pipe you just let the expanded water go back along the pipe as it won't reach another outlet.
BTW these water heaters aren't designed to deliver potable water so you don't need a potable water expansion tank, the check valve prevent none potable water from the heater getting back to a tap.
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Ah, now that's a more sensible explanation than my original one (which came from the installation instructions for one of these).
Thank you.
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dennis@home wrote:

Ah! I had assumed is was one of these fancy hot and cold and boiling water taps that you can make tea from. In that case it does not have to be a potable EV just one made from stainless steel but they might end up being one and the same. A heating/solar thermal type EV needs inhibitor to stop it rusting.
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I guess its also not going to work that well on mains pressure either though. Never did get this expansion vessel idea on such systems, Water is unique in expanding when cold and when hot! Brian
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On 04/02/2018 16:55, Brian Gaff wrote:

Yes, it's at its most dense at 4 degC - and expands above that *and* below that - more so when it turns to ice.
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On Sun, 04 Feb 2018 12:24:16 +0000, Chris Green wrote:

pack-87783101-cwp.html

Perhaps another expansion vessel?
<https://www.mrcentralheating.co.uk/water-storage/expansion-vessels
is my first hit.
Looks as though you could fit for about £37.
We had one added to the incoming mains for our combi because the manual said it was needed.
AFAICR it was much simpler - integrated unit which fixed into the cold supply.
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Dave R
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I don't think they're really intended for small unvented heaters, their capacity is as much (if not more) than the heater. Apart from anything else there's not really space for something that big, it'll be tight enough getting the heater in there.
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On 04/02/2018 12:24, Chris Green wrote:

You need less than that if you use 22mm pipe.
The pack includes PRV do you need one?
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That's a point, two 15/22mm couplings would be simple enough.

No, it's one of the reasons the Redring model above makes sense as it's rated for rather higher water pressure than most.
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On 04/02/2018 12:24, Chris Green wrote:

My Quooker doesn't have an expansion vessel. It has a combined check valve, which stops hot water going back into the cold mains, and a PRV which spills a small amount of water into the drain when expansion takes place. Can't you do something similar? Should work, provided the heater can stand a bit more pressure than the PRV is set to.
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On 04/02/2018 19:51, Roger Mills wrote:

Just to clarify, I'm using PRV here to refer to a pressure *relief* valve, not a pressure *reducing* valve.
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Any particular reason you can't supply it from the hot feed? There has to be more than 1.7m of pipe there.
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That's a point, removes the back feed issues as well, the hot water isn't potable.
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To expand on this, can anyone see any reason at all why I can't put an unvented water heater in the *hot* feed to the sink? It would solve virtually all problems in one go (at least the rules on how you plumb it problems):-
The hot feed is low pressure, we have a conventional old-fashioned header tank fed hot water cylinder
No back feed issues as it's non-potable water anyway
Minimum length of pipe to take-off is easy to fulfil (even though it's irrelevant)
It sounds a funny way to do things but it would actually work quite well, if we happened to want a *lot* of hot water then we'd eventually get hot feed from the cylinder. There will be a bit of heat loss from the pipes when we draw off small amounts but I don't see that as being a big issue. What the 'local' heater will do for us is give immediate hot water and stop the waste of water.
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