Hot water go slow

Symptom is hot water to bath starts off fine and then slows down - takes a long time to fill the bath. It's a mains pressure system.
I've just had the boiler serviced, which has fixed the central heating not working on the top floor (found a brilliant Valliant technician). He said the hot water is almost certainly a valve 'collapse' problem and will be giving me a quote to cut it out and put in a new one. But can't the valve just be repaired, and how to actually pin down the fault?
E.
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the answer to "can't the valve be repaired?" reminds me of a colleague whose Mini developed a gearbox fault. "Recon box" said the dealer, bu cooegue said "it's only 2 year old - mend it". So, they did and it cost more than fitting a recon box wuld have done.
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It could be the tap. Does the sink suffer the same problem? Dismantling the tap may be a simpler solution (if the cause) and likely DIYable.
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fred
it's a ba-na-na . . . .
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On 06/02/2015 10:03, eastender wrote:

When you say it's a mains pressure system, do you mean that you have a system which stores hot water in a mains-pressure cylinder, or a combi boiler which heats mains pressure water as it passes through?
Do you have other taps (wash-basins, sinks, etc.) served by the same system? Do they all exhibit the same symptom, or is only the bath tap affected?
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On 2015-02-06 10:59:23 +0000, Roger Mills said:

It's a system which stores hot water in a mains-pressure cylinder.

All taps affected and are served by same system, bath particularly so though.
E.
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I have just had my (different) boiler fixed due to a low DHW Flow. The fault was a collapsed O Ring next to the flow restrictor.
A previous boiler used to have a spring loaded reducer. I think that the normal flow was unrestricted - but as the tap was opened further there would be a "clunk" from the boiler as a valve closed in response to the flow and diverted the water through a restrictor orifice.
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On 06/02/2015 12:12, eastender wrote:

OK, I think I know what the problem is. Since all taps are experiencing it, it's a problem with the whole system and not just with one tap.
Since it starts off fine, it indicates that there's no problem with the pipework between the cylinder and the taps. Since it's a mains pressure cylinder, you'll have an expansion vessel somewhere - either an external pressure vessel or an internal bubble. When the cylinder heats up, the water expands, and a non-return valve stops it going back into the mains, so that it has to compress the air in the expansion vessel.
When you first open a tap, the initial flow is provided by the stored energy in the expansion vessel without needing any new mains water to come in. Once the pressure has reduced, new mains water is required for expelling the hot water out to the tap. If the flow of this mains water is restricted, the flow to the tap will be slow.
I mentioned a non-return valve just now. This will be on the mains input side to the cylinder, and may be combined with a pressure reducing valve - to prevent the pressure in the cylinder becoming too high if the mains pressure is more than about 3 bar. Or there may be two separate valves in series. Either way, that's where the problem lies. Something has gone wrong with one or other of these valves (or with the combined valve if that is what you've got) which is restricting the cold water flow more than it should.
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On 06/02/2015 15:18, Roger Mills wrote:

That sounds like a good diagnosis to me.
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On 06/02/2015 20:20, newshound wrote:

Yup that sounds like an eminently plausible explanation to me. See part 3 on the diagram:
http://wiki.diyfaq.org.uk/index.php?title=Unvented_DHW
Another (similar) possibility is some systems are built with a thermostatic blending valve on the outlet of the cylinder to limit the maximum temperature delivered to the taps. This is another single point of failure that could limit flow - although in this case it would be when that valve gets hot rather than when the pressure stored in the expansion vessel is released.
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On 2015-02-06 15:18:40 +0000, Roger Mills said:

Yes, that's an excellent explanation, many thanks. I've had a good look at the pipework and there are two Honeywell valves in series that look likely candidates. I don't think this is a DIY job (for me) so will chase up the boiler guy and see if he wants to fix it.
E.
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Would it really be the end of the world for a bit of hot water to go back into the mains?
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On 07/02/2015 22:21, Tough Guy no. 1265 wrote:

Might depend on your attitude towards prison, I suppose. <g>
Seriously though, it *is* illegal because your HW system could be harbouring all sorts of bugs (legionella, etc.) which could be fed back into other people's clean water supply. Might not be very likely, but the possibility is there.
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I've used the hot tap to fill a mug for hot chocolate loads of times. It's much quicker than waiting on the kettle.

No it isn't. Just think how much water would have to be fed back before it left your premises and got to the nearest junction point. Just how much expanding does your tank do?!
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Sure, but not all hot water systems can end up with legionella in them.

Corse it is.

They aren't necessarily talking about that, what can get into your own cold water supply as well.

Its not necessarily about expanding with a major water pipe burst, in that case they tank can empty into the pipe.
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In fact not many at all.

It won't go far enough to escape my property. That would require a larger volume of water than is produced by expansion. So it can only poison me.

My problem only.

Oh stop worrying about remote possibilities. Anyway with a water main burst, all sorts of shit gets into it.
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Sure, but the legislation is because some do.

But will still get into your own cold water feed.

We aren't just talking about expansion.

And anyone else in your house and visitors too.

And anyone else in your house and visitors too.

I'm not worried about anything, that is the reason for the requirement for a non return valve.

And is flushed after its repaired for that reason.
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Why make a fuss about what will probably never happen?

But I've never died from drinking the hot water directly.

What then?

Oh dear.

Oh dear.

You agreed with the reasoning, so you must be worried.

Funny how the water out of the tap is all muddy for a while afterwards.
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Because some prefer not to die.

Not everyone runs their hot water service at a high enough temperature to avoid legionella.
What you do is completely irrelevant.

The whole tank of hot water ends up in the cold water line when there is a really bad break in the mains in the street etc.

Oh cheap actually in your case.

Oh cheap actually in your case.

Nope.

Nope.
I JUST rubbed your nose in WHY its required.

Mine isn't.
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On 09/02/2015 00:54, Tough Guy no. 1265 wrote:

A typical large unvented cylinder may have a 20L expansion vessel on it. So if you lower the pressure of the supply for any reason, then without the non return valve, the cylinder could push 20L of contaminated water back into the main. Water that may harbour legionella, or be contaminated with other bacterial contaminates.
Many older properties have only 1/2" diameter supply pipes. That contains something like 0.1327 L per meter. So that 20L of expansion could push contaminated water 150M back up the supply - easily enough to affect other properties.

Indeed, but the containments entering that way are less likely to have been sat in the ideal bacteriological breeding grounds of some hot water systems.
The water regulations are quite specific about measures required to avoid back contamination, where there is considered to be a tangible risk.
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I find it hard to believe water expands anything like that much. Liquids don't expand as much as gases. Are you sure the whole 20L ever gets used in the expansion vessel?

It can't be much risk, as I've often drunk water from my hot tank. It didn't even taste off. It's mains chlorinated water in copper, just like the mains cold feed. It's just been sat longer. So also you should be thinking if you go away for a month on holiday, that you must run your cold water before drinking it. I doubt many people do that.
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