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
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.
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
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.
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.
Yup that sounds like an eminently plausible explanation to me. See part
3 on the diagram:
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.
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.
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.
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
The water regulations are quite specific about measures required to
avoid back contamination, where there is considered to be a tangible risk.
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.
What's soft and warm when you go to bed, but hard and stiff when you wake up?
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