Small plumbing snag: occasionally there's a bit of hot water coming out
of my cold water taps: I open a tap, the water runs cold for a few
seconds, then suddenly hot for another few seconds, and then cold again.
And if I turn off the water at the mains and open a cold water tap, the
water from my hot water tank eventually drains out through the cold
water tap. Shouldn't there be some kind of non-return valve ensuring
that the hot water doesn't drain back into the cold water side???
My En-suite basin cold tap does this also. It is because both hot and cold
pipes are close together under the floor so the heat from the hot pipe heats
up a short section of cold pipe and therefore the water inside.
Have you actually done this, i.e. EMPTIED the hot water cylinder via a cold
tap? Sounds a bit strange and costly IMO.
Are your taps in question mixer type or two individual pillar taps?
I must ask why you think this, If the main stoptap into the house is turned
off as the OP stated and a hot tap is opened then the cylinder would
eventually empty (apart from the dregs in the bottom), unless my
understanding of gravity is wrong!
I've been googling to find out exactly how it should work, it appears to
me that the hot water exits at the top of the cylinder, while cold water
enters the cylinder at the bottom. So the only way of actually draining
a hot water cylinder completely would be out of the cold-water inlet.
Some of the diagrams I've seen seem to have a figure like "-->|--" which
looks like a non-return valve to me (and the "><" is an expension valve):
Mains -- Press. reducer -- T to cold circuit -->|----><-- cylinder
So my suspicion is that the non-return valve is broken or missing, would
that typically be part of the cylinder itself?
You are right that it is not possible to empty your cylinder by turning on a
It is very unlikely that you have any kind of non return valve fitted.
What kind of work did your plumber carry out prior to the problem of hot
water from cold tap?
I think your understanding of gravity may actually be wrong! ;-)
Conventional HW cylinders draw off hot water from the top. The only way
to get it out of the top is to feed more (cold) water in at the bottom.
Hence it is never possible to empty it from the top connection - it
remains full no matter how much water you draw.
Usually the connection to the bottom of the cylinder from the cold
cistern is a dedicated one. I can just about visualise a plumbing error
that instead uses one feed to provide this and then continues to supply
other cold outlets in the house. In this case it might be possible to
drain a hot cylinder if you tied up the ballcock on the main cistern,
opened a hot tap to allow air in, and then opened a cold tap well below
the height of the hot cylinder.
The takeoff point for hot water is normally at the top of the cylinder (hot
water rises you see), so just opening a tap means you empty the header/cold
storage tank. To empty the cylinder itself you usually need to open the drain
at the bottom.
The symptom actually seen could be open the cold tap until it stops, then open
the hot tap and nothing happens. The cold header tank having been drained bey
the cold tap means there is nothing left in the cold tank to push water
It is not unreasonable to feed cold from the cold storage tank, as it means
pressures are even for a shower.
I was thinking that at first but it doesn't matter which tap I run and
it happens even when the central heating is off and I haven't ran any
hot water taps.
Yep. If I were to want to do a plumbing job on the cold water side these
days, I'd have to turn off the water at the mains, open a cold water
tap, and wait for the content of my hot water cylinder to have drained
through the cold water tap. Bizarre and annoying...
They are individual taps, apart from in the kitchen which is the usual
mixer. Doesn't matter where I experiment, same result :-( ...
Has this just started or have you had a plumber in recently?
I assume you have a normal gravity fed HW system as you have a cylinder in
an airing cupboard or the loft? If so can you not just turn off the main
stopcock and the cold feed to the cylinder then the water in the storage
tank cannot empty into the cylinder so the hot water within is not 'forced'
out to your taps?
It's actually a complete re-plumbing job and it doesn't have a gravity
feed, the problem is that several sets of plumbers were in on different
stages and I didn't get in again until a couple of months after it was
all done and appeared to be working OK :-( ...
I am not understanding something here. If it "doesn't have a gravity feed"
why have you got a HW cylinder? I am not a plumber but I was under the
understanding that Cylinder = gravity system (header tank in loft and No
Cylinder = mains fed HW on demand (combi boiler)?
But assuming your HW is hot, there will still be at least a hot pipe
from the boiler feeding the HW tank, no? So all that has to happen is
for this pipe to be in close proximity to the cold water pipe somewhere
along its length.
As other posters have said, a hot pipe is close to the feed pipe for the
cold water tap.
Hmmm. I would expect that that normally the hot tank would be fed by
a header tank above it. Typically in the loft.
Perhaps your hot water tanks cold feed is directly connected to the
incoming mains cold water. Thus, if you turn of the main stop cock
and open a cold tap the water in the hot tank could flow back out
of the feed to the cold tap.
Called the various plumbers and the guy who installed the hot water
cylinder came out. What I thought was just a T to a pressure relief
valve just before the cylinder was indeed a non-return valve as well. Of
course it was stuck open, however it was only a five-minute job to
replace it once the plumber got going. Thanks all for your replies!
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