Plumber screwed up? Hot water out of the cold water taps

[oh]

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???

[John]

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?

John

[Andy Burns]

You shouldn't be able to /empty/ your hot water cylinder through a hot tap, let a lone a cold tap.

[John]

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!

John

[Lobster]

Yeah, ours is the same. And the 'profile' of hot-cold-hot-cold is recognisably different in the winter when the central heating is on!

David

[oh]

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 :-( ...

[John]

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?

John

[oh]

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 ">

[oh]

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 :-( ...

[Andy Burns]

I /know/ it because I have recently had to drain my hot water cylinder to fit a side flange for a shower.

Given that the hot cylinder fills at the bottom, and empties from the top I suggest you try emptying one ... if it hasn't been fitted with a draincock near the inlet you'll need a siphon tube.

[Heliotrope Smith]

[John]

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)?

John

[Guy Dawson]

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 c*ck and open a cold tap the water in the hot tank could flow back out of the feed to the cold tap.

Guy

[John Rumm]

So this is a mains pressure stored hot water system then? Might have helped if you had mentioned this in the first place!

[John Rumm]

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.

[John Rumm]

This sounds like a result of the proximity of hot and cold pipes. Presumably it only happens if you leave enough time to allow it to reheat the adjacent pipe.

[Lobster]

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.

David

[Paul Matthews]

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 through.

It is not unreasonable to feed cold from the cold storage tank, as it means pressures are even for a shower.

P.

[oh]

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!