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

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???
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 18/11/2007 15:47, John wrote:

You shouldn't be able to /empty/ your hot water cylinder through a hot tap, let a lone a cold tap.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

turned
You are right that it is not possible to empty your cylinder by turning on a tap. 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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 18/11/2007 16:13, John wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John wrote:

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.
--
Cheers,

John.

/=================================================================\
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John wrote:

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.
--
Paul Matthews
snipped-for-privacy@cattytown.me.uk
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 :-( ...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John wrote:

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 :-( ...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
oh wrote:

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

John.

/=================================================================\
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
oh wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
oh wrote:

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.
Guy
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
oh wrote:

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.
--
Cheers,

John.

/=================================================================\
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
oh wrote:

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!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.