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Warm water from cold tap

Does what it says on the tin
Philholly1
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Joined: November 1st, 2020, 12:00 am

Warm water from cold tap

#352355

Postby Philholly1 » November 1st, 2020, 12:21 am

Hi, please please can anyone help. We have recently had a house renovation but having plumbing nightmares. The latest issue is warm water through the cold tap at random times.

We have a pressurised invented cylinder and flair mixer taps on the bath and basins, as well as in the kitchen and also mixer showers.

The plumber said it is down to water pressure and once the supply pipe is replaced to a larger 32mm one from the current 15mm one then it should be fine but he then said it could be a failing check valve on one of the taps or shower.

I have since found out there are no non return valves on any of them so I'm just wondering if it is the pressure and / or because of them not having the NRVs.

From things I've read it sounds as though it could be the pressure of the hot water is getting into the cold supply but I really have little idea and just very worried as we have been left to drink warm water. Sometimes it does run cold quickly but other times it takes several minutes to run cold, if at all.

Any help would really be gratefully received
Thank you

UncleEbenezer
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Re: Warm water from cold tap

#352374

Postby UncleEbenezer » November 1st, 2020, 5:54 am

Can't help with the technical detail.

But surely the usual explanation for warm (sometimes very warm) water from a cold tap is where it has accumulated in pipes that are warmed either by the sun warming their surroundings, or by proximity to hot water/central heating structures.

Do you observe any seasonal/weather effect that correlates it either with the sun or with boiler activity?

staffordian
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Re: Warm water from cold tap

#352380

Postby staffordian » November 1st, 2020, 8:23 am

At a previous house we had this to a limited extent in an en suite. It was simply down to the fact that under the sink, the hot and cold copper pipes were touching, so every time the hot tap ran, some residual heat in the hot pipe transferred to the cold.

I managed to persuade them apart enough to slip something between them. Just a minute air gap was enough to solve the issue.

Mike4
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Re: Warm water from cold tap

#352395

Postby Mike4 » November 1st, 2020, 9:36 am

Hard to comment in detail without seeing your house and inspecting the installation.

There are two common reasons for warm water from cold taps.

1) Cross-contamination. Where hot pipe has been installed very close or touching a cold pipe. When the hot tap being fed by that pipe is used, the pipe gets hot too (obviously), and it heats up the length of cold pipe next to it. Eventually both pipes will cool down but if a cold tap fed by that warmed-up cold pipe is used before it cools down again, the water from it will run warm for a while.

2) Back-flow from the expansion vessel on the unvented hot water cylinder. There needs to be a non return valve on the cold feed pipe to the HWC nor when a cold tap is turned on causing the mains inlet pressure to the house to drop, the expansion vessel pushes some hot water back into the cold supply pipe thereby contaminating the cold mains pipework with hot water.

Edit for speeling.

Philholly1
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Joined: November 1st, 2020, 12:00 am

Re: Warm water from cold tap

#352397

Postby Philholly1 » November 1st, 2020, 9:45 am

Thanks for the replies and taking the time.

It has only just been installed throughout but all taps do it. No pipes are touching but one thing I tried was turning off the hot water heating. When I do turn on hot water a certain pipe on the unvented cylinder gets hot but I think it should always be cold and hence thinking it is the pressure of the hot water when it's heating up.

Just a thought but no idea really

Many thanks

Mike4
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Re: Warm water from cold tap

#352407

Postby Mike4 » November 1st, 2020, 10:24 am

Philholly1 wrote:Thanks for the replies and taking the time.

It has only just been installed throughout but all taps do it. No pipes are touching but one thing I tried was turning off the hot water heating. When I do turn on hot water a certain pipe on the unvented cylinder gets hot but I think it should always be cold and hence thinking it is the pressure of the hot water when it's heating up.

Just a thought but no idea really

Many thanks


That's 2) above.

Philholly1
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Joined: November 1st, 2020, 12:00 am

Re: Warm water from cold tap

#352481

Postby Philholly1 » November 1st, 2020, 3:25 pm

Thanks for that and does make done sense.

If that is happening, do you know is it still safe to drink if it then runs cold or is it still cross contaminated?

Also, does that still mean we don't need Non return valves on the taps and showers or should those have still been put on?

Thanks again
Phil

DrFfybes
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Re: Warm water from cold tap

#352731

Postby DrFfybes » November 2nd, 2020, 4:19 pm

Philholly1 wrote:Thanks for that and does make done sense.

If that is happening, do you know is it still safe to drink if it then runs cold or is it still cross contaminated?

Also, does that still mean we don't need Non return valves on the taps and showers or should those have still been put on?

Thanks again
Phil


As your Hot Water is unvented, then it is effectively fresh cold water coming in and being stored in a (usually) stainless steel tank and heated to 55C or higher (to kill legionella).

Unlike in the old days when your hot water tank was fed from a header tank in the loft, often with a loose or missing lid and half full of loft insulation, cobwebs, and dead rodents, the unvented system is a lot less prone to killing you if you drink it.

I am by no means saying it is a good think that you are getting hot water back pressured into the cold system, in fact it is a Bad Thing however it is not likely to make you particularly ill if you clean your teeth with it or cook with it or use it to fill the kettle, after all you wash your cups in it (OK, with some detergent).

If you run it until it goes cold it should be fine to drink, but you should get the installer back if there is no NRV on the cylinder feed. I *THINK* NRVs are only required on mixer taps with unbalanced supplies (ie one side mains pressure and one side gravity fed) but in practice it is as easy to fit an isolation valve with a non return function on the inlet as easily as a normal isolation vale.

One thing to try is only to use the mixer taps with the hot OR cold on at once. Obviously not possible with the shower but it will show if one in particular is the problem.

Paul

Philholly1
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Joined: November 1st, 2020, 12:00 am

Re: Warm water from cold tap

#352913

Postby Philholly1 » November 3rd, 2020, 10:36 am

Hi Paul

Thanks very much for taking the time to reply, really appreciate it.

When the hot tap is on, the water is hot constantly so it only seems to be an issue with the cold supply and warm getting in to it.


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