hot water coming out of cold tap ....

Ada wrote in news: snipped-for-privacy@4ax.com:

I'm afraid your theory is not right - it's something worse.

Your HW heating is not being regulated, failed thermostat, ore something, and the tank is probably boileing and venting hot water into your cold tank (if it's that sort of system)

Please repost PDQ with more details, type of heating system etc, and let the experts get at it.

No way in the world can a hot pipe do more than vaguely warm a cold pipe next door to it, and that only if there's been no flow in it for ages, and the hot pipes been working hard

mike

I know nothing about these things, but my money would be on a fault with the shower mixer tap, is your shower a power-shower?, could a fault with the mixer result in hot water being fed back into the cold water system?

Anyway, I'm sure someone will come up with an authorative answer.

Regards

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Combi boiler, with cold water supplied from a storage tank? The hot water is under pressure and when the shower mixer is used, hot water goes up the lower pressure cold water pipes.

news: snipped-for-privacy@4ax.com:

Sounds exactly right, Mike, I was about to suggest the same thing.

Alan.

Curious: I get the same thing. Get to the bathroom in the middle of the night for some cold water and the cold tap runs rather warm for about 20 seconds before chilling out.

I assumed the same as the OP: adjacent central heating pipe heating standing cold water pipe when the cold isn't being used for periods. In my case I cannot see how your explanation fits as I have a combi boiler for hot water and the cold is fed straight from the mains - no tanks anywhere.

If the warm flow lasts only a few seconds before cooling and it only happens when the water has not been used for a while, then it probably is that your cold water pipe is being warmed by an external source. However, I cannot think of any explaination for what the OP reports except that the water in the cold water tank is being heated by venting from the hot water system or central heating.

Colin Bignell

Thats one possibility.

Another possibility is a buggered mixing valve in a shower,. I had the reverse - cold coming out of the hot - when installing some showers. I had the shower ON but the spray head blanked off...the cold pressure forced the cold into the hot pipework...:-)

So also look for places where hot and cold could physically mix.