Hot Water from the cold tap?

Recently I've found that in the morning my second floor bathroom sink will produce warm, almost hot water from the cold tap for about 15 sec. in the morning.
I'm thinking - "this is GREAT, the city is piping us out hot water! - this will sure help with my New England winter heating bills!". then I woke up... ;)
It's an old sink with separate taps, no mixing valve. Is this an indication of another mixing valve failing? My shower has a temp regulating mixing valve which seems to be function properly - is that the likely culprit? Would hot water run back into teh cold line and come out the cold side 5 feet away? - WHy? the pressure is the same in the hot and cold lines, isn't it? I've never heard of this symptom - any one else??
Many TIA
Martin
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My bet is you have someplace where it is mixing in the basement. Do you have something that uses hot and cold water there? Maybe a leaking washing machine valve? It could also be a bad dip tube in the water heater. Hot water will naturally rise in the piping so any interface of hot and cold water will show up in the 2d floor bathroom.
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wrote:

"Recently" as in "since you turned the furnace on for the season", I'll bet...

My guess is that the supply lines to that sink pass close to a heat source. Does your house by any chance have hot water or steam heat? If so, ten-to-one IMO that your supply lines (or the cold one, at least) run right next to a radiator in the next room.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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mf wrote:

I agree with Dough. It is likely picking up heat from a nearby heat source like a hot water pipe, heating duct or pipe, etc.
--
Joseph Meehan

Dia duit
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I have, first hand, a little bit for a little while. I got stuck on the same question you had. Isn't the hot water pressure no more, maybe even less, that the cold water pressure?

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It is not a pressure thing, but a temperature thing. We all know hot air rises. So does hot water. It can circulate on its own as long as there is a temperature differential and equal pressure.
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When no water is being drawn, the hot water pressure can rise above the supply pressure due to expansion in the heater along with a no-return check valve in the supply line. Expansion can back up the hot water into the cold lines if you have a regulator on the water supply and no functional expansion chamber. This can sometimes open the relief valve on the water heater. Thermo-syphoning can also occur between hot and cold water lines, particularly with mixing valves which can develop internal leaks between the hot and cold supplies without noticeable leaking out the spout. Not real common but also not real rare. Don Young
wrote:

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Mixing valve??? There is no such thing in that setup. It is simply a tee joint.
There is no back-flow, since the other tap is closed, and even if open is operating at the same pressure...

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glenn P wrote:

You probably have some hot and cold water pipes running alongside and in contact with each other causing the hot water pipe to transfer heat to the cold water pipe, heating the water inside.
-The Plumbinator
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This is quite commonly caused by internal leakage in any one of many kinds of single handle faucets. It can be aggravated by expansion of hot water in the heater where there is a check valve or regulator to prevent backup of expanding hot water into the supply. Sometimes hot water will back up into the cold water line feeding the heater, past where the cold water feeds somewhere else. It is often possible to find the source of the problem by careful feeling of the cold water lines. Don Young

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