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I live in the UK and it is our winter period, most mornings my down stairs cold water tap, which is one of the furthest from our combi boiler, runs warm as soon as I turn it on then runs cold.

Only my partner and I live in our home and we have not had the hot water tap on for this sink. How can a cold water tap can run warm as if you had just had the hot water tap on then turned the cold water top on?

This is in our downstairs toilet, not sure about the piping, no taps have been run I usually wash my face in there as soon as I get up and it runs warm straight away, both the hot and cold water taps come from our boiler. I would slightly understand it if it ran cold then warm then cold, instead of warm as soon as you turn the tap on.

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    When this happens, how long has the cold water been sitting in the lines between tap usages? If you try to trace the route that the cold water lines takes from where it enters your house to this tap, are there any places where the water could be heated up just from you keeping your house at a comfortable temperature? Jan 4 at 12:18
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    Does this happen every time you turn on the tap? Just the first time in the morning? Is there a hot-water recirculating loop to ensure that you get hot water out of the hot water tap right away? If the recirc loop was added after the rest of the plumbing, it will, most likely, push hot water into the cold water pipes instead of having a dedicated cold water return to the boiler.
    – FreeMan
    Jan 4 at 12:22
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    Is it as warm as...your house?
    – Huesmann
    Jan 4 at 14:09
  • Room-temp water is rarely described as "warm". It's almost 30 Fahrenheit degrees lower than body temp.
    – isherwood
    Jan 4 at 14:59

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This happens, for one example, where the heating pipes and cold water pipe share space (a pipe chase or in the same section of wall) in the building, so the cold water pipe is warmed by the heating pipes. Could also happen with non-pipe-heating if the cold water pipe shares space with a heating duct, for instance.

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  • This occurs in my home with forced-air heat. It's a frustrating result of a poor plan, sending wasted energy down the drain.
    – isherwood
    Jan 4 at 14:57
  • In my house the shared space is under the upstairs floorboards, via air, so it takes a while with the heating on for the cold water to get warm. Also in the airing cupboard where cold water pipes pass near the hot water tank and the pipes that serve it - even though those are all well insulated the airing cupboard stays really quite warm all the time (a common UK system, so possibly close to what the OP has)
    – Chris H
    Jan 5 at 7:04

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