My cold water from kitchen sink, bathroom sink and shower is warm in summer months during sunlight hours. It is the expected temperature (cool) in the morning and at night. Logic says to me that some how my inside water temperature is related to the outside air temperature. Is this possible, or could you help me to understand what could be causing this warm water situation?

  • 2
    If you let the water run for a minute or two, does it become cool again? Cold water pipes do tend to settle at room (or wall) temperature since they generally aren't insulated, and it can take a little while for the water running through them to cool them down again.
    – keshlam
    Sep 14, 2014 at 21:43
  • If you live in a house and have a cold water storage tank in the attic/loft, and the loft is uninsulated, heat from the sun may be warming the water in the storage tank during the day. And also putting the tank at risk of freezing when the weather is very cold...
    – Bamboo
    Sep 15, 2014 at 13:40
  • No. Water becomes warmer, but not hot, if I let it run.
    – Amy
    Sep 15, 2014 at 15:57
  • Water from the cold tap getting warmer instead of cooler if you let it run seems odd, unless your cold water pipes are in contact with hot water pipes, or unless you have a long run of pipe, either under your own property or from the city, that is buried too shallow and picking up heat from the ground, heated by the sun. Sep 16, 2014 at 14:42
  • Can you consider an experiment? Take the water temperature every thirty seconds for ten minutes and see how it changes. If it continually gets warmer other that period then there's a problem
    – Matthew
    Jun 23, 2018 at 5:23

2 Answers 2


If the cold tap is warm for a while and then cools back to normal, it is a real possibility that you have leak in the hot water line, under the slab. Overnight the hot water leak will beat up the area which includes (usally) the cold water pipe. Depending on where the leak is at, the cold water tap will be cold and then warm when first turned on and then cold again. This is called a slab leak. I have had three. It's almost always the hot line that leaks.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. Aug 8, 2020 at 2:16

How warm is warm?

It's very likely that the air temperature is playing a role in the water temperature in your cold water pipes. As your first questioner asked, does the water get colder if you leave the tap on for a little while? If it does, then you know that the water is colder at the source (your well, or the city water supply) than it is after it's been sitting in your pipes for a while.

It's likely that the pipes themselves are not insulated, especially since they are cold water pipes and there's less concern for losing energy (as there is when you're trying to conserve the energy you used to heat the water standing in your hot water pipes). So the pipes and the water resting in them will equalize at the temperature of their surroundings (air) fairly quickly.

The pipes could also be running through an area of wall (or garage or attic) that is uninsulated or underinsulated, or running through open space through joists in your basement or attic, or running through a south-facing wall, or even running through masonry (bricks, concrete), or in contact with masonry, that is directly exposed to the sun. You mentioned warm sunny days and I presume you're in the northern hemisphere. So the sun may be playing a direct role in heating up the pipes. You've noticed how surfaces that receive direct sunlight get a lot warmer (or downright hot, depending) than the surrounding air. The inside of a south-facing wall will be a lot warmer than the inside of a north-facing wall (in the northern hemisphere) when the sun is out in force.

It is also possible that along some length of the pipes, you have hot and cold water pipes touching each other, and your hot water pipes aren't insulated, so that heat is transferred from the hot water pipe to the cold water pipe. If that's the case and you have access to the pipes, you could wrap insulation around the hot water pipe, or at least wedge insulation between the pipes, because you're not only heating up your cold water, you're cooling down your hot water and bumping your hot water bill a little higher.

Whether the pipes are metal (copper, or maybe steel if they're old), or PEX (plastic tubing), the material that the pipes are made of is a good conductor of thermal energy and will not insulate the water in the pipes against gaining or losing energy.

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    I live in So. CA, USA where the temps go easily up to 30-35 degrees Celsius, sometimes above 37 degrees Celsius.
    – Amy
    Sep 15, 2014 at 16:00
  • I've been to a house where the water supply ran under asphalt. In the summer their water was always lukewarm. You most likely have a case where the water supply line is just getting warmed up by the sun.
    – diceless
    Sep 15, 2014 at 23:03

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