We are having an addition done, and they just ran the hot and cold water lines to the addition. The lines are not used yet, so the water in the hot water line is cold because hot water never moves through it.

Whenever we run hot water anywhere else in the house it is hot at first and then only lukewarm.

Here's a picture showing where they connected the new PEX line to our existing copper hot water line. When we don't run the water for a while that copper T connection is warm. As soon as we turn on a hot water tap the copper becomes very cold there - so it must be mixing in the cold water from the new PEX line.

What could be going on? Is it just that there is cold water in the unused line that is mixing with the hot water and making it cool? Even after running hot water for 20 minutes that connection is really cold. Or could there be a hot/cold crossover in the addition?

enter image description here

Update: I found a hot cold crossover at the shower mixing valve in the addition.

  • I dont see a hot shutoff, but you can shutoff the cold to the tank and try the hot.... do you get continuous water flow? if so, you have a crossover.
    – noybman
    Jan 16, 2019 at 5:28
  • In a house we were renovating the plumber roughed in a shower valve and had the handles removed so that it was not obvious that the valves were open. So crossover. Jan 16, 2019 at 10:31
  • Hmmm... I am curious as to what that 1/4" copper line leads to.... Jan 17, 2019 at 3:24
  • maybe the dip tube in the water heater broke off
    – jsotola
    Jan 17, 2019 at 4:08
  • @JimmyFix-it leads to the (non-functional) humidifier. Feb 6, 2020 at 23:47

1 Answer 1


Posting suggested test as an answer since the follow up identified it as a crossover.

When testing hot water flow and a crossover is suspected, an easy test is to turn off the water flow from the hot side at thetank to remove hot availability. When the hot tap is opened downstream, there should be no pressure, so very little water will come out. If pressure seems normal or near normal such that water consistently flows without ceasing, it can only be a crossover OR a bad shutoff valve *(but if it was a bad shutoff valve, the water would be hot for a good while if provided from a tank).

At this point, its a matter of following the various faucets or mixing valves to verify their settings. These valves (mixing vales) can go bad, or be installed wrong, so it's usually worth it to turn off the cold supply at each suspect location until flow stops.

Congratulations on solving the issues.

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