# flush toilet COLD water in shower

**URL:** <https://boards.straightdope.com/t/flush-toilet-cold-water-in-shower/386375>\
**Category:** Factual Questions\
**Created:** [January 2, 2007, 3:01am UTC](https://boards.straightdope.com/t/flush-toilet-cold-water-in-shower/386375 "2007-01-02T03:01:06Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Sigene](https://avatars.discourse-cdn.com/v4/letter/s/df788c/32.png) [@Sigene](https://boards.straightdope.com/u/Sigene)\
**Post date:** [January 2, 2007, 3:01am UTC](https://boards.straightdope.com/t/flush-toilet-cold-water-in-shower/386375/1 "2007-01-02T03:01:06Z")

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I can understand if I’m in the shower and someone flushes, I would get scalded. But why does my MIL shower get COLD when someone flushes or turns on the cold water in the sink? We checked, the toilet is correctly attached to the cold water line.

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**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\
**Post date:** [January 2, 2007, 3:03am UTC](https://boards.straightdope.com/t/flush-toilet-cold-water-in-shower/386375/2 "2007-01-02T03:03:35Z")

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She needs to have a mixing valve installed in the toilet feed. This allows hot and cold water to mix together, which reduces the fluctuation in the shower. I can flush my toilet and not suffer any discomfort whatsoever. It will also keep your toilet from ‘sweating’.

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**Author:** ![Rhubarb](https://avatars.discourse-cdn.com/v4/letter/r/59ef9b/32.png) [@Rhubarb](https://boards.straightdope.com/u/Rhubarb)\
**Post date:** [January 2, 2007, 3:49am UTC](https://boards.straightdope.com/t/flush-toilet-cold-water-in-shower/386375/3 "2007-01-02T03:49:25Z")

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It would appear that your MIL has an antiscald shower valve of the pressure balance type. This device senses the pressure difference between the hot and cold water lines. If there is a sudden pressure drop in the cold water supply, the valve closes off the hot water to prevent scalding. You can Google “antiscald” for much more information.

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**Author:** ![FRDE](https://avatars.discourse-cdn.com/v4/letter/f/e68b1a/32.png) [@FRDE](https://boards.straightdope.com/u/FRDE)\
**Post date:** [January 2, 2007, 9:34am UTC](https://boards.straightdope.com/t/flush-toilet-cold-water-in-shower/386375/4 "2007-01-02T09:34:50Z")

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My shower is straight off the hot water, which is heated on demand by a gas boiler.

If someone turns on another tap, then the supply of cold water into the heat exchanger is reduced - and the shower goes cold.