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I am planning to improve my bathroom by installing a bidet. I do not like the idea of cold water and I am planning to add a mixer to constantly get lukewarm water.

Proposed plumbing scheme:

  1. Insert a three-way elbow pipe (with valve for emergencies) to cold water pipe, and second one to hot water pipe.
  2. Connect a water mixer (either with or without a thermostat) to hot and cold water.
  3. Connect a bidet hose with a button on the end to start the flow.

My plan is to hide the water mixer in a wall behind inconvenient mini-door and keep it in opened position with 20%/80% hot/cold ratio (lukewarm) and control the bidet only using the button on the hose. This means mixer is being constantly open and I am afraid that when bidet end valve is closed and mixer is still on, hot and cold water might start to mix in the main pipes. Is this true?

Also, a follow up question, is the mixer even necessary in this situation? Couldn't I control the flow and the temperature by opening the hot water valve in three way pipe by 20% and cold water in the three way pipe by 60%, join those pipes and connect to the bidet hose without any mixer? The hose is 100cm long, so water should mix properly, I am just wondering is there some additional mechanism in the mixer to prevent water going opposite direction when opened, but output closed?

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    Depends if just a simple mixture valve or a anti-scalding valve. Just cold water gives you a pick up in the mornings, almost as good as coffee.
    – crip659
    Jan 7, 2023 at 1:58
  • IDK about plumbing codes, but check valves do exist which prevent liquids going in the wrong direction.
    – user253751
    Jan 7, 2023 at 2:40
  • That would depend on your heat tolerance, just in case you get hit by full hot water stream
    – Traveler
    Jan 7, 2023 at 2:43
  • Do you have a tankless heater very close to your bidet, with recirculation? Otherwise, you're gonna get only cold water at first, and by the time you get warm, your business will be complete. My recommendation would be to simply get a powered bidet toilet seat.
    – Huesmann
    Jan 7, 2023 at 13:15
  • I do have hot water with recirculation nearby (40cm away not including the hose shower).
    – Augustinas
    Jan 7, 2023 at 16:30

2 Answers 2

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Your concern is correct.

To "pre" mix hot and cold, you need a mixing valve with built-in check valves to prevent cross-over flow.

Most "bidet" mixing valves have this cross over prevention built in. Check the specifications.

Here's an example:

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Ref: amazon.com

What won't work is if you build your own mixer from two valves (hot & cold) followed by a T connection. This will have cross over.

Also, a shower mixing valve is not suitable because (generally) it will not have cross over prevention since it is not needed. The shower head itself has no shut off, and the shower hose or head will not build up enough pressure for cross over to happen.

A pressure balancing mixer valve (PBV) may also prevent cross-over flow. If not specified as such, it likely will still reduce cross-over.

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  • The problem here too is having actual hot water to work with, which won't happen unless the OP somehow has some kind of hot water recirc system.
    – Huesmann
    Jan 7, 2023 at 13:16
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There are bidet seats that have a holding tank that warms the water. You are always "greeted" with water at a temperature of your choosing.

I have not encountered full bidets that have this holding tank, but A search for the same may be your best bet.

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