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Why it is necessary to use a check valve when installing a mixer with bidet spray?

A check valve is used to shut off reverse flow automatically. Check valves allow water to flow only in one direction and prevent back flow automatically.
It is important to install check valve if you use a mixer with bidet spray. Hot water pressure is higher than cold water pressure that is why when bidet spray is in “closed” position but mixer is in the “opened” position, hot water flows into a cold water line. Therefore when you open a cold water, you will get hot water flow. Your neighbors will have the same. At the same time your water counter will show a record water flow rate which you will have to pay for.
To prevent a hose breakage from high water pressure it is necessary to close mixer after using bidet spray.