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All  you need to know about a toilet mixing valve. 

What Is a toilet mixing valve, and what does it do?
It mixes a little hot water to the cold water line going to the toilets, to prevent the toilet tank from sweating.
Where can I find my mixing valve?
Most of the time you can find your toilet mixing valve above the water heater or boiler.
But some foolish people put them in walls or other places that are hard to find.
If a toilet mixing valve fails, it can cause a number of problems.
It can cause hot water to go into the toilet.
It can cause a loud noise every time water is used in the house or sometimes only when a toilet is flushed.

But it can also make noise when any other fixture is used. It allows cold water to get into the hot water lines and hot water in to the cold water lines.

This is what many toilet mixing valves look like, it has a hot water inlet, a cold water inlet and a mixed outlet that goes to the toilets only. The amount of hot and cold water mixed water line can be adjusted by the two valves on either side of the mixed water line. This type of mixing valve can fail, this is why it needs to be installed in an assessable location, when put into wall or other places, it makes it impossible to find and replace.

Because I have found this type of toilet mixing valve to be unreliable, I don't use them, if a toilet mixing valve fails I replace it with this setup. 2 spring check valves 1 for the hot water line, and one for the cold water line, 1 half inch copper tee, 1 half inch sweat ball valve, half inch copper pipe and what ever 1/2" copper fitting are needed to make it all come together. Click on any image to see a larger image.

This is what it all looks like after I have replaced the old toilet mixing valve with my new setup Please note the little arrows on the pipes and the little notes. I placed the 2 inline spring check valve, just before the 1/2" copper tee that ties the hot and cold water lines together, this way the hot water cannot backup into cold water line, and the cold cannot go back into the hot water line. Also I put the 1/2" ball valve on the hot water pipe going into my new toilet water mixing valve. The ball valve can be tweaked, to allow just the right amount of hot water, into the mixed water line, the cold water valve can be full open. The mixed water line, only goes to the toilets, if it is tied to any other faucet, it will cause all kinds of water temperature problems in your home.