Thinking of making my own home hot water mixing valve

Hardware
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I have 4 branches on my home water lines. On each branch under a sink, I have a hot water recirculating mixing manifold with the pump on the water heater. These keep going bad and I need to replace about once a year. At 80 a pop, they are expensive to keep doing this.

My idea is to build my own mixing valve with a 12v solenoid in the middle and a T on the cold water side and a 4 way + on the hot water side. On the side of the hot 4 way + have a threaded resistance temp sensor, similar to an automotive temp sensor. I assume I can use this with an ESP32 on the analog lines to read the resistance to figure out the temp. So when the hot water side drops to 96 or something, then turn on the solenoid to mix it over and I can also turn off the pump if all four branches are up to temp via esphome to HA…

Has anyone done this before? Being that the mixing valves are $80 a pop and 4 a year sucks to keep squeezing under the sinks to do it. I should be able to build all 4 mixing assemblies for the price of a single mixing valve. lol.

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It’s probably a bad idea. Failure on the cold side during a shower is not something you want to take a chance with. I work in the field, that failure rate is insane. If these purpose built valves fail so rapidly, what makes you think that an adhoc system would fair any better?

Idk what brand you are using but quite a few of them can be rebuilt. Off the top of my head watts and honeywells definitely have repair kits. Is this a hard water problem? Maybe consider a whole home softener.

What is the hot water temp? If this is a steam combi boiler, hot water can exceed 200° F, too hot for a mixing valve. In that case you’d need a tempering valve right at the source to bring the temperature down before it hits the mixing valves. Watts LF70A is good to 210° F.

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