Do These Hot Water heater Mixing Valves Work?

[Scott Weaner]

Do THESE do anything? Thinking about one for my own home?

[Lucas Evans]

These are used to limit the top temp of the water that comes out of the tap. This prevents burns if you have little ones. I could see how you could up the temp of the hot water tank this would "water down" the hot water coming out giving you hot water at the tap for a little longer. But I doubt that would be worth the additional cost of gas compared to the length of time you could run the hot. 

@Stanley Parsley is right I have seen them used with a solar installation as well to introduce the pre-warmed water to the hot water tank to reduce the utilities cost.