Can anyone tell me why would you have a mixer valve here on a water heater, Standard 50gal gas water heater, no other use.
More than 90% of scalding incidents occur in the home. The tender skin of very young children and the slow reaction time of the elderly and the handicapped make them most vulnerable to serious hot water burns.
Scalding injuries are tremendously painful, and the effects can last for years.
Scalding occurs for a variety of reasons. In some cases, water heater thermostats are faulty, or set too high. In others, temperature regulating valves at the domestic hot water source are either malfunctioning or missing altogether.
Water heaters are normally set to temperatures above 131°F (55°C) to prevent development of harmful bacteria, such as Legionella, in the water supply. Water at temperatures above 106°F (41°C) are painful.
At a temperature of 131°F (55°C), a child can be scalded in less than 4 seconds.
I’ve seen them installed so they can run the water to the dishwasher hotter (unmixed) and then mix to a lower temp for taps.
If you have a home with older style bathtub / shower fixtures, this is the way to go. You know the type. Hot knob, cold knob and shower diverter knob. Because of the lack of mixing valve like the newer single faucet fixtures, the only choice is a whole home mixing valve.
Had this exact issue with a remodel I did. We wanted the original style 3 knob to stay with the time period. In order to meet code, the city code inspector gave us the choice of newer faucet mixing valve or whole house mixing valve.
What if there is radiant heating? How about a daycare section? Recirculating system?
One picture does not tell enough in this case.
Beauchemin,Marc-Andre - BMAinspection.com
New Home, no radiant heat, no daycare section, no recirc.
3 bath 1 dishwasher,
But it did have a independent hot and cold out door faucet supposedly to wash your car with hot water.
Just never seen a mixer valve on water with so few appliances.
With radiant heating ,Yes
I thought I would ask.
Don’t know what to say about Marcels Answer though.
Burn prevention might have been enough.:roll:
Pretty much Standard recommendation now in all Water Heater Installation Instructions.
Marcell said it all quite nicely IMO. 
Agree! It has been code in Ontario for the past few years and became code in Quebec only this year.
In Ontario, the tempering valve controls temperature of hot water to every fixtures in the home while in Quebec it controls the temperature of hot water to only some fixtures like bathtub and shower.
Some like to wash clothes, dishes with hot water.
Attached is a document from Hydro Quebec from a few years ago on the topic.