There are two type of valves with the same concept but different operation.
A 'Tempering valve ' will attempt to modulate the water to a preset temperature.
Should it get hot water , but NO cold it will pass the hot.
An anti "Scald valve" will shut off the hot if it can not maintain the temperature.
Difference in price as well as operation. CAVIAT EMPTOR!
AS to weather the HW heater that is installed has an engine coolant slowing feature , google the mfg literature, there are many different units.