Mike, What kind of tempering valve is it?
Temperature control valves are meant to cool the hot water from the tank to a desired temperature for use within the home.
Why is it necessary that you have one? Do you have young children in the home? At what temperature do you set the heater?
If the temperature control valve is made by Watts, remove it, for it is not suited for your application. Instead keep the heater set at a desired temperature that will not be dangerous to your household, something around 125F.
Good Luck,
Bill
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com