Here is the Illinois code say so on using mixing valves if the temp is set higher than 120 at the heater.
Originally Posted by GabeS
A) A thermostatic mixing valve, conforming to ASSE 1017-1999, shall be installed on the hot water line to the plumbing fixtures. (The mixing valve shall be set to prevent temperatures exceeding 120°F from reaching the plumbing fixtures.)
B) A single check valve shall be installed in the cold water line supplying the water heater. (This will prevent hot water backing up from the heating unit to the plumbing fixtures.)
C) A properly sized and approved expansion tank shall be located on the outlet side of the check valve in the water heater's cold water supply with no shut-off valve between the heater and expansion tank.