The new ones from Honeywell come with a nylon check valve built into both the hot and cold side, I guess for idiots like me who put them in wrong in the first place!
I originally mounted mine in a high location far from the boiler without a heat trap. It would on occasion deliver scalding water for a few seconds before regulating the temp which is obviously not desirable. I doubt a heat trap would have worked on my system at the boiler coil anyway with copper piping. My cold water lines got mighty hot due to conduction with a 170+ boiler. Heat traps only stop convection.
Since I mounted mine in the wrong place the first time, I moved it lower than the boiler dhw outlet and changed the supply and mixed outlet piping to pex when I re-arranged the utility room, then put in a check valve to stop the thermal transfer to the cold side (which didn't work well because it was bronze), then realized the mixing valve was encrusted and pitted, with worn o-rings and just not doing its job well, then replaced the mixing valve, then replaced a section of the copper hot supply pipe to the mixing valve to stop the conduction from the boiler dhw.
Then, and only then, did I stop all undesirable thermal transfer.
(Now I am thinking about a bronze pump to maintain a higher mixed line temperature for quicker hot water at the tap-----NOT!)