Shower mixing valve

What will fix the problem is a check valve in the hot water pipe at the shower. Due to pressure losses in the piping, the cold water will back-fill the hot water piping to the hot water heater. The check valve will give your problems when it comes time to winterize, as you cannot blow down the hot water line to where it branches from the sink. The mixing valves John linked, are real nice for S&B, but present the same problem when time to winterize. I have found that once the water is hot, turn it down as low as possible, and mix in the cold, then switch to the shower head. This, for me, prevents the large temperature swings that happen when shutting off the water flow. The DW now is a different story, she turns the hot wide open, then mixes the cold, and complains about being out of hot water.