When I had to replace mine I bought a new set of Allen wrenches at HF because I needed longer ones. One was too small, another too big, the one in between didn't work either. Then I compared the long one to the short one in the same set and it was a different size! If there is an Allen set screw on your handle one size will fit it if it is made properly. I have mine set all the way to hot, my water heater only goes to 140, in winter I turn it full hot until it is hot to save water, then adjust and step in.

try metric and sae sizes. it might be torx but i don't think so.