I'm pretty sure the "mixing valve" is central boilers attempt at boiler protection. Basically their version of a danfoss valve. I have never seen one personally so I don't know how they are designed to be plumbed. To go back to the original post I thought the question was why the op was losing 20 degrees of heat from the boiler to the valve. I'm still guessing he's not and the water leaving the boiler is the same temp as it is hitting the valve. Too bad he can't get an accurate reading with the ir gun this could be solved in 2 minutes! Lol!!