Suggested Radiant Mixing Control

I have an addition with radiant heat and a 15,000BTU heat loss.  I want to use an Everhot heat exchanger off my steam boiler to heat the water.  The Everhot also heats a small room with baseboard and a 10,000BTU load.



I need a control for a mixing valve that will take 180* water in the Everhot tank and temper it between 120* and 155* for the radiant loop, depending on the outdoor temperature.  My room with radiant needs the 155* when the outdoor temp is below 15*F.  I have been manually changing the temp when it gets real cold. Using Onix staple-up, which is why I need the higher temps.



Thanks!