How to operate DHW part in Triangletube ?

We have installed triangle tube boiler for both CH and DHW in January, 2010. Boiler temperature was always higher than water heater set temperature in colder days. The DHW part of the circuitry was working like another CH system. Now that the outside temperature is high, the boiler temperature is less than 90F and the DHW temperature is never above 85F. Taco SP504 is the switching relay being used in the system. I am trying to find out how to make the triangle tube recognize that the Zone 4 is DHW system and not a CH system. Thanks for any and all help.