PT100/1000 Temp Control

Good Afternoon, 

I have put together a heating program using the AM2 RTD and i'm a little stuck with the Temperature control, the unit is working and in a basic program its works so there is nothing wrong with the connections its a software issue, what i'm trying to do is have two boilers running until the temperature reaches say 50 Degrees then one boiler is to shut down and the other keep going until the desired temperature. 

The issue is that I don't know which analogue FB to use I have tried threshold and it works but in the wrong direction.... sequencing the boilers is sorted as I have used this for pumps but i'm struggling with the temp

"Then When the Temperature drops below say 45 Degrees the additional boiler kicks back in"

Sorry forgot to add that bit.