Hobbiest need help with a differental thermo mod

[cbiblis]

This attachment is the schematic for my differential thermostat i have on my solar hot water heater system. It works great but i need to add another feature. How the circuit works is that when the value of the sensor in the collector is higher than the sensor in the water storage tank the circuit turns on the pump. The problem is that if the temp gets below freezing then my pipes will burst. So i need to modify this design to switch the pump on when the temperature of the collector is just above freezing. this in turn will circulate hot water from my storage tank to the collectors there the circuit once sensing the temp is above that value should shut off again. the collector will store the heat for a while before it reaches freezing temps again.
any help would be appreciated.

 

[cbiblis]

eagle sch

here is the eagle sch file.

 

[MikeMl]

You need to add a third LM339 which compares the voltage across the collector sensor to a trip point which corresponds to a temperature just above freezing.

 

[cbiblis]

I'm kinda new to electronics. I'm not to sure of the lm chips. sense i will be rebuilding the pcb can i substitute the lm393n for a lm324n? The reason that i ask is that i have 10 of then in my shop. Thanks