Have a radiant heating system question about thermostats!?

I have a duplex that was built in 1960. Seeing as it is located in florida they went with radiant heating in the ceiling. I need to replace the thermostats and all I can find are ones that are for 4-5 wire systems. I need to know what I can do about replacing all of the thermostats. I believe the heat is on a 240V line with a 50 amp breaker. Any help would be much appreciated.