I have a three-zone hydronic heating system in my house and I want to replace my old thermostats with a programmable one. The problem is, I can't determine which thermostat to use. The current thermostats have three wires labeled 4 (open valve), 5 (power), and 6 (close valve). In talking with white-rodgers, a model that would work is the IF90-371, but they are $120 a piece (I need three - one for each zone). Is anyone aware of an alternative that I could use?