I have a 2 zone hot water system. I want to replace one of the old CEMCO LC120 manual thermostats with a newer programmable model which will handle 24V systems. The old system has a red wire and a green wire, if I put a voltmeter across them it measures 24V. If I short them the boiler comes on and the zone valve opens (I can hear the water circulating). But there is also a 3rd white wire (might be yellow). The voltage from that wire to the green wire is 50V! The other thermostat in the house is wired the same way.
Has anyone run into this type of system before? What is the function of the 3rd 50V wire? Do I need it (which would not allow me to use most of the newer programmable thermostats). Thanks!