I am starting to install a thermostat on a 1972 furnace with heating and air. I have four wires two green wires and one red and one black not sure how they hook up to the new thermostatwhich is atype 1f56w-444. If someone could please shed some light on this for me please help. Ron

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Originally Posted by ironhorse

I am starting to install a thermostat on a 1972 furnace with heating and air. I have four wires two green wires and one red and one black not sure how they hook up to the new thermostatwhich is atype 1f56w-444. If someone could please shed some light on this for me please help. Ron

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One of the four wires is likely the low-voltage power supply and the other three go to the indoor fan relay, outdoor unit relay, and primary heat control. I would start with the red wire and connect it to black, observing if any part of the system comes on. Proceed with each possible pairing of wires until you have determined which one is the low-voltage power supply (connect to "R" terminal) and which of the remaining wires go to the indoor blower control ("G" terminal), outdoor unit ("Y" terminal), and primary heating control ("W" terminal). Any pairing that does not produce some function of the system excludes the low-voltage "hot".