I'm upgrading from an "unknown" older analog thermostat to a Honeywell RTH7400 programmable for our heat. The old thermostat had only two wires hooked up, red and green. This must be a single stage unit, since we have a separate thermostat to control A/C (heat pump). The heat is a gas furnace.

The red is in the R slot, which is jumped to Rc. Green is in the G slot. I get no results with the FAN AUTO. With FAN ON, the heat works, but runs constantly with disregard to the temperature setting.

Three other wires, blue, yellow and white were not being used.

Any help would be greatly appreciated.

After some additional tinkering around, I was able to figure out the issue. The green wire needed to go in the W slot. I thought I had already tried that based on some previous advice here, but maybe the connections weren't good.

Problem solved!