Hi there -

I have a Mitsubishi HVAC system. I removed the old thermostat and now have 4 wires. Red, Green, White and Blue. However, the old stat was only using two; red and green. I followed the instruction from the new stats manual and other info found here and other sites but I cannot get it to work. I did get the fans to work at one point but that's all. I looked at the other end of the wiring at the unit above my bathroom but the 4 wires aren't labeled with R,G,W etc. There is a label that suggests that Blue and White go to "Transmission Out Door Unit". There are also two spare terminals.

Would really appreciate some advice/help.

Thanks!
Nick

Ps The new stat is a Honeywell RTH 6350D

How many wires are coming out of the wall?

The Red wire is going to be your "24v" power and green should be "fan", there should be a yellow wire and at least one more wire also.

Note: Some Honeywell t-stats do not work with certain systems; but in any-case after your thermostat is wired correctly, it has to be programmed for "heat pump", "gas", "straight cool", or "electric heat coils" depending on what you have.

Thanks for your reply !I have four wires. Red, Green, White and Blue. The old stat only use the Red and green though. Its heating and cooling with a heat pump. I programmed the new stat but noting worked. I called the Honeywell helpline and they said it's not compatible with a 2 wire system. However, I have two additional wires in the wall which are connected at the other end...