Smart Thermostat that works with 4 wires?

I have older type Heater. I only have 4 wires ( red, white, green, blue). I understand that new thermostats require 5 wires. So, which of the thermostats compatible with ST will work with four wires?

Thank you.

I just installed the Ecobee3 about three weeks ago. I only have four wires, but it comes with a converter that installs at the furnace. So you connect one end with 5 wires and the other with 4. I installed it myself in about 30 minutes. Very easy and works perfect.


You only need 2 if you only have heat (or AC) and the thermostat can be powered via battery. Check out the Ct100

1 Like

Yes, I only have Heat, no AC.

Does Ecobee works off of the battery? I am not sure I want to rely on the HC unit that is powered by the battery (I guess its my mental thing).

If you have 4 wires and are only controlling a heater then you can reuse the AC wire as the C wire that you would be missing otherwise and most any thermostat would work and you wouldn’t need batteries.

Check how the wires are hooked up. Like posted above you only need 4 for heat. Usually Red/power, Blue/common, green/fan, white/heat. Any thermostat will work with these 4.

Sounds good. I am leaning towards Zen thermostat, for its look and price.

I have a Nest (1st gen) wired red=power, white=heat, yellow=cool, green=fan, blue=24V common and it has been working fine for over 2 years now. So if you only have heating, no AC you should be fine without the yellow.

I also have another nest (1st gen) working with 4 wires: red=power, white=heat, yellow=cool, green=fan (without the blue=24V common) and it is working without problems.

No - Ecobee 3 does not have an option to use a battery.