Wiring guide
4-Wire Thermostat Wiring (No C Wire)
R, W, Y and G is the classic furnace + central AC setup found in millions of US homes, one wire short of what most smart thermostats want. The fix is usually an adapter at the furnace, not new wire.
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (4th gen) | Works: direct swap | No C wire: Nest usually runs on power sharing. If it loses Wi-Fi or its battery drains, add the Nest Power Connector. |
| ecobee Premium | Works with an adapter | No C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler. |
| ecobee Enhanced | Works with an adapter | No C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler. |
| Honeywell T9 | Works with an adapter | No C wire: install a Honeywell wire saver at the furnace. It frees up a wire to become C, with the Y wire moving to K. |
| Amazon Smart Thermostat | Works with an adapter | No C wire: use Amazon’s C-wire power adapter. Per Amazon’s guide, the G wire moves to C and the Y wire moves to K. |
Where each wire goes, thermostat by thermostat
| Old wire | Nest Learning (4th gen) | ecobee Premium | ecobee Enhanced | Honeywell T9 | Amazon Smart Thermostat |
|---|---|---|---|---|---|
| R (red) | Rc or Rh | Rc | Rc | R | R |
| W (white) | W | W1 | W1 | W | W |
| Y (yellow) | Y | PEK | PEK | K | K |
| G (green) | G | C | C | C | C |
Colors are typical, not a rule: always go by the letter the wire was connected to. Click a thermostat for its full diagram, parts and app settings.
How to confirm this is your setup
Four wires on R, W, Y and G: one furnace, one single-stage AC, a fan, and no C wire. This is the most common setup in North American homes built before smart thermostats. Check the cable for a fifth conductor folded back inside the wall first, because that solves the problem for free.
Mistakes to avoid
- Installing a thermostat that needs a C wire without its power adapter, then seeing it reboot or drain its battery.
- Putting an adapter in but leaving the old wire on the old terminal at the furnace. Each adapter moves at least one wire at the furnace board, so follow its diagram exactly.
- Jumping R to C to “make power”. That shorts the transformer and usually blows the furnace’s fuse.
Our advice
Each brand solves this differently. ecobee ships a Power Extender Kit, Honeywell uses a wire saver (the K terminal), Amazon sells a C-wire adapter, and the Nest Learning Thermostat can often run by drawing small amounts of power. The table below shows what each one needs.