1. The 10 terminals
The backplate in ecobee’s ecobee3 lite installation guide has two rows. The left row is Rc, G, Y1, Y2 and O/B. The right row is Rh, C, W1, W2 and PEK. There are no accessory (ACC) terminals, which is the main wiring difference from the ecobee Premium.
- Rc and Rh: 24 V power. One R wire goes on Rc. Two R wires stay on Rc and Rh separately.
- C: the common wire that powers the thermostat. See our C wire terminal page.
- G: the fan.
- Y1 and Y2: cooling, or the compressor on a heat pump, first and second stage.
- W1 and W2: heating, first and second stage. On a heat pump these become auxiliary heat.
- O/B: the heat pump reversing valve.
- PEK: used only with the Power Extender Kit.
The guide rates the thermostat for 24 VAC at 2 A maximum and says not to connect it to line voltage or millivolt systems. If your old thermostat is marked L1 and L2, or 110, 120 or 240 VAC, the ecobee3 lite isn’t compatible.
2. The diagrams as one table
ecobee’s ecobee3 lite install page links four wiring diagrams. This table reads across three of them. Each cell is the wire that lands on that terminal, named by its old label. Second-stage wires (Y2, W2) apply only if your equipment has them; the diagrams draw them as optional.
| Terminal | Furnace and AC | Furnace with PEK | Heat pump |
|---|---|---|---|
| Rc | R | R | R |
| C | C | G wire | C |
| G | G | Empty | G |
| Y1 | Y or Y1 | Empty | Y or Y1 (compressor) |
| Y2 | Y2 | Empty | Y2 (compressor stage 2) |
| W1 | W or W1 | W or W1 | Aux heat stage 1 |
| W2 | W2 | Empty | Aux heat stage 2 |
| O/B | Empty | Empty | O/B (reversing valve) |
| PEK | Empty | Y wire | Empty |
The PEK diagram shows a basic four-wire system (R, G, Y, W) only, so second-stage and O/B terminals are left empty there. The fourth diagram covers a boiler or radiant system with an air handler and separate cooling or heating; follow it directly if that is your setup.
3. Wiring with a C wire
These steps follow Guide 1 in ecobee’s booklet.
- Turn off power to the HVAC system at the switch or breaker. Confirm by calling for heat or cooling and checking that no air comes from the vents after a few minutes.
- Remove the old cover and photograph the wires on their terminals.
- Label and disconnect the wires one at a time with the supplied labels. Leave any jumper between R terminals on the old thermostat; label only wires that come out of the wall.
- Mount the backplate, with the trim plate if you need to cover old holes. ecobee specifies a 3/16 inch bit for the anchors.
- Insert the R wire first. One R wire goes on Rc. If you have both Rc and Rh wires, put each on its own terminal.
- Insert the rest into the side, not the front, of the matching terminals. The lever drops when the wire is held. Tug each one gently.
- Push the extra wire back and cover the wall hole, since drafts affect the temperature reading.
- Press the thermostat on until it clicks, restore power and follow the setup on screen.
Not sure your spare wire is really a C wire? Our multimeter guide shows how to check for 24 V between R and C.
4. No C wire: the Power Extender Kit
The ecobee3 lite ships with a Power Extender Kit. The guide’s checkpoint says it needs four wires (W or W1, Y or Y1, G and R) or three wires (Y or Y1, G and R). ecobee’s PEK support article adds that it isn’t compatible with heat-only or dual-transformer systems. With the power off:
- Open the furnace or air handler and photograph the control board.
- Label the R, Y (or Y1), G and W (or W1) wires from the thermostat cable. If a terminal also holds wires going to the outdoor AC unit, leave those alone and label only the thermostat wires.
- Check for two transformers. The guide says wires on both Rc and Rh at the control board may mean a two-transformer system, and to call ecobee support before going on.
- Move the labeled wires from the board to the matching gray terminals on the PEK.
- Connect the PEK’s five white wires to the board’s R, C, G, Y and W terminals.
- Mount the PEK without straining the wires and close the cover. Some systems won’t run with the panel loose.
- At the thermostat, connect:
- R wire to Rc
- G wire to C
- Y wire to PEK
- W wire to W1 (if you have one)
- Restore power and follow the setup on screen.
The ecobee3 lite labels the terminal PEK. Newer ecobee models call it PEK+, and the moves are the same. If the kit won’t suit your system, our no C wire guide compares the other options, and the 4-wire no C wire setup shows the same case for other brands.
5. Heat pumps: O/B and aux heat
ecobee’s heat pump diagram covers air-source and geothermal heat pumps with auxiliary heat. It shows:
- Y1 to the compressor, with Y2 as an optional second compressor stage.
- O/B to the reversing valve at both the heat pump and the air handler.
- W1 to the air handler’s first auxiliary heat stage, with W2 as an optional second stage. A wire your old thermostat labeled AUX therefore goes on W1, not W2.
- R and C shared by the heat pump and air handler, with R on Rc.
The diagram doesn’t say whether your reversing valve is energized in cooling (O) or heating (B); that is set during setup. Our O/B terminal page explains how to tell, and the heat pump wiring guide covers E wires and other labels. The guide’s PEK checkpoint lists only R, Y, G and W, so if you have a heat pump with no C wire, check your system in ecobee’s compatibility checker before buying.
6. When the ecobee3 lite is the wrong fit
- Line voltage or millivolt heat: not supported. See our line voltage thermostat guide.
- A third heating or cooling stage (W3, Y3): the backplate stops at two of each.
- Humidifier, dehumidifier or ventilator wired to the thermostat: there are no ACC terminals.
- Two wires, heat only, no C wire: the PEK doesn’t apply. Look at running a new cable or a model with its own solution.
7. If it won’t turn on
ecobee’s install page and guide list four checks:
- Every wire is fully inserted in its terminal at the thermostat. Tug each one.
- The HVAC cover panel is closed properly.
- A single R wire is in Rc, not Rh.
- If you used the PEK, R is on Rc, G is on C and Y is on PEK.
If the screen stays blank after that, check the furnace fuse and door switch with our troubleshooting checklist, and see thermostat wire not detected if setup misses a wire.
8. Frequently asked questions
Where does a single R wire go on the ecobee3 lite?
On Rc, whatever the old terminal was called: R, Rc or Rh. ecobee’s guide says not to jumper Rc and Rh, because the thermostat handles that itself. If your old thermostat has a metal jumper between R terminals, leave it on the old thermostat. See our Rh and Rc terminal page.
Which wires move when I use the PEK on an ecobee3 lite?
At the thermostat, R goes on Rc, the G wire goes on C, the Y wire goes on PEK, and W goes on W1. The G and Y1 terminals stay empty. At the furnace, the PEK sits between the thermostat cable and the control board. Our ecobee PEK wiring guide walks through the furnace side.
Does the ecobee3 lite PEK work with only three wires?
The ecobee3 lite guide says the kit works with four wires (W or W1, Y or Y1, G and R) or with three wires (Y or Y1, G and R). If you are missing any of those, ecobee asks you to contact its support. Two-wire heat-only systems are not covered by the PEK.
Where does the aux heat wire go on an ecobee3 lite with a heat pump?
In ecobee’s heat pump diagram for the ecobee3 lite, the thermostat’s W1 runs to the air handler’s heat strips, and W2 is drawn as an optional second stage. So a first-stage aux wire goes on W1, not W2. The reversing valve wire goes on O/B. See aux heat vs emergency heat.
Can the ecobee3 lite control a humidifier?
Not through dedicated terminals. The ecobee3 lite backplate has no ACC+ or ACC− terminals, unlike the ecobee Smart Thermostat Premium. If you need humidifier or dehumidifier wiring at the thermostat, compare our ecobee Premium wiring diagram.