Thermostat Wiring Diagrams: Every Setup from 2 to 8 Wires

1. How to read a thermostat wiring diagram

Nearly every central heating and cooling system in the United States is controlled by a 24 V AC low-voltage circuit. A transformer in the furnace, air handler or boiler supplies 24 V on the R terminal. The thermostat is a set of switches: when it wants heat, it connects R to W; for cooling, R to Y; for the fan, R to G. Relays and valves in the equipment do the heavy work. That is why thermostat cable is thin and why a thermostat swap is a reasonable DIY job.

A thermostat wiring diagram therefore has three parts:

• The thermostat on the wall, with lettered terminals (R, W, Y, G, C and so on).
• The indoor unit (furnace, air handler or boiler), where the same letters appear on a terminal strip on the control board. This is the “thermostat wiring at the furnace” end.
• The outdoor unit (AC condenser or heat pump), connected to the indoor board with a separate, smaller cable.

Read every diagram the same way: follow the letter, not the color. The W wire at the thermostat lands on W at the furnace, Y on Y, and so on. Colors are a convention installers usually follow, but they are not required by code, and houses with replaced or extended wiring often break the pattern.

2. Thermostat wire color code

This is the standard thermostat wire color code used by most US installers, with the exceptions that cause the most wiring mistakes. Use it to understand a diagram, but connect wires by the letter they came off.

Two letters deserve extra care. B means the reversing valve on Rheem and Ruud heat pumps but the common wire on many Trane and American Standard systems. X varies even more by brand. If you can’t tell, look up the old thermostat’s manual or check where the wire lands at the indoor unit. The full chart with photos is in our thermostat wire colors guide.

3. Thermostat wiring diagram: gas furnace + central AC

This is the most common system in American homes: a gas furnace indoors and a split air conditioner outside. With a C wire it uses five wires; without one it uses four.

What each wire does in this system

• R brings 24 V from the furnace transformer up to the thermostat.
• W starts the heating sequence: the furnace runs its inducer, proves the flame and, after a short delay, its own blower.
• Y calls for cooling. At the furnace board, the thermostat’s Y wire and the outdoor unit’s Y wire share the Y terminal, so 24 V reaches the condenser’s contactor, which starts the compressor and outdoor fan.
• G runs the indoor blower for cooling and for “fan on”.
• C completes the circuit for anything powered, including the contactor coil outside and a smart thermostat’s electronics.

Thermostat wiring at the furnace control board

Open the furnace’s lower (blower) door, which presses a safety switch, so power must already be off. On the control board you’ll find a row of screw or push-in terminals marked R, C, W, Y and G (sometimes W1, Y1, Y/Y2, DHUM or HUM too). Look for:

• Two wires under Y and C. One goes up to the thermostat, the other out to the condenser. That is normal.
• A spare conductor folded back along the cable jacket. If the thermostat end is also unused, it can become your C wire: connect it to C at both ends.
• The blade fuse, usually 3 A (violet) or 5 A (tan). If it is blown, something shorted. Replace it with the same rating, never a larger one.

Setup-specific diagrams with each thermostat’s terminals: 5-wire with a C wire and 4-wire, no C wire.

4. Thermostat wiring diagrams by number of wires

The quickest way to identify your system is to count the wires connected to the old thermostat. Don’t count a short jumper between two terminals, and don’t count spares folded back into the wall.

2-wire thermostat wiring (R and W)

Two wires mean heat only: no cooling, and no fan control from the thermostat. You will find this on hot-water and steam boilers, older gravity or heat-only furnaces, and zone valves. The thermostat closes R to W and the boiler or furnace does the rest.

A smart thermostat needs constant power, which two wires can’t provide on their own. Your options are a new cable with a C wire, a plug-in 24 V AC adapter (as long as the thermostat accepts it), or a thermostat designed for 2-wire heat. Before buying anything, rule out two lookalikes:

• Thick wires and wire nuts on a baseboard heater are line voltage (120/240 V), not 24 V.
• A gas fireplace or wall furnace with a standing pilot is often a millivolt system that a 24 V smart thermostat can’t use.

Diagram and compatibility: 2-wire heat-only wiring.

3-wire thermostat wiring

Three wires come in three versions, so read the letters:

• R, W, G: heat and fan, no AC. Common on furnaces in mild climates. See 3-wire heat and fan.
• R, W, C: heat only, but with a C wire, so a smart thermostat has power.
• R, Y, G (or R, Y, G plus C): cooling only, with heat provided by something else. See cooling-only wiring.

On some heat pumps, three wires plus a fourth unused one are a sign the installer ran a cable but wired only the basics. Check the air handler before assuming.

4-wire thermostat wiring (R, W, Y, G)

The most common thermostat wiring in the US: heat, cooling and fan, but no C wire. Old mechanical and battery-powered thermostats didn’t need one. For a smart thermostat, check in this order:

1. Look for a spare wire in the wall and at the furnace. Many 4-wire installs use 18/5 or 18/6 cable with one or two unused conductors.
2. Use the thermostat’s own solution: ecobee’s Power Extender Kit, a Honeywell wire saver (Y moves to K), Amazon’s C-wire adapter, or Nest’s ability to run without C on many systems (with the Nest Power Connector as a fix if power is unstable).
3. Run a new cable (18/5 or larger) from the thermostat to the furnace. This is the most reliable option.

Full diagrams: 4-wire thermostat wiring with no C wire and the no C wire guide.

5-wire thermostat wiring (R, W, Y, G, C)

Four wires plus a common: the setup every smart thermostat is designed for. The diagram is exactly Figure 1. If your old thermostat had five wires but one was never connected, it is probably a spare, not C. A true C wire is connected to C at the furnace board.

6-wire thermostat wiring

Six wires usually mean one of three systems:

• Heat pump with auxiliary heat: R, C, Y, G, O/B and AUX (or W2). See heat pump with C wire.
• Two-stage furnace or two-stage AC: R, C, W, Y, G plus W2 or Y2. See two-stage heat and cool.
• Dual transformer: Rh and Rc as separate wires plus W, Y, G and C. See Rh and Rc dual transformer.

The letters tell them apart instantly. Our 6-wire thermostat guide walks through each.

7-wire and 8-wire thermostat wiring

Seven or eight wires are almost always a heat pump with extra stages: R, C, Y, G and O/B, plus some of Y2 (second compressor stage), AUX/W2 (backup heat), E (emergency heat), or a dehumidify or humidifier terminal. Some thermostats on these systems also have an L or S terminal for a fault light or outdoor sensor. Make sure the thermostat you buy supports every function you use: the Amazon Smart Thermostat, for example, handles only a single-stage heat pump with one aux stage. See heat pump with emergency heat.

5. Heat pump thermostat wiring diagram

A heat pump heats and cools with the same compressor, so its wiring differs from a furnace in two important ways: Y runs the compressor in both modes, and an O/B wire switches the reversing valve that decides which way the heat moves. Backup heat, usually electric heat strips in the air handler, is a separate stage.

O or B: choose the right setting

The wire goes on the O/B terminal either way. If the house gets cold air when heat is on (after allowing for the compressor’s delay and defrost cycles), flip the O/B setting. Rheem and Ruud specifics: Rheem/Ruud heat pump B wire.

AUX vs. E

AUX (also W2, W1 on ecobee in heat pump mode, or X2 on older thermostats) adds backup heat automatically when the heat pump can’t keep up or during defrost. E is emergency heat: it locks out the compressor and runs backup heat only, for when the outdoor unit is broken. Many systems have no separate E wire; the thermostat then uses the AUX wire for emergency heat too. The details, including why aux heat runs too much, are in our heat pump and dual-fuel wiring guide and aux heat vs. emergency heat.

6. Boilers, zone valves and other systems

Boiler thermostat wiring

Most hot-water boilers use a simple 2-wire circuit. On a single-zone boiler the thermostat wires land on terminals often marked T T or TT (or R and W) on the boiler control. On zoned systems, each thermostat wires to a zone valve or to a zone control panel (such as a Taco or Honeywell zone panel), which then fires the boiler. A 24 V C wire is usually available at the zone panel or transformer, even if it isn’t run to the thermostat.

Some older zone valves use three wires to the thermostat and need a matching thermostat or an isolating relay. Check the valve’s manual before replacing that kind of thermostat.

Line-voltage heat (120 or 240 V)

Electric baseboard and wall heaters are controlled by a thermostat that switches the full heater current. Those wires are thick, joined with wire nuts, and can kill. A 24 V smart thermostat must never be connected to them. Use a line-voltage smart thermostat instead. See line-voltage thermostats.

Communicating systems

Premium variable-speed systems such as Carrier Infinity, Bryant Evolution, Lennox iComfort and Trane ComfortLink use a digital connection on terminals marked A, B, C, D (or 1, 2, 3, 4). They don’t follow the diagrams in this guide. A standard thermostat works only if the equipment is set up for conventional 24 V control.

7. Old thermostat to new: terminal conversion

Smart thermostats rename a few terminals. W becomes W1 on ecobee, Y becomes Y1, and O and B merge into one O/B connector. This chart shows where each old label goes on the four most popular smart thermostats in the US. These names come from each manufacturer’s installation guide.

The Rh–Rc jumper

Many old thermostats have a short wire or metal tab linking Rh and Rc. That is a jumper, not a wire from the wall. Don’t move it to the new thermostat: smart thermostats link Rh and Rc internally when only one R wire is connected. Only two separate wires from the wall (one on Rh, one on Rc) mean a dual-transformer system.

For any combination not listed, tick your old labels in the wiring tool. It checks the result against each thermostat’s published wiring rules and draws the diagram.

8. Honeywell, Nest, ecobee and Amazon thermostat wiring

Honeywell thermostat wiring

Honeywell and Resideo (Honeywell Home) thermostats use classic labels: R, Rc, W, Y, G and C on conventional systems, with O/B, W2/Aux, E, Y2 and on some models L/A (fault light) for heat pumps. Two Honeywell-specific terminals cause the most questions:

• K is a combined fan-and-cooling signal used by the Honeywell wire saver (THP9045). One wire on K carries both calls, and the module at the furnace splits it back into G and Y, which frees the old G wire to become a C wire. See K terminal and wire saver wiring.
• R and Rc with a jumper or slider: older Honeywell models use a metal jumper or a switch (R/Rc “jumpered” vs. separate). Leave it in the jumpered position unless you have two transformers.

Model guide: Honeywell Home T9 wiring.

Nest thermostat wiring

The Nest Learning Thermostat (4th gen) has push-in connectors labelled Rh, Rc, C, W, W2, Y, Y2, G, *OB, AUX and E (the exact set varies by Nest model). A single R wire can go in either Rh or Rc. Nest can often run without a C wire by drawing small amounts of power through the heating or cooling wires. If that causes problems (a clicking system or a thermostat that loses power), Google’s Nest Power Connector or a C wire fixes it. See Nest Learning (4th gen) wiring.

ecobee thermostat wiring

ecobee thermostats use Rc, Rh, C, W1, W2, Y1, Y2, G and O/B, plus PEK for the Power Extender Kit and, on the Premium, ACC+ and ACC− for a humidifier or dehumidifier. A single R wire goes on Rc. On a heat pump, the W1 terminal becomes AUX for backup heat. Without a C wire, ecobee’s Power Extender Kit installs at the furnace and connects to the PEK terminal. See ecobee PEK wiring and ecobee Premium wiring.

Amazon Smart Thermostat wiring

The Amazon Smart Thermostat keeps things simple: R, C, W, Y, G and O/B, plus K for its C-wire adapter. It supports one stage of heat and cooling or a single-stage heat pump with one aux stage, so it is not a fit for two-stage, dual-transformer or emergency-heat wiring. Without a C wire, Amazon’s adapter is required. See Amazon Smart Thermostat wiring.

9. How to wire a thermostat, step by step

This sequence works for any 24 V thermostat. Allow 30 to 60 minutes.

1. Check compatibility first. Pull the old faceplate off and note every label that has a wire. Enter them in the wiring tool to confirm your new thermostat supports your system before you disconnect anything.
2. Turn off the power at the furnace or air handler service switch or breaker. Set the old thermostat to heat and confirm nothing turns on.
3. Photograph the wiring from straight in front, so every letter and wire is readable.
4. Label each wire with the letter it is connected to, using the stickers in the box or tape. If two wires share one terminal, label both.
5. Disconnect the wires and remove the old base. Wrap the cable around a pencil so it can’t fall into the wall.
6. Check the bare ends. Each needs about 1/4 to 3/8 inch of straight, clean copper. Trim and re-strip any that are bent or corroded.
7. Mount the new base, leveling it if the model needs it, and patch or cover old screw holes with a trim plate.
8. Connect each wire to its matching terminal using the conversion table above. Tug each one gently to make sure it holds.
9. Attach the display, restore power and follow the setup prompts. Answer the equipment questions carefully: system type, heat pump or furnace, O or B, stages, and whether the fan is controlled by the furnace (gas) or the thermostat (electric).
10. Test every mode. Run heat, cool and fan on their own. Expect a few minutes of delay before the compressor starts.

Detailed walk-through with photos: how to replace a thermostat, then settings to check after installing and the best everyday thermostat settings.

10. Thermostat wire: gauge and conductor count

Thermostat cable is named by gauge and number of conductors: 18/5 means five 18-gauge solid copper wires in one jacket. 18 AWG is the standard for residential runs; 20 AWG is found in older or bargain cable and works for short runs.

When you run new cable, buy at least one more conductor than you need today. A spare costs pennies and saves another pull through the wall later. Rules for running new wire, and why you should never share the cable with line-voltage wiring, are in the thermostat wire gauge guide.

11. Thermostat wiring mistakes and how to fix them

For deeper checks with a meter, see how to test thermostat wires with a multimeter, thermostat wire not detected and thermostat clicking but AC not turning on. Our troubleshooting checklist covers the rest.

12. Frequently asked questions