2-Wire Thermostat Wiring Diagram

2-Wire Thermostat Wiring Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable 2 wire thermostat wiring diagram: download the diagram as SVG or open it and print to paper or PDF.

A 2-wire thermostat wiring diagram uses only the R (power/hot) and W (heat call) terminals to control a single-stage heating system such as a gas furnace, boiler, or electric heater with no cooling or fan control.

A 2-wire thermostat is the simplest thermostat wiring configuration and is used exclusively for heating-only applications. It connects just two conductors between the thermostat and the heating appliance: R (or Rh) and W.

TERMINAL FUNCTIONS: - R (or Rh): This is the 24 V AC power feed from the furnace or boiler transformer. It is often called the hot wire. In 4-wire and 5-wire systems, Rc and Rh are separate feeds for cooling and heating; in a 2-wire system there is a single R terminal used for heating. - W (or W1): This is the heat call wire. When the thermostat calls for heat, its internal contacts close and complete the circuit between R and W. This 24 V AC signal energises the gas valve relay, ignition board, or heat contactor in the furnace or boiler control board, starting the heating cycle.

There is no C (common) wire in a basic 2-wire system. The circuit is completed through the furnace control board internally — the thermostat acts as a simple switch in the 24 V AC control circuit loop.

APPLICATIONS: Two-wire thermostats are found on: - Older gas forced-air furnaces (heat only, no air conditioning) - Hot water and steam boilers without zone valves - Electric baseboard heaters wired for low-voltage control - Millivolt heating systems (certain gas fireplaces and gravity furnaces use a self-generating millivolt circuit — these also use only two wires but operate at millivolt levels, not 24 V AC, and require a thermostat rated for millivolt service)

MODERN SMART THERMOSTAT COMPATIBILITY: Most Wi-Fi smart thermostats require a C (common) wire for their continuous power supply. A 2-wire system without a C wire will not power a modern smart thermostat without either running a new wire, using an add-a-wire adapter, or using a power-stealing thermostat model rated for 2-wire installations.

Wire colour convention (not universal): R is typically red; W is typically white. Always verify against the appliance's wiring label before connecting.

Wiring a 2-Wire Thermostat to a Heat Pump: The O/B Reversing Valve Terminal

Heat pumps require a dedicated reversing-valve wire that a standard heating-only 2-wire thermostat does not provide. If you are upgrading from a 2-wire system on a heat pump, you will need to run additional conductors. The reversing valve is a 4-way solenoid valve that switches refrigerant flow direction between heating and cooling modes. Most heat pump brands — Carrier, Trane, Lennox, Goodman, York, and Daikin — energise the reversing valve in cooling mode and wire to the O terminal. Rheem and Ruud energise the valve in heating mode and wire to the B terminal. Using O when the system needs B (or vice versa) causes the system to heat when you call for cooling and cool when you call for heating. Always check the equipment label on the outdoor unit before configuring or replacing a thermostat.

Wiring a Thermostat to a Central AC Unit: Y to the Contactor, C for Power

For a split-system central AC (air handler plus outdoor condensing unit) the minimum wire set is four conductors: R (24VAC hot from the control transformer), Y (compressor), G (indoor fan/blower relay), and C (common return). When the thermostat calls for cooling it closes R to Y, sending 24VAC to the contactor coil in the outdoor unit. The contactor closes its high-voltage contacts and starts the compressor and outdoor fan. Simultaneously R closes to G, energising the air handler blower relay. The C wire completes the 24VAC circuit back to the transformer's secondary winding and is required by most modern digital thermostats. Without C, some thermostats attempt power-stealing through the G or Y wire, which can cause nuisance blower operation or interfere with the control board. For a heat-plus-cool system add a W wire (white) for the furnace gas valve or heat strips — five wires total.

Rc vs Rh: The Jumper Question and the Risk of Two Transformers

Rc (R-cooling) and Rh (R-heating) are both 24VAC hot terminals but may originate from different transformers. In the most common configuration — a single-transformer system such as a heat pump or gas-furnace-plus-AC combination with one shared transformer — Rc and Rh carry identical voltage and are bridged by a factory jumper wire in the thermostat sub-base. Only one red wire is needed in the cable. In a dual-transformer system, such as an older oil boiler paired with a central AC unit, each appliance has its own 24VAC transformer. The two transformers are not phase-synchronised. If you install a jumper between Rc and Rh on a dual-transformer system both low-voltage fuses blow immediately, the thermostat loses power, and both heating and cooling stop. Before installing any new thermostat, count the transformers: if only one unit in your system has a transformer, use the jumper; if both the furnace/boiler and the air handler have their own transformer, keep Rc and Rh wired separately with no jumper. Nest thermostats handle this electronically — never insert an external jumper wire into a Nest sub-base connector.

How to wire 2 wire thermostat wiring diagram

Switch off power to the furnace or boiler Locate the furnace disconnect switch or circuit breaker and switch it off. Some older furnaces also have a separate 24 V transformer — confirm power is off at the control board with a multimeter before touching any wiring.
Remove the old thermostat and photograph the existing wiring Take a clear photograph of the existing thermostat base with wires labelled. Note the terminal they connect to and the wire colour. This is your reference for reconnecting correctly.
Identify the R and W terminals at the thermostat base Most thermostat bases have terminal labels moulded or printed next to each screw. Confirm which screw is R (or Rh) and which is W (or W1). If a bare wire labelled C, Y, or G is present but not connected, it is spare — leave it unused.
Connect the wires to the new thermostat Connect the wire previously on R to the new thermostat's R terminal, and the wire previously on W to the W terminal. Strip wire ends to 10–12 mm if needed. Ensure no copper strands touch adjacent terminals.
Secure the thermostat base and mount the thermostat Anchor the thermostat base to the wall — use a level to ensure it is horizontal, as some mechanical thermostat types use mercury switches that require level mounting. Snap or screw the thermostat body onto the base.
Restore power and configure the thermostat Switch the furnace back on. Set the thermostat to a temperature above the current room temperature to call for heat. The furnace should start within one to two minutes. Confirm the heating system fires and that the thermostat stops the furnace when set below room temperature.

Specifications

Control circuit voltage	24 V AC (most systems); 250–750 mV DC (millivolt systems)
Transformer output (typical furnace)	24 V AC, 40 VA or 75 VA
Thermostat cable size	18 AWG (0.75 mm²), 2-conductor
Maximum thermostat cable run (recommended)	Up to 30 m (100 ft) on 18 AWG without significant voltage drop
Heat call signal	Closed contact between R and W; ~24 V AC drops to near 0 V when closed
Terminals used	R (or Rh) and W (or W1) only
System type	Single-stage heating only (no cooling, no fan control)

Safety warnings

Even though thermostat wiring is low-voltage (24 V AC), the furnace itself operates at mains voltage (120 V or 240 V). Always switch off the furnace circuit breaker and confirm power is off at the control board before opening the furnace or touching any wiring.
Gas furnaces and boilers involve combustion systems. If you smell gas at any point during the work, do not proceed — evacuate the premises and contact the gas utility. Do not operate electrical switches.
Millivolt systems look similar to 24 V systems but are electrically different. Connecting a 24 V thermostat to a millivolt system will not damage the thermostat, but it will not function. Verify the system type before purchasing a replacement thermostat.
Do not short R and W at the furnace control board as a test method unless directed by the appliance service manual. Sustained shorting without a load can damage some control boards.
Ensure any new thermostat cable is routed away from mains voltage wiring to avoid induced interference. Many building codes require thermostat wire to maintain separation from line voltage conductors.

Tools needed

Flathead and Phillips screwdrivers
Digital multimeter (AC voltage function)
Non-contact voltage tester
Wire strippers (suitable for 18–20 AWG)
Spirit level (for mounting thermostat base level)
Drill and wall plugs (if mounting to masonry)

Common mistakes

Connecting both wires to R terminals (two power wires) — one wire must go to R and one to W. Doubled R connections will not call for heat.
Using thermostat cable not rated for the temperature range — in attics or near heat sources, use cable with appropriate temperature rating.
Purchasing a smart thermostat that requires a C wire without verifying C wire availability — results in a thermostat that fails to power up or loses Wi-Fi connectivity due to insufficient power.
Reversing R and W — the system will typically not operate. Some smart thermostats will display an error; older mechanical thermostats may simply not respond.
Leaving stranded wire ends not fully inserted into terminals — a partial connection creates intermittent heat calls, causing the furnace to short-cycle.
Configuring O when the system needs B (or vice versa) — always check the outdoor unit data plate for the brand before setting the reversing-valve polarity in the thermostat menu.
Installing a jumper between Rc and Rh on a dual-transformer system, immediately blowing both low-voltage fuses.
Omitting the C wire when installing a smart or digital thermostat, causing intermittent power-stealing that interferes with the air handler control board.
Connecting the Y wire to the outdoor condensing unit but forgetting to connect it at the air handler control board, preventing the compressor contactor from receiving the 24VAC signal.

Troubleshooting

Furnace does not start when thermostat calls for heat: Cause: No 24 V AC present at R terminal, open circuit in thermostat wire, or thermostat contacts not closing. Fix: Measure voltage between R and W at the thermostat base (expect ~24 V with contacts open). If voltage present, the thermostat is not closing — replace it. If no voltage, trace back to the furnace transformer and check its output and any protective fuses on the control board.
Furnace runs continuously and will not stop: Cause: R and W are permanently bridged — shorted thermostat contacts, touching wires in the cable run, or a control board relay stuck closed. Fix: Remove both wires from the thermostat body. If the furnace stops, the thermostat is faulty. If it keeps running, the control board relay is stuck — disconnect power and inspect or replace the board.
Thermostat display does not power on: Cause: Smart thermostat requires a C wire for continuous power; 2-wire system does not provide one. Fix: Confirm the thermostat model's minimum wiring requirements. If C wire is needed, install an add-a-wire adapter at the furnace control board, or replace with a thermostat compatible with 2-wire systems.
Heat pump heats when set to cool and cools when set to heat: Cause: The O/B reversing-valve terminal is configured for the wrong polarity — set to O on a Rheem/Ruud system that needs B, or set to B on a system that needs O. Fix: Access the thermostat's installer menu and change the O/B setting to match the brand. Rheem and Ruud require B (valve energised in heating); all other common brands require O (valve energised in cooling). Confirm the brand on the outdoor unit data plate.
Both heating and cooling stopped working after replacing thermostat, no power to thermostat: Cause: A jumper was installed between Rc and Rh on a dual-transformer system, shorting the two transformers and blowing both 3A or 5A low-voltage fuses. Fix: Locate and replace both blown fuses on the furnace/air handler control boards. Remove the Rc-to-Rh jumper. Wire Rc and Rh separately to their respective transformer outputs and do not add a jumper.
AC compressor does not start when thermostat calls for cooling: Cause: Y wire is disconnected, the C wire is absent causing the thermostat to reset, or the outdoor contactor coil has failed. Fix: Verify Y wire is secured at both the thermostat and air handler control board terminals. Check that the C wire is connected and providing 24VAC to the thermostat. Use a multimeter to confirm 24VAC appears between R and C at the thermostat sub-base when the cooling call is active.

Frequently asked questions

Can I install a smart thermostat with only 2 wires?

Some smart thermostats support 2-wire installations by power-stealing a small current from the R-W circuit to charge an internal battery. Check the thermostat manufacturer's compatibility list for your specific heating system. Alternatively, install an add-a-wire adapter at the furnace control board to derive a C wire from existing wiring.

What is the voltage between R and W in a 2-wire thermostat system?

Approximately 24 V AC when the thermostat contacts are open (no heat call). When the thermostat closes the R-W circuit, the voltage drops to near 0 V because the circuit is completed and current flows through the furnace control board's heat relay coil. This is the expected behaviour — it is not a fault.

My thermostat has R and W but also a G terminal — is this still a 2-wire system?

No, G is the fan control terminal. If G is wired, you have at least a 3-wire system. A 2-wire system connects only R and W. However, if G exists on the thermostat body but no wire is connected to it, the system remains functionally 2-wire and the fan runs under furnace control only.

What is a millivolt thermostat and how does it differ from a 24 V 2-wire thermostat?

A millivolt system uses a thermopile (a self-powered thermocouple stack) to generate a low DC voltage, typically 250–750 mV. The thermostat acts as a millivolt switch, not a 24 V switch. Using a standard 24 V thermostat on a millivolt system will not work — use only a thermostat rated for millivolt service.

Why does my furnace run continuously and not respond to the thermostat?

If the furnace runs without stopping, the R and W terminals are permanently shorted — the thermostat contacts are stuck closed, or the two wires are touching somewhere in the cable run. Remove both wires from the thermostat and check if the furnace stops. If it does, the thermostat is failed. If it keeps running, the fault is in the furnace control board.

How do I wire a 2-wire thermostat to a heat pump?

A standard 2-wire thermostat (R and W only) does not support a heat pump because heat pumps require at minimum R, C, Y, G, and O or B wires for proper operation — five conductors. You will need to replace the thermostat wire with an 18 AWG multi-conductor cable and use a heat-pump compatible thermostat. A 2-wire setup can only be used on a simple gas or oil heating system with no cooling.

What is the O/B terminal on a heat pump thermostat and which brands use B instead of O?

The O/B terminal controls the reversing valve solenoid that switches the heat pump between heating and cooling modes. Most brands — Carrier, Trane, Lennox, Goodman, York, Daikin — energise the valve in cooling mode, so wire to O. Rheem and Ruud energise the valve in heating mode and use the B terminal. Configure the thermostat menu to match your brand, as a wrong setting causes the system to heat and cool in reverse.

Do I need an Rc to Rh jumper on my thermostat?

Yes, if your system has a single transformer. Most homes with a gas furnace plus central AC, or a heat pump, use one transformer and the Rc and Rh terminals must be bridged so both heating and cooling share the same 24VAC power source. No, if your system has separate transformers for heating and cooling — bridging them will blow both low-voltage fuses and shut down the system. If only one red wire came off the old thermostat, you almost certainly have a single-transformer system.

What happens if I connect the Y wire to the wrong terminal when wiring a thermostat to an AC unit?

The Y wire energises the outdoor contactor coil. If Y is left unconnected the compressor will not start even if the fan runs. If Y is accidentally wired to G instead, the outdoor compressor may run without the indoor blower, causing the evaporator coil to freeze. Always verify terminal connections match the labelling on the air handler control board before restoring power.