Rh and Rc Thermostat Wires: Understanding Function, Differences, and Wiring Tips

The Rh and Rc wires are common thermostat terminals that power heating and cooling systems. For many homeowners, understanding how these two wires work, when to jumper them, and how to wire modern thermostats can prevent inefficient setups and false readings. This guide explains the purpose of Rh and Rc, the differences between them, and practical wiring tips for typical homes in the United States.

What Do Rh And Rc Do?

The Rh terminal generally provides power from a heating transformer to the thermostat. Rc, or cooling power, provides power from a separate cooling transformer to the thermostat. In systems with a single transformer that handles both heating and cooling, Rh and Rc are typically connected together with a jumper, so one power source runs both functions. When a separate transformer exists for heat and cool, Rh and Rc receive power independently. The thermostat uses these terminals to energize relays that control the furnace or air conditioner.

Key point: Rh and Rc are both sources of 24-volt AC power, but their presence or absence, and whether they are jumped, depends on the HVAC system configuration. Many modern thermostats are designed to work with either a single-transformer or a dual-transformer setup and can bridge Rh and Rc automatically or manually when needed.

When Do You Use Rh Or Rc?

Rh is designated for heating circuits, and Rc for cooling circuits. If a home has separate heating and cooling equipment each with its own 24-volt transformer, Rc and Rh are usually separate, and a jumper is not used. If there is a single transformer powering both heating and cooling, a jumper links Rh and Rc so the thermostat can send a call for heat or cool using the same power source. Some thermostats automatically bridge Rh and Rc, while others require a physical jumper installed on the thermostat base.

Understanding your system matters for efficiency and safety. If the thermostat lacks a jumper but your system requires one, the thermostat may not respond to calls for heat or cool. Conversely, forcing a jumper on a dual-transformer system can backfeed power and damage equipment. A quick diagnostic by a professional or a careful check of the furnace and air conditioner wiring diagrams helps confirm the correct configuration.

Two-Transformer Systems And Jumpers

In a dual-transformer setup, the heating system and cooling system each have their own 24-volt power supply. In this case, Rh and Rc are separate, and a jumper should generally not be used. This allows heat and cool calls to be powered independently, which can improve efficiency and reduce cross-talk between devices. Some older thermostats require a jumper even on dual-transformer systems, so refer to the thermostat’s installation manual before wiring.

In single-transformer systems, the thermostat draws power from a single 24-volt source. A jumper between Rh and Rc is usually installed (or the thermostat’s internal jumper is enabled) so both heating and cooling can be controlled with a single powered circuit. When replacing a thermostat, verify whether the current setup uses a jumper and whether the new thermostat needs to emulate that connection.

Practical tip: If you replace a thermostat and aren’t sure about jumper requirements, remove power from the HVAC system, inspect the existing thermostat base for a jumper, and consult the equipment’s service manual or a licensed HVAC technician before reconnecting power.

Common Wiring Scenarios And Color Codes

Wiring color codes can vary by manufacturer, but several standard conventions help with quick identification. The following are typical wire assignments for Rh, Rc, and related terminals:

• R: 24V power from the transformer. If a single transformer powers both heat and cool, this may be labeled R with a jumper to Rh or Rc.
• W or W1: Call for heat. Sometimes labeled W on the thermostat side of the control board.
• Y or Y1: Call for cooling. Connects to the compressor contactor in the outdoor unit.
• G: Fan control. Powers the blower relay in the furnace or air handler.
• O/B: Heat pump reversing valve control (not part of Rh/Rc, but common in heat pump configurations).

When wiring, ensure that the Rh and Rc terminals receive power from the correct transformer and that any required jumpers are correctly placed. If a thermostat supports additional equipment like humidifiers or dehumidifiers, there may be dedicated terminals for those loads as well. Always label wires before disconnecting and follow the thermostat’s wiring diagram exactly.

How To Diagnose And Wire Safely

Wiring a thermostat involves working with low-voltage power. Safety is essential: shut off power to the HVAC system at the service panel before handling any wires. Steps for a safe diagnosis and wiring assessment include:

• Verify the HVAC system type (single-transformer vs. dual-transformer) by checking the furnace and air conditioner control boards or the service manuals.
• Check for an existing jumper between Rh and Rc on the old thermostat or at the furnace control board; confirm whether the jumper is required for the new thermostat.
• Use a multimeter to confirm 24-volt AC between R and C if the thermostat has a common wire; many modern thermostats require a C-wire for power.
• Match each wire to its corresponding terminal on the new thermostat according to the manufacturer’s wiring diagram. Do not guess; miswiring can blow fuses or damage equipment.
• After wiring, restore power and test each function: heat, cool, fan, and any auxiliary equipment. Ensure the thermostat can call for heat and call for cool without cross-talk.

Note: If there is any uncertainty about transformer count or jumper requirements, consult a licensed HVAC technician. Incorrect wiring can void warranties or cause equipment failure.

Tips For Modern Smart Thermostats

Smart thermostats often simplify wiring but may add new requirements. Consider these recommendations:

• Many smart thermostats no longer require a jumper between Rh and Rc because they operate with a built-in internal bridging feature when a single transformer powers both heat and cool.
• Ensure a C-wire is available or provided with an external power adapter if the thermostat needs constant power. Without a C-wire, some thermostats may experience power outages or reset during operation.
• Smart thermostats may support O/B heat pump control, which improves performance on heat pump systems. Verify compatibility with your furnace and outdoor unit.
• When replacing equipment, review energy efficiency settings and schedule configurations to maximize comfort and savings.

In summary, Rh and Rc are central to how a thermostat powers heating and cooling. Understanding whether a jumper is needed hinges on whether the system uses one transformer or two. Proper identification of wires, adherence to the manufacturer’s wiring diagram, and careful testing ensure reliable operation and optimal HVAC efficiency.