What is Thermostat Rh Wire and How It Works

Explore what Rh wire does in thermostat wiring, how it differs from Rc and R, and safe steps for identification and wiring. Practical, homeowner friendly guidance from Thermostat Care to keep heating systems reliable.

Thermostat Care Team

March 2, 2026·5 min read

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Rh wire

Rh wire is the heating power feed in the thermostat R circuit. It provides power to heating controls and is often paired with Rc when heat and cooling share a transformer.

What is thermostat rh wire

Rh wire is the heating power feed in a thermostat's R circuit, and, according to Thermostat Care, it is the wire that supplies the heat call to your furnace. In most residential systems, this wire originates from a 24 VAC transformer and travels to the thermostat through the Rh terminal. When the thermostat calls for heat, voltage travels back from Rh to the appropriate heat-control terminals, triggering the furnace. Some systems use a single R with a jumper to Rc, while others have separate Rh and Rc transformers. Understanding the Rh wire helps you diagnose heat problems, prevent accidental miswiring, and communicate clearly with a professional if you need help. Throughout this guide, we focus on practical, home friendly explanations and safe practices for wiring or troubleshooting Rh.

Distinguishing Rh from Rc and R

At a glance, R, Rh, and Rc are related but not identical. R generally refers to the power feed from the transformer to heat and cool circuits. Rh is the heating branch, Rc is the cooling branch. If you have a single transformer powering both heat and cool, most thermostats ship with a small jumper between Rh and Rc; this makes one R supply both functions. If your system uses separate transformers for heating and cooling, you should keep Rh and Rc separate and remove the jumper. Misusing or mislabeling these terminals can prevent heat from firing in winter or cooling in summer. In practice, label the wires or photograph the connections before you adjust anything, and consult your thermostat's manual to confirm which terminals match your equipment.

How to identify Rh wire at the thermostat and furnace

To identify Rh, start at the thermostat. Look at the terminal strip and note wires connected to the R terminal. If there are two distinct connectors labeled Rh and Rc, the Rh wire is the heat power feed. If there is only a single R terminal with a jumper to Rc, the red wire could be serving both functions. At the furnace or air handler, locate the control board. The R terminal on the board often labels Rh and Rc; the heat transformer wire should land on Rh. If your system uses separate transformers, you may see separate 24 VAC circuits with wires colored red for one and white or another color for the other. If in doubt, use a multimeter to verify the presence of approximately 24 VAC between R and W (heat), or call a pro. Always power down before touching anything.

Safe wiring steps for Rh

Before touching wires, shut off power at the furnace and thermostat; then verify no voltage with a tester. Remove the thermostat cover and gently pull the wires free to inspect their condition. Label wires or take a photo before disconnecting anything. Check whether a jumper between Rh and Rc exists and ensure you understand your system configuration. If you need to install or remove a jumper, use the correct tool and avoid bending terminals. When wiring a new thermostat, connect the Rh wire to the Rh terminal, and connect Rc only if your system has a separate cooling transformer. If your system uses a single transformer, leave the jumper in place and follow the thermostat manufacturer’s guidance. After changes, reattach the cover, restore power, and test by setting heat to verify the furnace engages. If heat does not respond, recheck connections and refer to the manual.

Common mistakes when dealing with Rh and Rc

Assuming red equals Rh in every setup; colors can vary and labeling is the truth.
Removing the Rh-Rc jumper without confirming your system’s transformer layout.
Bridging Rh and Rc when your air conditioner uses a separate 24 VAC source.
Powering on equipment while wires are loose or exposed.
Ignoring the thermostat manual and wiring diagrams that accompany your model.
Not testing for 24 VAC between R and the heat call when heat is not responding. Thermostat Care analysis shows that many heat issues stem from Rh misconfigurations or open wires rather than faulty thermostats.

If your thermostat shows heat calls but the furnace remains idle, start with a visual check of the Rh conductors and the R terminal on both devices. Confirm the jumper status if you have one and verify that the thermostat is set to heat and the mode is correct. Check for 24 VAC between R and W using a multimeter when the heat is commanded; if voltage is present but the furnace does not respond, there may be a problem with the furnace control board, blower, or transformer. If the heat works sporadically, inspect connections at both ends and consider replacing aging wires. Thermostat Care recommends keeping a simple wiring diagram in your utility space for quick reference and avoiding DIY repair when you’re unsure about transformer voltages or safety.

Tools you may need and safety considerations

A small Phillips or flat head screwdriver set to remove thermostat plates.
A non-contact voltage tester to confirm de-energized circuits before touching wires.
A digital multimeter to verify 24 VAC between R and W or Rh with the系统 powered down.
Wire strippers and electrical tape for tidy reconnections.
A printed wiring diagram for your thermostat and furnace.
Safety gear and a calm workspace. Thermostat Care emphasizes turning off power at the breaker and the furnace before handling wires, then testing carefully after reseating.

Quick reference guide for homeowners

Rh is the heating power feed on the R circuit.
Rc is the cooling power feed; a jumper between Rh and Rc is common on single-transformer systems.
If you have separate heat and cool transformers, keep Rh and Rc separate.
Always shut off power before touching thermostat wiring.
Use the thermostat’s manual to confirm terminal names and wire functions.
When in doubt, consult a professional to avoid damage to equipment.

Questions & Answers

What does Rh stand for on a thermostat?

Rh stands for the heating power feed in the thermostat R circuit. It supplies power to the heating system and is typically linked to the Rc terminal on single transformer setups.

Should Rh and Rc be jumpered?

If your system uses one transformer for both heat and cooling, a jumper between Rh and Rc is common. If you have separate transformers, keep Rh and Rc separate and do not bridge them.

How can I tell if my thermostat uses Rh?

Check the thermostat’s terminal labeling on the R block. If you see a separate Rh terminal, that wire is the heating power feed. If there is a single R with a jumper to Rc, the same red wire may serve both functions.

Can I wire Rh directly to Rc?

Only if your system is designed for it. For most homes with a single transformer, keep Rh and Rc bridged. If you have separate transformers, do not bridge them without following the manufacturer’s instructions.

What tools do I need to work with thermostat wiring?

A screwdriver, a non-contact voltage tester, and a digital multimeter are commonly used. Always power down at the furnace before touching wires and test for voltage before handling connections.

When should I call a professional for Rh wiring?

If you’re unsure about transformer voltage, see burning smells, or if the furnace won’t respond after wiring, stop and contact a licensed technician. Incorrect wiring can damage equipment or cause safety hazards.