RH Thermostat Wire: Definition, Uses, and Wiring Tips
Learn what RH thermostat wire is, how it carries the heating signal, gauge considerations, color coding, and practical tips for DIY installation and troubleshooting.
RH thermostat wire is a 24-volt control conductor in a two-wire heating circuit that delivers the heating call from the thermostat to the furnace control board. It is a type of thermostat wiring used for heat in many residential systems.
Understanding RH Thermostat Wire
RH thermostat wire is the heating circuit conductor that carries the 24-volt call from the thermostat to the furnace when heat is requested. In many traditional setups, this dedicated leg translates a thermostat action into a burner or heat exchanger response. According to Thermostat Care Team, this wire is the essential link that ensures reliable operation and quick, safe calls for heat. When the thermostat closes the circuit, the furnace receives power and begins heating. For DIY homeowners, recognizing the RH wire and keeping it isolated from other circuits reduces the risk of short circuits or accidental energization. While modern thermostats may combine functions on a single R terminal, older systems often use a dedicated RH leg. The key is to ensure the wire is properly connected to the R or RH terminal, and that the corresponding W or heat call is connected correctly on the furnace board. Practically, you want a clean, labeled run from the thermostat to the equipment, with no splices or damaged insulation. Safety first means disconnecting power at the breaker before inspecting wiring, and using a voltage tester to confirm that the line is de-energized before touching any conductors.
Color Codes and Common Gauges
Thermostat wiring uses a color and gauge convention that helps you identify wires quickly. In many systems, Red is the primary power wire that feeds both RH and RC terminals. White usually controls heat (W), Green for fan (G), Yellow for cooling (Y), and Blue or Common (C) for the common side of the 24-volt transformer. When you see RH, Rc, or R, make sure you know whether your thermostat requires a jumper between R and RH or RC. If your thermostat is already bridged, you may not need separate RH and RC wires. Common gauges for thermostat cable range from 18 to 22 American Wire Gauge, with 18 AWG offering a robust balance of durability and flexibility in most residential runs. If you need to replace a damaged segment, match the gauge and insulation rating, and keep the conductor count consistent with your thermostat. Proper labeling at both ends reduces confusion during future maintenance.
Questions & Answers
What does RH stand for in thermostat wiring?
RH stands for heating power on the thermostat wiring. It is the hot side of the R circuit that supplies the 24 volt signal to the furnace for heat calls.
RH stands for heating power in thermostat wiring, the hot side that carries the 24 volt signal to the furnace for heat calls.
Can I use the RH wire for other functions?
Do not repurpose RH as a miscellaneous conductor. Use the designated terminal for heat calls and avoid crossing or tying lines that could energize the system unexpectedly.
RH is a heat call conductor. Do not repurpose it for other functions.
What gauge should RH wire be?
Choose a wire gauge that matches the existing thermostat cable and the furnace control board rating. Common residential thermostats use 18 to 22 gauge wires.
Common gauges are 18 to 22 gauge for thermostat wiring.
Is it safe to DIY thermostat wiring?
DIY wiring is possible for simple replacements if you are confident and follow safety steps. Always disconnect power first, and verify there is no voltage before touching any conductors; consult a professional if you are unsure.
DIY wiring can be safe if you follow safety steps and disconnect power first.
What should I do if RH and RC are bridged on my thermostat?
Many thermostats bridge RH and RC internally. If bridging is required for your model, ensure the jumper is correctly installed; if not, remove the jumper as directed by the manufacturer.
Check if your thermostat requires a jumper between RH and RC and install or remove as directed.
Where can I find the wiring diagram for my furnace?
Refer to your furnace or air handler’s installation manual or the control board labeling. If you cannot locate it, contact the manufacturer or a licensed HVAC technician for guidance.
Check the equipment manual or label on the furnace control board for wiring diagrams.
What to Remember
- Close the RH circuit only when heating is commanded
- Match wire gauge and insulation to the existing cable
- Label wires at both ends for future maintenance
- Power off before inspecting or wiring
- Consult a professional if unsure about safety or codes