RC and RH are two thermostat wiring terminals that confuse homeowners fast. You see both labels on almost every thermostat, and it feels like you need an electrical degree to guess where each wire goes.
The truth is simple. RC and RH send power to different parts of your heating and air conditioning system. This guide explains what each terminal does, how the wiring works, and how you can tell which wire belongs where.
What Do RC and RH Mean on a Thermostat?
Most thermostats label their power terminals RC and RH. Each label points to a different part of your HVAC system. The thermostat uses both terminals to control heating and cooling based on your commands.
RC Terminal (Cooling Power)
The RC terminal stands for “Red Cooling.” The RC terminal sends 24-volt power to the cooling circuit. The wire comes from the cooling transformer inside your air conditioning system. If your AC will not start, the RC terminal and wire are the first things to check.
RH Terminal (Heating Power)
The RH terminal stands for “Red Heating.” The RH wire sends 24-volt power to the heating circuit. This wire comes from the heating transformer inside your furnace or air handler. If your heat does not run, the RH terminal is often the issue.
Does Your HVAC System Use One Transformer or Two?
This is the detail that decides if RC and RH stay separate or get linked with a jumper. Most homes use one transformer, but some older homes and heat pumps use two.
One-Transformer Systems (Most Homes)
Homes with one transformer send power to both heating and cooling through the same source. RC and RH act the same in this setup, so manufacturers connect them with a metal jumper. This is why you often see one red wire feeding both terminals.
Two-Transformer Systems
Some systems use a transformer for cooling and a second one for heating. These systems keep RC and RH separate.
If you connect them together, you create a short that blows the low-voltage fuse or damages the control board. This is where many wiring mistakes happen, especially during DIY thermostat wiring.
What Color Wires Go to RC and RH?
Most homes follow the same color pattern, but you should still confirm your wiring at the furnace panel. Color does not guarantee function, especially in older homes.
RC Wire Color
The RC wire is usually red. This wire brings cooling power from the cooling transformer into the thermostat.
RH Wire Color
The RH wire is also red. This wire brings heating power from the heating transformer. Because both wires are red, homeowners often mix them up.
Exceptions You Might See
Old wiring, spliced wiring, and heat pumps add confusion. Some installers use different colors when they run out of wire.
This is why you always confirm the wire’s origin at the furnace or air handler before you attach it to RC or RH.
Wiring RC and RH on a Nest Thermostat
A Nest thermostat uses a simplified wiring layout, but the logic does not change. The thermostat senses the type of system you have and adjusts the RC or RH rules on its own.
Single-Transformer Homes
If your home uses one transformer, you put the single red wire into either RC or RH on the Nest. The Nest handles the jumper logic internally. You do not need a metal jumper like on older thermostats.
Two-Transformer Homes
If your home uses two transformers, you insert the cooling power wire into R. You insert the heating power wire into RC or RH depending on your setup.
Many two-transformer setups need a Nest Power Connector to keep stable voltage.
Signs RC/RH Are Wired Wrong on a Nest
A wrong RC or RH connection shows up fast:
- Delayed message
- No cooling
- No heating
- Thermostat rebooting
- Warm air in cool mode
These signs point to a power supply issue in the heating or cooling circuit.
RC and RH Troubleshooting: Simple Steps
Wiring mistakes trigger predictable symptoms. Here are the common problems you will see.
No Cooling
A loose or miswired RC wire stops the AC contactor from receiving power. The air conditioning system will not start.
No Heating
A loose or miswired RH wire stops the heating transformer from sending power. The furnace or heat strips stay off.
Thermostat Says “No Power”
A blown low-voltage fuse, a triggered float switch, or a wire on the wrong terminal triggers this message. The thermostat cannot detect stable voltage.
System Short Cycling
A missing jumper in a single-transformer home causes fast on-off cycles. Cross-wired R terminals in a dual-transformer home cause the same behavior. Both cases stress the HVAC system and shorten equipment life.
When You Should Call a Professional
You call a technician when the wiring includes dual transformers, heat pumps with backup heat, blown low-voltage fuses, or wires with unknown paths. These setups can damage a control board or transformer fast.
Frequently Asked Questions About RC and RH
Do I need both RC and RH wires?
You need both if your home uses two transformers. You need one if your home uses one transformer.
What happens if RC and RH are swapped?
Swapping them in a two-transformer system can blow a fuse or damage the board.
Why are RC and RH sometimes connected with a jumper?
The jumper joins them when the system uses one transformer.
How do I know if my home uses one transformer or two?
Check the furnace panel. If you see one transformer feeding both circuits, you have a single-transformer setup.
What color wire goes to RC or RH?
Both wires are usually red, but always verify the origin at the furnace.
Can I put the red wire in R instead of RC or RH?
Yes. Many new thermostats combine RC and RH into a single R terminal.
How do RC and RH work on a Nest thermostat?
The Nest identifies system type and manages jumper logic or separate circuits automatically.
Need Help With Thermostat Wiring in Fort Worth? Call Comfort Experts.
If you want help with thermostat wiring, Nest installation, or HVAC troubleshooting, our team can handle everything for you.
We serve Fort Worth and nearby areas with same-day service and clear answers. Comfort Experts keeps wiring simple.
