Once, I tried to change the thermostat in my home, but got confused by wires that looked identical in color and had no idea what their purpose was or why they were there. So, I thought of contacting a technician, but my family member suggested that I research it, and I ended up changing the thermostat myself.
These two wires often confuse homeowners because they both look the same & both are generally red in color, carry 24-volt power, and sit together adjacent to each other on a terminal board, which is why most people get confused.
But both these wires have absolutely different destinations, which makes it very important for you to understand which one among them is called RC and which one is called RH. RC stands for Red cooling and is responsible for carrying the power from your AC to thermostat, whereas RH stands for red heating and carries power from your furnace/heating system to thermostat. One feeds your cooling system, and the other feeds your heating system. Both have the same color, same voltage, but completely different jobs. Let us understand these wires one by one and why they are called RC and RH.
What is the RC wire on a thermostat?
Among these two, one is called RC and supplies power to your cooling systems, such as your AC. This wire is fitted in the cooling transformer inside your AC. The main purpose of this wire is to signal your thermostat that the AC is powered and ready. The moment you press the cold button on your thermostat, it sends a signal to our AC compressor via the Y wire, telling it to start. The power is constantly sitting in your AC transformer waiting to be used, and the moment you press the button, it travels via RC wires to the thermostat.
No RC means no instruction, so AC remains switched off.
What is an RH wire, and what is its purpose?
Just like RC wire is used for cooling, RH wire is used for heating devices in your home. The RH wire works with devices such as furnaces. Your furnace also has power sitting there, waiting to be used, which travels via the RH wire to the thermostat the moment you press the button.
No RH power means the furnace can not start. It’s not broken; it’s just silent because no instruction has reached or been provided to the thermostat, so it sits silent.
How do RC and RH wires work?
Now you know what RC and RH wires are. Let’s find out how they work with an easy explanation, step by step:
Step 1: Power is always waiting
Our HVAC systems have transformers, sometimes one and sometimes two. This transformer is always generating power close to 24 volts constantly. This power is always waiting at terminals to be used. If it travels through the RC wire to the thermostat, it is for cooling; if it travels through the RH wire,, it is for heating.
Step 2: Button pressed by you
You need to turn on your AC or Furnace for, which you press the button on the thermostat. Now the thermostat needs to send instructions to either the AC or the furnace.
Step 3: Thermostat utilizes the power
The thermostat now uses that sitting power to send the signal. RC power is picked up by the thermostat to generate and send cooling and RH power signals for heating.
Step 4:Signals travels through different wires for cooling and heating.
- Signal sent for cooling travels through the Y wire.
- The signal for heating travels through W wire.
Step 5: AC or Furnace switches on
The AC or furnace receives and accepts the signal received from the wires sent by thermostat. The air starts blowing, and your AC or furnace is working now.
Short Summary: The RC and RH wires supply 24 volts to the thermostat. The thermostat then uses that power to send instructions via wires W and Y to your cooling and heating devices. The thermostat is helpless without the RC and RH power; it has nothing to do.
How much power do RC and RH wires carry, and are they dangerous?
Both wires carry the same r, which is not dangerous and will not even give you an electric shock, but it is still advisable to switch off the power before engaging in any activity with these wires.
Difference between RC and RH wires
At first glance, these wires look very hard to differentiate from each other because both are identical and both carry a very negligible amount of risk, as the power they carry is just 24 volts, which is not good enough to be called risky, but what separates them from each other is their destination. RC wire transfers instructions to cooling systems and RH to heating systems. Color is the same, and so is the power, but the destination is different.
What are the signs/indications that your RC and RH are not working properly?
Signs of RC wire not working properly
- No cooling on a hot afternoon:
- Even after pressing the button, your AC does not throw any sort of cool air
- Sometimes not starting and throwing any kind of air, but our thermostat shows the right temperature. This is one of the common signs that an RC wire isn’t working and deserves your attention.
Signs your RH wire is not working properly
- Furnace not responding on a cold day:
- When you have done everything right and pressed the button as required to switch on your. furnace, but it is still not responding.
- The house stays cold even after switching on the furnace
Thermostat screen shows no power.
If your thermostat is blank and not showing anything on the screens, it is not receiving any signal from either RC or RH. Potential causes of the thermostat not showing power:
- A wire may have come loose from its terminal over time
- A wire may have been inserted in the wrong terminal during the time of installation
- A wiring short cause blown low-voltage fuse
System turning on and off rapidly.
If your heating and cooling devices switch off rapidly, it indicates a short circuit. This is called short cycling. In a home with only one transformer, this indicates a missing jumper; in a home with two transformers, it means the RC and RH wires are not properly connected.
How to fix the RC and RH wire issue in a simple and easy way?
Whenever you encounter these situations or symptoms, which can indicate that your RC and RH wires are not working properly, you should always turn off the system and check that each wire is properly connected. Most of the time, the only issue is the wire, which has loosened up over time and just needs 30 seconds of attention to be fixed. We end up paying technician fees when we skip reviewing the wires and go straight to a call. This small rule can save service call fees. But if the issue is still unresolved, the problem lies deeper, and a call to the technician is warranted.
Why do I use RC and RH on a Nest Thermostat?
First, we need to understand what the nest actually does. The Nest simplifies handling RC and RH wires, which can look more complicated in homes with older thermostats that rely on jumpers. If you forgot a step, for example, connecting a jumper between the RC and RH wire without any explanation, your system stops working.
The Nest is comparatively easier and smarter than the old thermostat. It helps you guide and makes things less complicated. The main difference is that in homes with a single transformer and an old thermostat, you always need a jumper between the RC and RH wires, but with a Nest, it’s not required because the Nest handles it internally. No jumper is required , no extra steps, and nothing to forget.
How can you tell if your home has one transformer or two?
- Before moving to the next installation, this is the question a homeowner should find out.
How to find out:
- Look at your old thermostat before removing it
- If it has one red wire going into a single R terminal, then understand your home has one transformer
- If you see two red wires, one connecting to RC and one to RH, and no jumper, then it means you have two transformers
- This difference tells you everything you need to know before jumping to any conclusion.
Why does this question matter?
This question matters because in one transformer home, RC and RH must be joined; if they are not, your cooling and heating systems will never receive power.
In two transformer systems, RC and RH cannot be joined together, as doing so could cause a short circuit that could destroy your system in seconds.
This is the exact reason why every thermostat brand, whether it is Honeywell, ecobee, or nest will ask you these questions first before anything else.
RC and RH on Honeywell
Honeywell is one of the most popular brands among customers right now, used at home. It works differently from Nest, which operates and manages the RC and RH wires internally. Honeywell uses a physical slider tab on the thermostat’s body. Think of the slider tab as a switch that tells your thermostat whether it is dealing with one transformer or two.
Slide up: One wire position.
Slide down: two-wire position.
RC and RH on an Ecobee Thermostat
Ecobee is another brand that takes a very different approach to its thermostat. It keeps both RC and RH as two separate terminals on all premium models in the market. No internal joining, no slider tab, just two separate terminals.
Rule of C wire on Ecobee
Ecobee needs a C wire to function properly. In a dual-transformer system, the C wire must come from the cooling wire bundle, not the heating wire bundle. In homes with single transformers where no C wire is available, Ecobee includes a Power Extender Kit.
Wire Manual
Wire and Function
- C wire(Common wire) : 24-volt power to run the thermostat
- Y wire :Cooling instruction to AC compressor
- W wire : Instruction to Heating systems, such as a furnace
Conclusion
These two wires are not complex to understand, but they are very easy once you have the right information. Most people sit at home for years without knowing how RC and RH work. Learning the functions of these helps you avoid irrational expenses and saves your time by avoiding service calls and waiting for a technician to come, only to find out it was something you could have fixed yourself. Your home is your comfortable place. It’s a very simple story; you just need the right explanation.
Frequently Asked Questions
Difference between RC and RH wire?
RC wire is used to send cooling instructions, whereas RH wire is used to send heating instructions from the thermostat.
Do I need both RC and RH wires at my home?
No, only when your home has two transformers, one for heating and one for cooling. In homes with single transformers, the single red wire handles both functions.
What is the fusion of the Honeywell slider tab?
The slider tab has a built-in jumper switch that is used to differentiate between one or two transformer systems. If the slider is up, it joins RC and RH internally in homes with a single transformer. Slide down separates them for two transformer homes.
My AC and Furnace are not working. Could it be an RC and RH issue?
Yes, a loose wire or miswired RC and RH means the thermostat is receiving no instruction, and therefore it is unable to send any instruction to your AC or furnace. So, you can start exploring the issue by visiting your RC and RH wire with due process.
