When you are troubleshooting an HVAC fault or have to replace a thermostat you might often get lost between all the wiring and their connections to the thermostat’s terminals. Especially the terminals marked “RC” and “RH” can be confusing. What are ”RC” and “RH” and what are the differences between the two?
“RC” and “RH” are terminals on an HVAC thermostat, each with a specific function. They are used with HVAC systems with two separate transformers – one for cooling and one for heating. The RC terminal supplies the 24-volt cooling power, and the RH terminal is the 24-volt heating power supply.
In this article, we’ll look at why most modern thermostats have RC and RH terminals and some (mostly older) thermostats have only one terminal which is marked “R” and how they work. We’ll also discuss how they have to be wired.
Basic functions of RC and RH terminals
Many homeowners want to replace a faulty HVAC thermostat themselves but are not electricians or technically trained people. They can sometimes get confused with the terminals marked “RC” and “RH.”
You will find that if your thermostat has an RC terminal it will have an RH terminal as well, and vice versa. The RC terminal passes power to the cooling circuit for the air conditioners in your home. The RH terminal passes power to the heating circuit when there is a call for heat.
Terminals and two transformers
Many homes have two different transformer 24-volt control systems – one for heating and another one for cooling. If your HVAC installation has two systems the regulating thermostat will have two separate terminals marked “RH” and “RC.”
The reason is that with separate terminals (RC and RH) it is possible to use only one thermostat to control both systems, while the thermostat is still in the position to keep the control circuits isolated from each other.
Terminals and only one transformer
If your HVAC system doesn’t have separate transformers for the heating and cooling circuits, but your thermostat has an RC terminal and a separate RH terminal, the two terminals can be jumped out. In other words, there will be a wire running between these two terminals.
And sometimes there are no RC or RH terminals but only one terminal marked with an “R.” This R terminal is the 24-volt hot terminal. The power originates at the 24-volt transformer and supplies power to the thermostat and then passes power to the device it is controlling. The R terminal will also pass power to all the other terminals except the C terminal which powers the thermostat.
Wire the RC and RH terminals correctly when installing a new thermostat
You will most probably only worry about the terminals and what the markings on them mean when you are either troubleshooting or replacing an existing thermostat.
Understand terminal designations
When you have to install a new thermostat that is comparable to the old one, it is usually easy to connect the wires to the new thermostat’s terminals. If the markings on both thermostats are the same, you just have to copy carefully from one to the other. To make it even simpler, you can label the wires with small tabs before disconnecting the old thermostat.
Marked wires and an understanding of what the terminal designations are on your old and your new thermostat will make it easy to connect the wires and terminals correctly.
Some thermostats only require two wires
But sometimes, especially when your old thermostat which you are replacing is very old, the new, modern thermostat’s markings can differ significantly from the old one. Some modern thermostats, for example, only need two wires.
If your new thermostat only requires two wires, you can usually just leave the other wires unconnected and the new thermostat will work without any problems. Consult the new thermostat’s manual to determine which two wires have to be connected to the two terminals.
When you have only one wire to connect
If you have only one wire (R-wire) to connect to your thermostat and the thermostat has both an RH (for powering the heating) and an RC (for powering the cooling) terminal you can connect the R wire to either RH or RC.
Terminals other than R, RC and RH
It is always good to know what the terminal indications, other than R, RC and RH mean. The other most commonly terminals found on thermostats are:
- G terminal – for the indoor blower fan relay
- Y terminal – for the cooling relay in the condensing unit for your air conditioner and heat pumps
- W terminal – controls the heating system
- C-terminal – necessary to power the thermostat as the thermostat needs a source of power to operate.
- Terminal – control the reversing valve in most brands of heat pumps
- B terminal – also control aspects of the reversing valve.
What happens if terminals are wired incorrectly
With all this said, you might wonder what happens if you wire a thermostat incorrectly. Potential consequences include electric shock, blowing a circuit breaker, and in severe cases damaging the thermostat unit, the electrical system or even the furnace or air conditioning units themselves.
Conclusion
When you understand the function of all the terminals on your thermostat you don’t have to get lost between wires and terminals when you are troubleshooting an HVAC fault or installing a thermostat. If you understand why your thermostat has only one R terminal or two terminals marked RC and RH the way you have to connect the wires to the terminals becomes much more logical.
The bottom line is that if there is only one R terminal on your thermostat your HVAC system uses only one transformer, or all the transformers are simultaneously used. When there are two wires and one is connected to the RH terminal and the other one to the RC terminal your home has a two-transformer system – one for cooling and one for heating. The RC terminal is the 24-volt cooling power supply, and the RH terminal is the 24-volt heating power supply.
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