Thermostat Terminal Designations Explanations - You are wiring a new thermostat or about to wire a new thermostat and you want to understand what the terminal designations are on your old and your new thermostat. Why is it such a mystery with these terminal designations? Once you get into seeing how this works it all becomes common sense. This is as long as whoever wired the thermostat originally followed the common sense code of designations.

A good example of this is I have opened thermostats and seen black used on the R terminal. Green used on the W terminal and red used on the Y terminal. Once you see the thermostat wire and the colors of thermostat wire that are available you will understand. As far as those unconventional wired thermostats I’ve seen, I can only think that the person was color blind. And that is giving them the benefit of the doubt. Now, on to the designations.

Thermostat Terminal Designations

Here we will explain the designations and where they go and what they control. We will also reference another article about the colors for the thermostat designations. We get tons of emails and comments about this subject so we thought we would try to help you out. Hopefully, you will gain a better understanding of how your thermostat works by learning these thermostat terminal designations.

Thermostat Terminal Designations Explanations

Terminal Strip in an Air Conditioner Air Handler
First we start off with the thermostat terminal board itself. This is hidden away behind the actual thermostat and on a small plate referred to as the sub-base of the thermostat. It has little letters on it that differ from thermostat to thermostat but the most common designations are: R or RH and RC, G, Y, W, and C.

For heat pumps there are typically O and B terminals also. There are others and we will explain these as we go along here. What are these mystery terminals and where do they go and what do they control?

The List

Thermostat Terminal Designations Explanations

Thermostat Terminal Designations Explanation
R Terminal

The R terminal is the 24 volt hot terminal. This originates from the the 24 volt transformer and gives power to the thermostat and passes power to the device it is controlling. For example – If you turn on the fan only switch on the thermostat a switch inside the thermostat closes completely the circuit and powering up the fan relay. So the 24 volt power would come from the transformer and passed to the G terminal which controls the fan relay. If your thermostat only has an R terminal (versus separate RH and RC terminals) then the R terminal will pass power to all the other terminals except the C terminal.

RC/RH TerminalsThe RC terminal is the terminal for 24-volt hot and passes power to the cooling circuit for air conditioners. If you have an RC terminal then you also have an RH terminal. The RH terminal passes 24-volt power to the heating circuit on a call for heat. If you have these terminals jumped out ( a wire that runs between these two terminals) then you have a single transformer. If you have separate red wires on these terminals then you have a two transformer systems – one for the heating system and one for the cooling system. See this page for an explanation of why your system would have two transformers.
G TerminalThe G terminal is for the indoor blower fan relay. This gets its 24 volt power from the R terminal and energizes the indoor fan on a call for the fan to run or a call for cooling. The fan is controlled differently for the heat.
 Y TerminalThe Y terminal is the terminal for the cooling relay in the condensing unit for your air conditioner (and heat pumps). When the thermostat calls for the air conditioner the contacts inside the thermostat close and Y gets the 24-volt power and passes it to the compressor contactor inside the condensing unit. This in turn, energizes the compressor and the condenser fan motor. The same thing happens for a heat with one other additional occurrence explained below. An addition to this terminal is a Y2 terminal for second stage cooling if the system is so equipped with 2 stage cooling.
 W TerminalThe W terminal controls the heating system. If you have a furnace or boiler then these appliances get there signal of 24 volt power from the W terminal in the thermostat.  With boilers it is a little different depending on the type of boiler you have. A hot water boiler is controlled by an aquastat which cycles the boiler on and off based on the boiler temperature. There are many other ways to control a boiler but typically on a call for heat the W terminal in the thermostat closes and gets its power from the R terminal. This power usually energizes a relay that turns on the circulator pump for boiler piping loop. The water circulates through the loop and you have heat. The boiler turns on and off based on the water temperature and not from the thermostat. Furnaces are turned on by the W terminal by energizing a relay. Gas furnaces will automatically turn the indoor fan on by another control in the furnace. An addition to the W terminal is the W2 terminal which is for second stage heating. Some systems and thermostats even have a W3 for third stage heating.
C TerminalThe C-terminal is the 24 volt common (as opposed to hot). This terminal is necessary to power thermostat as the thermostat needs a source of power to operate. This terminal is kind of like the neutral side of a 120-volt circuit. A basic circuit needs a source, a path, and a load. In this case, the source is the transformer, the load is the thermostat, and the path is the R terminal and the C terminal which provides the path back to the 24-volt transformer.
O TerminalThe O Terminal is the control for the reversing valve in most brands of heat pumps. When the heat pump thermostat calls for cooling then this terminal closes and energizes the reversing valve causing it to switch from a default position of heating to a position of cooling. This is done as a failsafe for heating which is considered more important than cooling. If the reversing valve or its solenoid would fail for whatever reason you would likely still have heat. Its safer to go without the air conditioner in most cases than it is to go without the heat.
B TerminalThe B terminal is also for the reversing valve except in this case it reverses from a default position of cooling to a position of heating inside the heat pump. This is usually on Rheem and Ruud heat pump systems.
 S1,S2 or T TerminalsThese terminals are used for an outside air temperature sensor. This type of set up is usually reserved for heat pumps but it can also be on an air conditioning thermostat also.

5-wire thermostat wiring

Thermostat Terminal Designations - Conclusion

There other terminal designations in different thermostats from different manufacturers and the terminals are usually designated to control various things in the HVAC system such as a humidifier or an electronic air cleaner. Always read the owners manual and take photos of how the current thermostat is wired before undoing the wiring. Additionally, always turn the power off before touching the wires. First for safety and secondly because you can cause harm to the system by touching the wrong wires together. Hope this basic list of thermostat wiring terminal designations helps you.

High Performance HVAC Air Conditioning and Heating Systems

Thermostat Terminal Designations