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Thermostat Wiring Information
Posted 1/3/2008

Thermostat Wiring Information

You want to purchase a new thermostat, but you're not sure where to start. Well, we understand your confusion and we're here to help. The first step is determining what type of thermostat would be compatible with your existing thermostat and the equipment it controls. Easier said than done, right? No worries, we've written this article to help you correctly identify your type of heating and cooling system. The main types of heating and cooling systems we'll be covering are single stage, heat pump and multistage.

Typically, the easiest way to determine thermostat compatibility is  to look at the wires present in your existing thermostat . Start by taking off your thermostat cover and counting the wires. The next step is determining which terminals are connected to the wires. If you're able to do this, then you're well on your way to finding a replacement thermostat that will suitably fit your needs.

The color of the wires in your thermostat is meaningless. You're probably wondering how that's possible, but let us explain. Most of the time it's typical for installers to use certain color wires for certain functions (i.e., red wire usually goes to the "R" terminals), but that's not always the case. A wire is just a wire, and its color is not necessarily indicative of the function of the wire.

Single Stage or 1 Heat/1 Cool Thermostats

About 65 percent of heating and cooling systems are a conventional forced air system with a gas furnace and an air conditioner. If this applies to you, then the wiring inside your thermostat should look similar to the picture below.

Thermostats that accept the same 4 wires shown above are often called "1 Heat / 1 Cool" thermostats or "Single Stage" thermostats. These systems are essentially "on-or-off" systems with only one level of heating or cooling.

These wires have the following functions:

1)     Wire coming to the "RH," "RC," or "R" terminal (usually red): This wire is the source hot wire from the transformer on the heating or cooling equipment. The function of the R terminal is to energize the other terminals. It all depends on what mode the thermostat is in or whether the thermostat is calling for heating, cooling, and/or fan operations. If you see a RH and RC terminal, you may also notice a short jumper wire between them.  In most cases, the heating and cooling system share a single transformer. The only time this jumper wire would be removed is if the heating and cooling systems have their own transformer. For instance, if a central air conditioning system was added to an old home.

 

2)     Wire coming to the "G" terminal (usually green): This wire is the fan relay - when energized, it will turn on your system fan/blower.

 

3)     Wire coming to the "Y" terminal (usually yellow): This wire is the compressor relay for cooling. When energized, it will turn on your air conditioner.

 

4)     Wire coming to the "W" terminal (usually white): This wire is the heating relay. When energized, your heating system will start up.

You're quite lucky indeed if your thermostat wires are the same as those mentioned above. There are LOTS of possible replacement thermostats for single stage heating and cooling systems. It all depends on what features you're looking for in your new thermostat. Some of the options we have available are programmable vs. non-programmable, manual or auto-changeover, as well as many others.

Heat Pump and Multistage Thermostats

You've checked your thermostat and you have way more than the 4 wires mentioned in the single stage heating and cooling systems. First off, you'll be glad to know there's nothing strange going on with your thermostat. Your heating and cooling system is either a heat pump or multistage system. These types of heating systems require special thermostats capable of controlling their advanced functions. No cause for concern though, because we've got you covered!

One quick way to determine if your heating system has a heat pump is by checking the terminals "O," "B," or "O/B." If you have wires coming from these terminals, then your heating and cooling system is a heat pump. These terminals relate to the reversing valve that controls the flow of refrigerant in both heating and cooling operation. Also, if you have an "emergency" or auxiliary" heat function on your current thermostat - you've probably got a heat pump.

The picture below shows the wiring block from a thermostat that controls a heat pump with auxiliary and emergency heat, plus two stages of cooling.  You'll notice there are 8 wires making connections in this thermostat - quite a contrast from the 4 or 5 wires you'll find on a conventional single stage thermostat.


Let's go over the meaning of some of the terminals you see on the thermostat pictured above:

1)     Wire coming to the "RH," "RC," or "R" terminal (usually red): This wire is the source hot wire from the transformer on the heating or cooling equipment. The function of the R terminal is to energize the other terminals. It all depends on what mode the thermostat is in or whether the thermostat is calling for heating, cooling, and/or fan operations. If you see a RH and RC terminal, you may also notice a short jumper wire between them. In most cases, the heating and cooling system share a single transformer. The only time this jumper wire would be removed is if the heating and cooling systems have their own transformer. For instance, if a central air conditioning system was added to an old home.

2)     Wire coming to the "G" terminal (usually green):  This wire is the fan relay - when energized, it will turn on your system fan/blower.

3)     Wire coming to the "Y" terminal (usually yellow): This wire is the compressor relay. When energized, it will turn on the compressor. This terminal works in tandem with the "O/B" terminal, which energizes to control the reversing valve. If you have a heat pump, you know the compressor runs in the heating AND cooling season. 

4)     Wire coming to the "W2" terminal (usually white): This wire is the auxiliary heating relay. When the heat pump no longer heats efficiently, the thermostat will shut it down and energize this contact. In most cases, this turns on resistive electric heat strips to give the heating system the firepower it needs to hold a setpoint in very cold weather. Higher end systems might use this terminal to fire up a gas furnace in a dual fuel configuration. There are other components needed in a dual fuel situation, but we'll save that for later.

5)     Wire coming to the "E" terminal (could be any color): This is the emergency heat relay. It's more of an "on-demand" button the homeowner has the option of selecting. In many cases, there is a jumper wire installed between the "E" terminal and the "AUX" terminal for cases when there is no specific emergency heat relay on the furnace.

6)     Wire coming to the "C" terminal (this wire could be any color, but is usually black or blue): This wire is known as the "common" wire because it brings 24 VAC power to the thermostat. Many digital thermostats require a battery to work properly. The batteries provide power for a display backlight or the use of a remote sensor. It's nice to have the common wire present just in case your batteries die because reprogramming your digital thermostat can be a real pain.  

7)     Wire coming to the "O/B" terminal (usually orange or blue): This terminal is used to control the reversing valve on a heat pump. 

8)     Wire coming to the "Y2" terminal (could be any color): This is for the second stage of cooling if the cooling system is multistage. Anytime you see wiring connected to terminals like "Y2" or "W2" it is pretty safe to assume you're dealing with a multistage heating or cooling system.

Even though heat pump and multistage systems are more uncommon than single stage thermostats, ProThermostats.com has a variety of options for you to choose from. It all depends on what features you're looking for in your new thermostat. Some of the options we have available are programmable vs. non-programmable, manual or auto-changeover, as well as many others.

These "do-it-yourself" guidelines are provided as an overview and should not be your sole source of instruction for installing a thermostat. The guidelines included herein are just examples and may not apply to your specific heating system. Anytime you are working with electricity or HVAC systems you should consult a professional. You should always read and follow your manufacturer's directions before installing your thermostat. National Trade Supply, LLC cannot be held responsible for injuries or damages resulting from these instructions.