Replacing Carrier Thermostat with Nest

Nest thermostat on wall set to 69 degrees

Thermostats are the critical control elements of home heating furnaces or air conditioners. HVAC devices mostly run on the concept of a thermostat. One of the key concepts of these devices that every DIYer should know is thermostat wiring.

This can be helpful if you want to check what is wrong with your thermostat. In most cases, you can solve problems without necessarily having to call an expert.

In this post, we will look at thermostat wiring and try to answer the question of wiring smart thermostats to old devices. But before we venture further to answer this question, let's start by looking at basic hermit wiring and what the wires mean.

Thermostat Wiring and What the Wires Mean

There are different thermostat designs, with each design having a different wiring principle. The most basic thermostat has two wires, while the most complex ones have up to 16 wires.

The most common thermostats range between two and five wires. Let us look at the common types of thermostats and what the wires mean.

wiring for a thermostat

Thermostat Wiring Color Codes

In some thermostats, you may have up to 16 sockets. The sockets have the letters C, R, W1, W2, O/B, G, Y1, Y2, BK, 2x RS, 2x ODT, AUX NO, AUX C, and AUX.

With 2-wire, 3-wire, and 4-wire thermostats, you will not have the 16 sockets. You will also notice that not all sockets take wires. On 5-wire to 8-wire thermostats, you will have many sockets that are not useable.

Thermostat Black Or Blue Wire (“C” Wire)

The C-wire, or the "common" wire, is the wire that is either colored black or blue on the thermostat. The C wire is usually connected to the transformer and completes the 24V electric circuit. Almost all thermostats have the C wire.

Thermostat Red Wire (“R” or “Rc” Wire)

The red wire, or R wire, is the direct live power line. This wire originates from the transformer and provides 24-hour AC power to the thermostat.

All types of air conditioners have red wire for the thermostat. You may find terminals labeled Rc or Rh for the dual transformer systems. In such a case, you may want to consider allowing an expert to take over.

Thermostat White Wire (“W1” or “W2” Wire)

The thermostat white wire terminals are either indicated as W1 or W2. These wires are for heating and are normally located in the gas furnace thermostat. They are not available in air conditioner thermostats.

Orange Wire For O And Dark Blue Wire For B (“O/B” Wire)

The orange wires, or in some cases dark blue, are also part of the thermostat. They are indicated as O on the terminals or may be indicated as B. In some thermostats, they are denoted as O/B. The orange wire usually goes to the condenser located in the outdoor unit of heat pumps.

Thermostat Green Wire (“G” Wire)

The green wire, denoted as the G terminal on the thermostat, is used to run the fan. The G wire regulates the power input that goes to the fan. As a result, the thermostat helps determine the speed at which the fan runs at given times.

Yellow Thermostat Wire (“Y1” And “Y2” Wire)

The yellow wire goes to the Y1 or Y2 terminal of your thermostat. This wire mostly wires back to the air handler. Y1 is for ordinary one-stage cooling.

Other Wires (BK, RS1, RS2, ODT1, ODT2, AUX NO, AUX C, AUX NC)

There are many other terminals you may encounter on your thermostat. Each of these terminals has its purpose. However, most f these auxiliary terminals are rarely used.

screw gun removing thermostat

Removing The Old Thermostat

If you are going to replace an old thermostat, make sure you are well-versed in removing it first.

Before you start removing the thermostat, make sure you first take a photo of how it is wired. Once you have the photo unscrew the wires and remove them from the thermostat. Make sure you have completely turned off the source of power before you start working.

2 Wire Thermostat Wiring (Furnaces)

When it comes to wiring 2-wire thermostats, you will have to locate the red and nad white wires. This type of wiring is an idea for furnaces only and does not apply to air conditioners.

Nest Thermostat 2 Wire Hookup: What You Need To Know

The Nest Thermostat is one of the modern digital thermostats that has found a high following among home users. Thanks to its digital display and other cool features, more homeowners have been trying to replace their regular thermostats with the Nest thermostat.

The systems that are compatible with Nest thermostats are either heat-only or cool-only systems. The system must be low voltage at the wiring. You must make sure you have this information before you start wiring.

Do not wire your 2-wire thermostat as a replacement for a 5-wire or any other type of system. If you do this, you will affect the functioning of your AC or furnace. It may lead to failure in some of the key functions of the system.