Thermostat Cable Basics

It is (hopefully) in every building we walk into. We don’t give it any thought, yet its absence will most likely lead to climate discomfort. A thermostat cable is used regularly in the installation of building climate control systems. The number of thermostat cables and their length per system depends on the type and complexity of the system.

What is a Thermostat?

A thermostat is the control device that allows for simple user interface with the internal workings of a climate control system. By using an adjustable set-point, the thermostat’s purpose is to turn on/off the heating / cooling system.

Thermostat Types

There is a variety of thermostat types. Needless to say the older designs lack the newer ones’ tech aspects. Since we can still find them in older homes, let’s take a look at them as well.

Manual older styles include the mercury style with numbers and dials and the square with a dial on the bottom. Both of these older styles are no longer being installed in new projects. They are, however, still in existence in older homes.

Non Programmable thermostats are a step up from the manual mercury type. They usually include an LED display panel showing the temperature. Changing the selected temperature is done on the up/down buttons to the side of the screen.

Programmable thermostats allow you to program different set temperatures for different times of the day. Some allow to pre-set it for upcoming days. Some models allow for up to four different temperature settings in a 24 span.

Wireless Thermostats are the new guy on the street. They connect to your smart phone / tablet, and allow you to control the temperature in your home regardless of your proximity to it. You may turn the HVAC unit when you are on your way home so it is cooled/heated to your liking once you arrive.

Thermostat Cable

Thermostat cable

Regardless of the type of thermostat, they are all wired with the same concept in mind. Let’s look at a single stage heating cooling system. At the unit is a terminal block labeled with letters: R – W – Y – G – C with a screw next to each letter to attach a wire. The thermostat unit has the same letters as the terminal block, so to connect the wires by the colors (R to R, W to W, etc.).

R – Red terminal, traditionally used to power the thermostat
W – The White terminal is for heating. The wire goes directly to the heating source (furnace).
Y – Yellow terminal is used for cooling / air conditioning. It goes to the compressor relay.
G – The Green terminal is used for the fan relay to power the indoor blower fan.
C – The Common terminal does not have a universal color associated with it. This terminal connects to the transformer to complete 24 volt power circuit in the thermostat.