Your thermostat determines how often and how long your AC will run. It’s the brains of your home climate control. When it malfunctions, nothing will run properly.
The moment you take your thermostat off the wall to replace it, you’ll notice several wires. How do you know which wire goes where? What do all of these wires mean?
When you have finished reading this article you will understand thermostat wire colors and thermostat wire letters as well as how to wire thermostats.
Safety First
Before you begin any project, begin with safety. To rewire a thermostat, you’ll need shock-proof gloves, safety glasses, and insulated boots. If you do receive an electrical shock, your insulated boots will reduce the risk of receiving life-threatening injuries.
Also, read the owner’s manual for your thermostat before you begin.
Thermostat Wire Colors
Each wire color generally means the same thing regardless of your thermostat brand. So when you see a blue wire with a Honeywell thermostat, it will do the same thing that it does with Lennox or Ecobee thermostat. Here is a wire color code for starters.
- Blue: connects to the B terminal and links to the power source and runs the thermostat
- Black: connects to the B terminal, is interchangeable with the Blue wire, links to the power source, and connects to a heat pump
- White: connects to the W terminal and connects to the primary heating unit and the air handler or furnace
- Red: connects to the R, Rh, or RC terminal and relays 24-volt power to the HVAC unit
- Rc: connects to the RC terminal and to the cooling power supply
- Rh: connects to the Rh terminal and hooks up to the heating power supply
- Green: connects to the G terminal and connects to the fan relay so your furnace or air handler fan runs
- Yellow: connects to the Y terminal, connects to the air conditioner, runs from the air handler, and ends at the compressor
- Orange: connects to the O terminal, exists in homes with an air source heat pump, connects to the outdoor condenser
All thermostats will have at least two wires. The more sophisticated your thermostat, the more wires you will have. Thermostats come in two, three, four, five, and six-wiring systems.
Two-Wire Coded
The two-wire thermostat is the simplest of thermostats. It should have a red and a white wire. You most likely have a thermostat that controls just the heating in your home and not the air conditioning.
The white wire goes to your heating unit. The red wire is for just the thermostat. Two-wire thermostats are also known as heat-only thermostats.
Three-Wire Code
A three-wire digital thermostat works well for controlling a heating system that is run by a water heater or boiler. With these, the red or hot wire connects to the R terminal. The White wire connects to your heating unit and links to the W1 or W terminal.
There will also be a green wire that connects to a G terminal. It completes the circuit.
Four Wire Code
A battery-operated and digital thermostat will use a four-wire thermostat. The four-wire thermostat still only works primarily with heating. If you’re looking at a thermostat like the Nest or Ecobee, you will have a four-wire thermostat.
The red wire connects to the R terminal and keeps the thermostat running. The blue wire will complete the circuit to the C terminal. The white wire connects to the W terminal and is the link for the heating unit.
The green wire then connects to the fan that links to the G terminal.
Five Wire Code
An HVAC system that manages cooling, heating, and climate control in an entire building will use a five-wire thermostat. You need that many wires to regulate the heating and air conditioning settings. The extra wire in the five-wire allows you to control an additional system such as a heat pump or air conditioner.
The red wire, as always, is the hot wire that connects to the R terminal and is the power source wire. The white wire connects to the W terminal and is the connection to the heating system.
The yellow wire connects to your cooling system and links to the Y terminal. The green wire regulates your fan and connects to the G terminal.
The blue wire is your extra wire, and it can run the extra application. it connects to the C terminal. Sometimes your blue wire will be a black wire.
Six-Wire Code
The six-wire thermostat allows a user to set temperatures with different states of heating. The device will switch to a low-heat setting when it first kicks in. If the room isn’t warm enough, though, you can have a pre-set interval that turns the furnace back on and heats the room more.
If you have a heat pump, then you will need an even more complicated thermostat. Typical heat pumps require an eight-wire system.
The six-wire system has all of the same wires as a five-wire system with the addition of a brown wire. The brown wire connects to the W2 terminal and gives you that second stage of cooling or heating.
If you don’t have a brown wire, you will have an orange wire. The orange wire will connect to the O terminal and will trigger the reversing valve on a heat pump.
Some thermostats do not have standard wire color codes. The codes above give you the general codes that most thermostats follow. if you discover your thermostat has a different set of colors, consult your owner’s manual or call in a professional service technician who knows how to wire thermostats.
Color Confidence
Thermostat wire colors exist for a reason. Each one indicates what the wire does. Once you understand this, then you can better understand your thermostat and HVAC system as a whole.
You can also purchase a thermostat with more confidence knowing you have the one you need. Then you can contact a professional electrician or HVAC specialist to install your thermostat for you.
Do you need someone to fix your thermostat? If so, contact us.
