Thermostat Wiring: What Every Letter and Colour Does

By Ray Bennett · Updated 2 October 2026 · 9 min read · Checked against the sources at the end

Thermostat Wiring: What Every Letter and Colour Does

Short answer

Thermostat wiring is 24 volts and a row of lettered screws. R is the power in, C is the return that lets the screen stay on, W calls heat, Y calls cooling, G calls the fan, and O or B is the heat pump changeover. The colours are a habit, not a rule, so photograph the old thermostat before a single wire comes off.

Thermostat wiring is five or six thin wires and a row of lettered screws, and nearly every job that goes wrong goes wrong the same two ways. The wires drop back inside the wall, or a colour gets trusted instead of a letter.

So before anything comes off the wall, photograph the old thermostat with every wire still screwed down.

Before you pull the old one off

What to carry: a small flat screwdriver, your phone and a strip of masking tape. Twenty minutes, and most of that is reading rather than working.

Turn the power off at the machine, not at the thermostat. On a gas furnace there is a switch on the side of the cabinet that looks like a light switch, and a breaker behind it. On an electric air handler it is the breaker alone. Twenty four volts will not hurt you, but touching the wrong two wires together blows a fuse on the control board and leaves the whole system dead, which then looks like a much bigger fault than it is.

Photograph it, then write it down as well. A photo of the terminal block, then a list on the masking tape: white on W, red on R, yellow on Y, and so on. Once the wires are off the screws, that list is the only truthful record you have.

Stop the cable escaping. Wrap the bundle round a pencil, or tape it flat to the wall, before you loosen a single screw. A cable that slips back inside the wall turns twenty minutes into a hole in the plaster.

Two systems where a normal thermostat does not belong. Thick wires joined with wire nuts rather than small screws means line voltage, 120 or 240 volts, feeding electric baseboard or an electric wall heater. A 24 volt thermostat wired onto that is destroyed in a second and is a fire risk. It needs a line voltage thermostat rated for the load.

Two thin wires and nothing else, on a gas wall furnace or a gas fireplace, is often a millivolt system. It makes its own power from the pilot flame, well under one volt, and there is no transformer and no C wire to find. Neither of these takes a smart thermostat.

What each letter means

The letter is the job, the colour is only a habit. Calling for heat simply joins R to W inside the thermostat.

Everything on an ordinary thermostat runs on 24 volts, made by a small transformer inside the furnace or the air handler. The thermostat heats nothing and cools nothing itself. It is a set of switches that join the 24 volt wire to one of the others, and the furnace does the rest.

Terminal	What it does	Usual colour
R, or Rh and Rc	24 volts in from the transformer. This is the wire everything else gets joined to	Red
C	Common, the return side of the transformer. It switches nothing. It is what lets a thermostat run on mains power instead of batteries	Blue or black
W, and W2	Calls heat. W2 is second stage on a two stage furnace	White
Y, and Y2	Calls the compressor, which means cooling	Yellow
G	Calls the indoor fan on its own	Green
O or B	The changeover valve on a heat pump. O is live in cooling, B is live in heating	Orange for O, often brown for B
Aux, and E	Backup heat and emergency heat on a heat pump	No habit worth trusting
L	A fault light fed back from a heat pump. Nothing is broken if it is empty	Brown or tan

The colours are a habit, not a rule

Nothing stops an installer pulling a cable and using whatever conductor comes to hand. On a house that has been re-wired once, colours often mean nothing at all.

Two catch people out more than the rest. Orange on a heat pump is usually O, the changeover valve, and it belongs nowhere near W. People see orange, read it as heat, land it on W and then wonder why the system blows cold in winter. Blue is usually C, but on some heat pumps it is B, which is the opposite end of the same job.

If your photograph and a colour chart disagree, the photograph wins every time.

Which wires your system actually uses

Three systems, three wire counts. A heat pump has no W for the compressor, which is the one that surprises people.

Heat only. A furnace or a boiler with no air conditioning uses R, W and usually G, with C if the installer ran one. Nothing outside to switch on, so there is no Y.

Furnace and air conditioning. The common five: R, W, Y, G and C. This is the usual set for a furnace with central air, and it is what a smart thermostat expects to find.

Heat pump. R, Y, G, C and then O or B, plus Aux or E if there are electric strips behind the coil. The compressor does the heating as well as the cooling, so Y runs in both seasons and there is no W for it. Getting O and B the wrong way round gives you cold air when you ask for heat and warm air when you ask for cooling, with nothing actually broken.

Wiring it, in order

Check 1

Fit the new base plate with the wires through it

Feed the bundle through the opening in the plate, level it, mark the screws and fix it. Stuff a scrap of insulation into the wall opening afterwards. Cold air creeping up inside the cable hole makes a thermostat read low and run the heat longer than it should.

Check 2

Land the wires by letter, from your own photo

Strip the length the new thermostat’s guide gives, at least a quarter inch on a Honeywell RTH6360 and a third to half an inch on a Nest, and put each wire on the terminal it came off at the old thermostat. Any conductor you are not using gets folded back and left in the wall opening, not cut off. It is the spare you will want later.

Give every wire a gentle pull when it is down. A strand that has not clamped properly works loose later and the fault looks random.

Check 3

Set the Rh and Rc link

If the new thermostat has both Rh and Rc, it wants to know whether one transformer feeds the whole system or two do.

One red wire coming out of the wall means one transformer, so the little metal jumper stays in, or the terminals are bridged in the settings. Two red wires, one from the furnace and one from a separate cooling unit, means the jumper comes out and stays out. Joining two transformers together is how both of them get killed.

Check 4Five minutes

Power on and call one thing at a time

Switch the breaker and the furnace switch back on and let the thermostat start up.

Ask for the fan on its own first, which is G, and you should hear the indoor blower and nothing else. Then ask for heat, which is W, and stand at the furnace for one full cycle. Then ask for cooling. On a heat pump in winter, run it once in cooling for a few seconds as well, just to hear the changeover valve clunk over.

Give a compressor five minutes between calls. Most thermostats hold it off deliberately to protect it, and people take that pause for a fault.

No C wire, and what to do about it

A plain battery thermostat never needed a C wire, so plenty of houses do not have one landed. Sensi says most Wi-Fi and touchscreen thermostats may need it, though a few run on batteries. Others, such as a Nest, borrow power through the heating and cooling wires instead, and Google lists a system that turns itself on and off, clicking, and a display that will not turn on among the signs that this is falling short.

There are three honest ways out of it, in the order worth trying.

1. Look for a spare conductor. Pull the cable gently at the furnace end. Cables are usually run with more conductors than the job needed, and there is often a blue or a brown folded back and taped at both ends. Land it on C at the control board and on C at the thermostat and the job is done in ten minutes for nothing.

2. Fit an adapter. A small box at the furnace end runs two signals down one wire and frees a conductor for C. Venstar, Ecobee and Honeywell all sell one, and it is a sensible answer when the cable genuinely has no spare.

3. Run a new cable. Eighteen gauge, eight conductor thermostat cable, and you never think about it again. Easy in a basement or a crawl space, not so easy in a finished two storey wall.

It is wired and nothing happens

A dead screen after a rewire is usually one of three things, and all three are quick.

The board fuse. There is a small blade fuse on the furnace control board, usually three or five amps, the same kind a car uses. Touching R against C or against any other wire blows it instantly. It is cheap, and a blown one is a clear brown or broken stripe through the window in the middle.

The furnace door. Most furnaces have a switch behind the blower panel that cuts everything when the door is off. The panel has to be properly clicked shut, not just leaned into place.

The breaker or the switch on the cabinet. Easy to leave off, and easy to knock off while you are working near it.

If the thermostat lights up and the fan runs but the burners never fire, that is not wiring any more, that is the furnace. Furnace blowing cold air sorts it by when the air goes cold, and a furnace that lights and dies after a few seconds is usually a dirty flame sensor rather than anything you just touched.

Questions people ask

How many wires do I need for a smart thermostat?

Five for a furnace with air conditioning: R, W, Y, G and C. Four is enough for heat only, and a heat pump wants six or seven once O or B and the backup heat are counted.

Can I just use the G wire as my C wire?

You can, and you lose the fan button, which means no fan for cooling on many systems. The adapter costs a little more and keeps everything working.

What if there is no wire on C at the thermostat but there is one at the furnace?

Then somebody landed it at one end and not the other, which is common. Put it on C at the thermostat and you are done.

Orange and dark blue are both there. Which is the changeover?

Check the old thermostat photo and the terminal it sat on. Google warns that a blue wire on B is often really the common, and that a common wire on any terminal but C can blow a fuse, so trace both to the board or the outdoor unit before choosing.

My old thermostat had two wires only.

Heat only, on R and W, and no common. A modern battery thermostat will run on that. A smart one will not without a new cable or an adapter, and if it is a fireplace or a wall furnace it may be millivolt, which needs a thermostat rated for 750 millivolt. Honeywell says those are typically battery powered.

Do I need to do anything about the system type setting?

Yes, and it is a step people often skip. Tell the thermostat whether it is a furnace, a heat pump, and whether the changeover is O or B. Wiring it correctly and leaving it set to the wrong system type behaves exactly like a wiring fault.

Sources: Google Nest Help "Identify a thermostat wire" (labels, colours, the B wire) and "Learn about the common or C wire"; Sensi (Copeland) "What is a C wire or common wire?"; Honeywell RTH6360 quick installation guide 33-00201ES (line voltage, strip length, R slider, O/B setting), RTH2300 operating manual 69-2326EF (five minute compressor protection) and "Thermostat Compatibility: 750 Millivolt Systems"; White-Rodgers 50A55 and 50D50U furnace control instructions (3 and 5 amp board fuse)