How thermostat wiring works

Read the letter, not the color. That is the first rule of thermostat wiring and one the makers repeat. Honeywell Home: there is no standard for which color wire controls each function, and each wire should be identified by the terminal it connects to, never by color. Google’s Nest help says the same, that thermostat wire colors can vary and you should not rely on color alone. The letters on the terminals are the map. The rest of this page is how to read it.

A switch on a 24 volt loop

A wall thermostat on a central system does not carry the power that runs the furnace or the air conditioner. It carries a signal. A training paper from Mainstream Engineering compares it to a light switch circuit: power goes from a source to a load, and here the source is a low-voltage transformer, usually 24 volts AC, whose two wires are labeled R and C (common). The transformer is typically at the air handler.

The R side of that transformer runs up to the thermostat, and the thermostat connects it to one wire or another. In the simplest system Mainstream draws, each of those wires leads back to the coil of a relay or contactor in the equipment. Close the switch and the coil gets its 24 volts between that wire and common, and the relay turns on the part of the system that does the heavy work. Where the equipment has a control board, the thermostat wires land on it: Honeywell Home says you should see the same terminal labels there as on the thermostat, and Google says the board’s connectors typically carry labels such as R, W, G, Y, C or O/B.

What the letters mean

The same few letters are the ones Google’s wire guide lists as the common thermostat wires. The descriptions below are from Mainstream’s paper, Google’s wire guide and Honeywell Home’s wiring page.

• R is the power wire, the 24 volt feed from the transformer. Where a furnace and an air conditioner each have their own transformer, there are two: Rh from the heating transformer and Rc from the cooling one, and they are kept apart. With a single transformer, Rh and Rc are the same wire and are typically jumpered together in the thermostat. Whether a new thermostat wants that jumper is its maker’s call: Resideo’s guide and Google both say theirs do not use jumpers.
• W calls for heat. It completes the circuit from R to the heating control. Some systems have W1 and W2 for stages of heat, and on a heat pump Google notes that W2 may control auxiliary heat.
• Y calls for cooling. It completes the circuit to the contactor that runs the compressor and the outdoor fan. On a heat pump, Google notes, the Y wire controls the compressor.
• G runs the blower, the indoor fan.
• C is common, the other side of the transformer. See the next section.
• O/B is the heat pump terminal. It controls the changeover valve, the reversing valve, which Google says tells the system when to switch between heating and cooling.
• E turns emergency heat on or off on a heat pump system.

The O/B terminal earns its two letters. Mainstream explains that some heat pumps have a reversing valve that rests in the heating position and is powered for cooling, wired to O, and others rest in the cooling position and are powered for heating, wired to B. For the reversing valve a system uses one or the other, not both. One trap: Google and Honeywell Home note that some systems have separate O and B wires, and that on some a wire labeled B is not a heat pump wire at all but the common. Google warns that a common wire put into any connector but C on its thermostat may blow a fuse on the system or damage the thermostat. The two kinds of valve are why Resideo’s installation guide for its T5 thermostat makes the installer choose whether O/B should energize on cool or on heat.

The C wire

A thermostat that only switches does not need the common wire. Mainstream: the C wire need not be run to the thermostat, but if the thermostat requires power it should be, and a thermostat without it has to run on a battery and stops working when the battery dies.

A thermostat with a screen and Wi-Fi uses power of its own, and the two makers we read set different rules for their own thermostats. Honeywell Home describes the C wire as what gives a Wi-Fi thermostat continuous power from the heating and cooling system, and says it is required on all Resideo Wi-Fi thermostats. Google says the C wire helps deliver power in case the other wires cannot provide enough, that its Nest thermostats were engineered to use as little power as possible, and that in many cases they do not need one, though some systems may require a C wire or Google’s own power connector. So the new thermostat’s maker decides, and its compatibility check is where to find out.

To see whether you have one, Honeywell’s check starts with the power, not the wires: for your safety, turn off power at the breaker box or switch that powers the furnace or air conditioning, and confirm the system will not start. Only then does it say to take the thermostat off its wall plate, leaving the wires connected, and look for a wire in the C terminal. If the thermostat is labeled 110, 115, 120 or 240 VAC, or what sits behind it is thick wires or wire nuts, stop and disconnect nothing: that is a line voltage thermostat, described below, and a professional’s job. If there is no C wire, Honeywell lists three options: check for an unused wire in the cable, use a C-wire adapter, or contact an HVAC pro. A spare wire is not a C wire yet: Honeywell says using one means connecting it at the HVAC equipment as well as at the thermostat, and that the adapter also requires access to the equipment. That end of the cable we leave to a technician.

One more reason we leave that end to a technician, from Resideo’s guide: not all heating and cooling equipment uses the C label for the 24 volt common, and where the equipment has no C terminal, the guide says to check the system manual or contact the manufacturer to find out which terminal is the common.

When it is not low voltage

Everything above describes a 24 volt system. Some thermostats switch the full household voltage instead, and they are a different animal.

Most of the signs are behind the thermostat’s cover, so the power comes off first, at the breaker box or switch, and is proven off by the 5-minute test in the numbered steps further down. Switching the thermostat itself off does not do it: Honeywell Home notes that this leaves the power to the heating equipment on. Honeywell Home’s sign: if you see thick black or red wires, you have a line voltage system, which requires a line voltage thermostat and is not compatible with low voltage thermostats. Google’s description adds detail. High voltage thermostats are often built into the wall, typically have 2 to 4 thick wires, often joined with wire nuts, and are usually labeled 110, 115, 120 or 240 VAC. Resideo’s guide names electric baseboard heat at 120 to 240 volts and says its low-voltage thermostat does not work with it.

Google’s instruction for these is the one we pass on: high voltage thermostats should only be installed or uninstalled by a professional, because of the voltage they carry.

Two other kinds do not fit the scheme above either. Google describes millivolt systems, typically wall or floor heaters, and proprietary systems, which you can recognize by terminal labels that are not the standard letters, such as 1, 2, 3 or A, B, C. Resideo’s guide says its thermostat does not work with millivolt systems. For either kind, check compatibility with the thermostat’s maker before buying anything.

The old thermostat may hold mercury

The Thermostat Recycling Corporation (TRC) says that up through the early 2000s manufacturers used mercury in thermostats, and that if yours uses a dial or lever to set the temperature, chances are it contains a small amount. Connecticut’s environmental agency describes what you would see with the face plate carefully removed: one or more glass ampoules containing a silver mass of mercury. Take the power off before the cover: Google’s compatibility guide lists turning off the system’s power, and testing that it is off, before removing a thermostat’s cover. If it is labeled 110, 115, 120 or 240 VAC, or thick wires or wire nuts are behind it, stop there and disconnect nothing; that is a line voltage thermostat and a professional’s job.

TRC says a mercury thermostat is safe as long as the ampule stays intact, and that the law requires anyone replacing one to dispose of the unit properly. TRC does not say which law. The one we found described is Connecticut’s: its environmental agency says that after July 1, 2014 Connecticut residents may not throw mercury thermostats in the trash, on a page last updated in January 2015. Connecticut puts the handling rule in capitals: do not break or remove the ampoules. It sends its residents to a household hazardous waste collection or to an electrical wholesaler in the TRC program, and TRC’s site has a locator for recycling sites near you. Resideo’s guide says the same thing in its own way: do not place the old control in the trash, and ask your local waste management authority how to recycle it.

Before you touch a wire

The makers’ instructions agree on the order of work, and it is short.

1. Turn off the power. Honeywell Home and Resideo both say to turn off power at the breaker box or at the switch that controls the heating and cooling system. Google gives the reasons: it protects you and avoids blowing a fuse in your equipment, and the HVAC system uses high voltage, which can be dangerous. Then prove it: both makers say to change the temperature on the old thermostat, and if the system does not start within 5 minutes the power is off. Google adds that a system can have more than one breaker, and to turn them all off, and that where you have both heating and cooling you should try to start each, since each part can have its own power switch.
2. Look before you disconnect. Resideo’s guide asks two questions here: is the thermostat rated 120 volts or higher, and are there thick black wires with wire nuts? Google adds that high voltage thermostats are usually labeled 110, 115, 120 or 240 VAC. A yes to either question, any of those labels, or thick wires or wire nuts on their own, means stop and disconnect nothing: that is a line voltage thermostat, and Google’s rule is that only a professional should install or uninstall one.
3. Take a picture of the wiring, with the terminal letters visible. Honeywell Home and Resideo both ask for it.
4. Label each wire with the letter of the terminal it came from, never its color, as Honeywell Home says. Where a terminal carries two letters, such as W and O/B, Honeywell Home labels the wire with both; jumpers and wires that were not connected to a terminal get no label. Then follow the new thermostat’s own guide at the wall. Where it sends you to the furnace or air handler, stop: that end of the cable is a technician’s, as below.

What a setback is worth

The wiring is the same whether or not you ever change the setting, but the Department of Energy’s Building America Solution Center gives the reason people bother with a programmable thermostat: a household can save as much as 10 percent a year on heating and cooling by turning the thermostat back 7 to 10 F for eight hours a day. For a heat pump it recommends a thermostat with adaptive recovery, which brings the house back to temperature while keeping the use of costly auxiliary heat to a minimum. One caution before any winter setback: the November almanac page has the frozen-pipe advice, and in cold weather it comes first.

What is yours, and what is a technician’s

Yours: reading the terminals, taking the photograph, and replacing one low-voltage thermostat with another at the wall, by the maker’s guide, with the power off and proven off.

A technician’s or an electrician’s: any line voltage thermostat, millivolt and proprietary systems, a heat pump whose O/B setting you cannot confirm, a B wire whose job you cannot confirm, a two-transformer system with both Rh and Rc wires (Google strongly recommends a professional, to prevent damage), a heat pump backed by a gas or oil furnace (Honeywell Home says to pause and call an HVAC contractor), and anything at the furnace or air handler end of the cable, including finding or adding a common wire when the equipment has no terminal marked C.

Related

• Almanac: November, the winter thermostat routine
• What a furnace does when the thermostat calls for heat and how central air and a heat pump move heat
• Glossary: thermostat, C wire, line voltage thermostat, reversing valve, air handler