HVAC · Lesson 9 of 13Intermediate9 min read
Controls and Thermostats
A thermostat is a switch with a thermometer attached. It doesn’t carry the power that runs the furnace or the compressor; it closes small 24-volt circuits that tell relays and control boards in the equipment what to do.
The 24-volt control circuit
Most central heating and cooling systems use a low-voltage control circuit. A small control transformer inside the furnace or air handler steps the house’s 120-volt power down to 24 volts AC. One wire, R, brings that 24 volts up to the thermostat. When the thermostat wants heat, it connects R to W; the 24 volts travels back down the W wire to a relay or the control board, which starts the heating sequence (Lesson 3). For cooling it connects R to Y, which closes the contactor in the outdoor unit, and for the fan, R to G.
The C, or common, wire completes the circuit back to the transformer without switching anything, which gives a smart thermostat steady power for its screen and Wi-Fi. Because the thermostat only switches 24-volt signals, small thin wires are enough.
Not every thermostat works this way. Electric baseboard heaters often use line-voltage thermostats that switch the 120- or 240-volt heater circuit directly, as the Department of Energy notes. Those are a different, more dangerous kind of wiring.
A simplified model of a single-stage system. Real thermostats add delays, staging and settings that vary by brand; the equipment’s wiring diagram and the thermostat’s instructions are the final word.
| Letter | What it usually connects | When it’s powered |
|---|---|---|
| R (or Rh and Rc) | 24-volt power from the transformer; some systems have separate heating and cooling transformers | Always |
| C | Common: the return side of the transformer | Always (it completes the circuit) |
| W or W1 | Heat | During a call for heat |
| W2 | Second-stage or backup heat | When the first stage can’t keep up |
| Y or Y1 | Cooling: the compressor contactor (and a heat pump’s compressor in heating too) | When the compressor should run |
| Y2 | Second-stage cooling | High-stage calls |
| G | The indoor blower fan | Fan calls, and most cooling calls |
| O or B | A heat pump’s reversing valve: O energizes it for cooling, B for heating | Depends on the brand |
Letters, not colors
The letters are a common convention, but there are no standard wire colors, and brands differ, especially for heat pumps (O versus B). The equipment manufacturer’s instructions and wiring diagram are the final word.
On, off and the deadband
A thermostat doesn’t switch on the instant the room drops a hair below the setting and off the instant it’s a hair above. It waits for a small difference, often called the differential or deadband, before it turns on, and it shuts off a little past the setting. Without that gap, the equipment would start and stop every few seconds. The widget shows the cycles: the room temperature drifts down, the heat comes on, the room warms past the setting, and the heat shuts off.
Programmable and smart thermostats
The Department of Energy says you can save as much as 10 percent a year on heating and cooling by turning your thermostat back 7 to 10°F (4 to 6 Celsius degrees) from its normal setting for 8 hours a day, for example while you sleep or are away. The savings are bigger in milder climates. DOE also corrects a common myth: a furnace doesn’t work harder to reheat the house than it saved, because a cooler house loses heat more slowly the whole time it’s set back.
Programmable thermostats make setback automatic, returning the house to normal before you wake or get home. DOE notes that ordinary setback isn’t recommended for heat pumps in heating mode unless the thermostat is designed for heat pumps (Lesson 6).
Smart thermostats add features that ENERGY STAR describes: they learn your preferences, can be controlled from a phone, use geofencing to know when you’re heading home, and report how your system runs. ENERGY STAR certified smart thermostats also drop into a low-power standby mode when idle. Make sure any thermostat you buy is compatible with your system.
Where a thermostat belongs
A thermostat can only control what it senses. DOE says it should be on an interior wall, away from direct sunlight, drafts, doorways, skylights and windows, and that furniture shouldn’t sit in front of or below it, since that blocks natural air movement. A thermostat in the sun or above a lamp reads too warm and shuts the heat off early; one in a draft reads too cold.
| Part | What it does | Signs of trouble |
|---|---|---|
| Thermostat | Senses room temperature and closes the R–W, R–Y and R–G circuits. | Blank screen; wrong mode; doesn’t match a separate thermometer. |
| Control transformer | Steps 120 volts down to 24 volts for the control circuit. | Nothing responds; a technician tests it. |
| Control board | Runs the furnace or air handler’s sequence and its safeties. | Blinking fault codes. |
| Contactor | The heavy switch in the outdoor unit that the Y signal closes. | Outdoor unit won’t start, or won’t stop. |
| Thermostat wire | Thin, multi-conductor cable from the equipment to the thermostat. | Loose or broken conductors. |
| Outdoor sensor | Some heat pump systems use one to manage backup heat and defrost. | Backup heat running at the wrong times. |
You canWhat you can safely do
- Check the basics first: the mode (Heat, Cool, Off), the setting, the fan setting (Auto or On), and the batteries if your thermostat uses them.
- Program a setback schedule, or use your smart thermostat’s schedule.
- Many homeowners replace a low-voltage thermostat themselves: switch off power to the heating and cooling equipment first, photograph and label every wire before disconnecting it, and follow the new thermostat’s instructions. If any wire doesn’t match the instructions, stop and ask a pro.
StopWhen to stop
- Line-voltage thermostats on baseboard heaters carry 120 or 240 volts. Leave them to an electrician.
- Wiring inside the furnace, air handler or outdoor unit, including the transformer and control board, is for a qualified technician.
- If the thermostat calls but nothing happens, check the breaker once (Reset a Tripped Circuit Breaker); if it trips again, leave it off and call.
Check yourself
Tap an answer to check it, or open “Show the answer.”
What voltage does a typical central-system thermostat switch?
Show the answer
24 volts AC. A control transformer in the equipment supplies 24 volts AC, and the thermostat switches it.
Which terminal does a thermostat connect to R to call for heat?
Show the answer
W. W is heat. Y is cooling (the compressor), G is the fan, and C is common.
What does the C wire do?
Show the answer
Completes the circuit to the transformer, giving the thermostat steady power. C is the common side of the 24-volt transformer.
According to the Department of Energy, turning the thermostat back 7 to 10 degrees for 8 hours a day can save up to:
Show the answer
10 percent a year on heating and cooling. Up to 10 percent a year. A cooler house loses heat more slowly, so setback saves energy.
Why shouldn’t a thermostat sit in direct sunlight?
Show the answer
It reads too warm, so the heat shuts off early. It senses its own warm spot instead of the room’s air.
Go deeper
- U.S. Department of Energy, Energy Saver: Programmable Thermostats DOE on programmable thermostats and setback.
- ENERGY STAR: Smart Thermostats ENERGY STAR on smart thermostats.
- Wikipedia (secondary): Thermostat How thermostats work, including the terminal letters (secondary source).
- Being an Adult 101: Reset a Tripped Circuit Breaker How to reset a tripped breaker safely.
Sources
A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.