Thermostat Wiring Terminals
Click any terminal with a guide badge to read the full explanation.
Run the numbers
HVAC Load Calculator computes live in your browser — free, no sign-up.
Wire colors are a convention, not a guarantee. Always identify terminals by the letter label, not the wire color.
39 terms
Term Guides
Full explanations with diagrams, settings references, and troubleshooting context.
The capacitor provides the phase-shifted starting current that lets the compressor and condenser fan motors start under load. A run capacitor stays in circuit to boost motor efficiency; a start capacitor briefly assists and disconnects. A failed capacitor typically causes the motor to hum without starting, or the AC to blow warm air.
Full guide →The contactor is an electrically-operated switch in your outdoor AC or heat pump unit that connects 240V line voltage to the compressor and condenser fan motor. The thermostat’s 24V Y signal pulls in the contactor coil; pitted or burned contacts cause the compressor to hum but not start.
Full guide →Aux heat is the electric resistance backup heat that runs alongside the heat pump compressor when outdoor temperatures drop below the balance point. It appears as ‘Aux’ on the thermostat display—distinct from ‘Em Heat’ (emergency heat), which runs the backup alone with the compressor off.
Full guide →The balance point is the outdoor temperature at which your heat pump’s heating output exactly equals your home’s heat loss. Below it, aux heat is needed to maintain the setpoint. For most US homes, this falls between 25°F and 40°F.
Full guide →The C-wire (terminal C) provides continuous 24V AC power to your thermostat from the HVAC control board. Without it, smart thermostats that need constant power will drain batteries, drop Wi-Fi, or fail to charge.
Full guide →The flame sensor is a grounded metal rod in the burner that confirms ignition by passing a small current (~2–4 µA DC) through the flame. A dirty or oxidized sensor reads low current and causes the control board to shut off the gas—usually after 2–3 ignition tries—to prevent an uncontrolled gas release.
Full guide →The limit switch is a safety device in your furnace that shuts off the burners if the heat exchanger gets too hot—typically trips above 170–200°F. When it opens during normal operation (rather than only on overheating), it usually signals restricted airflow from a clogged filter or blocked return.
Full guide →HSPF2 is the current US efficiency rating for heat pump heating, replacing HSPF in 2023. Higher is better; minimum HSPF2 for cold-climate heat pumps is 7.5 (Region IV). Like SEER2, HSPF2 test conditions are stricter than legacy HSPF, so a unit rates roughly 15% lower under the new standard.
Full guide →The O/B terminal controls the heat pump’s reversing valve. Most brands (Carrier, Trane, Lennox) use O—energized in cooling. Rheem/Ruud use B—energized in heating. Setting this wrong means the system heats when you call for cooling.
Full guide →The R terminal is the 24V AC hot wire that powers thermostat control circuits. Systems with one transformer use a single R wire; systems with two separate transformers for heating (Rh) and cooling (Rc) split them—a Rh-Rc jumper bridges the two when only one transformer is present.
Full guide →The reversing valve is a refrigerant-flow switch inside a heat pump that determines whether the system heats or cools. When energized (O-wire energized in cooling), refrigerant flows toward the outdoor coil to reject heat; when de-energized, it flows toward the indoor coil to absorb heat.
Full guide →SEER2 is the current US efficiency rating for central AC and heat pump cooling, replacing SEER in 2023. It uses a more realistic external static pressure in testing (0.5 in. w.g. vs. 0.1 in.), so the same equipment scores ~4–5% lower than SEER. Minimum SEER2 is 13.4 (North) or 15.2 (South) for central AC as of 2023.
Full guide →Short cycling is when your HVAC system runs very brief on-off cycles—usually under 3–5 minutes—instead of completing a full run. It wastes energy, causes excessive wear on the compressor and motor, and usually points to an oversized system, a low refrigerant charge, or a failing limit switch.
Full guide →The differential (also deadband or swing) is the temperature range around the setpoint within which the system stays off. A 1°F differential with a 70°F setpoint means heating kicks on at 69°F and turns off at 70°F. Too tight a differential causes short cycling; too wide causes comfort complaints.
Full guide →Quick Reference
Plain-language definitions for terms you’ll encounter across HVAC and thermostat content.
24V AC (actually 24–30V AC under load) is the standard control voltage for residential thermostat wiring in the US. It’s supplied by a step-down transformer in the furnace or air handler. All thermostat terminal wires—R, C, W, Y, G, O/B—carry 24V AC, not line voltage. Never use a multimeter in DC mode to check thermostat wiring.
Indoor air quality covers particulate matter (PM2.5, PM10), CO₂ concentration, VOCs, humidity, and temperature. Some smart thermostats (ecobee SmartSensor Air) include air quality sensors that trigger ventilation or alert you when levels drift.
The apparatus dew point is the effective surface temperature of the evaporator coil in a psychrometric sense—the theoretical temperature air would reach if 100% of it contacted the coil. A lower ADP means more dehumidification relative to sensible cooling.
A BTU is the amount of energy needed to raise one pound of water by 1°F. HVAC equipment capacity is rated in BTUs per hour (BTU/h)—a ‘2-ton’ AC has 24,000 BTU/h of cooling capacity.
The compressor is the pump in your outdoor AC or heat pump that circulates refrigerant by pressurizing the vapor leaving the indoor evaporator coil. Compressors run on 240V—thermostat wiring only signals the contactor to supply that voltage.
COP is the ratio of heating or cooling output (Btu or Watts of heat) to electrical input (Watts). A COP of 3.0 means the heat pump delivers 3 Watts of heat per Watt consumed. Most heat pumps have heating COPs of 2–4 depending on outdoor temperature.
The dew point is the temperature at which air becomes saturated and moisture begins to condense. A 70°F room with a 60°F dew point is moderately humid; below 55°F feels dry and comfortable. Your AC’s evaporator coil surface must be below the dew point to remove moisture from air.
Duct insulation R-value measures resistance to heat transfer through duct walls. Code in most US climates requires R-6 to R-8 on ducts in unconditioned spaces. Uninsulated flex duct in an attic at 130°F can raise supply air temperature by 5–10°F before it reaches the register.
EER measures AC cooling efficiency at a fixed 95°F outdoor / 80°F indoor test condition. Unlike SEER2 (a seasonal average), EER reflects peak-summer performance. Mini-split ratings often use EER or CEER. Higher is better; a 12 EER unit is 20% more efficient than a 10 EER unit.
Emergency heat bypasses the heat pump compressor and runs the backup heat source alone (electric strips, gas, or oil). It’s for emergencies only—when the compressor is failed or locked out. Running Em Heat continuously is 2–3x more expensive than heat pump operation and is not a long-term substitute.
Terminal G energizes the air handler’s fan independently of heating or cooling—used for circulate/fan-only mode. When G is absent, most systems run the fan automatically with heating and cooling calls only.
The furnace heat exchanger is the metal chamber where combustion gases are contained while house air flows around the outside to absorb heat. A cracked heat exchanger is a carbon monoxide hazard—never operate the furnace with a confirmed crack.
A hot surface ignitor (HSI) is a silicon nitride or silicon carbide element that glows to ~2,500°F on a call for heat to ignite the burners—replacing older standing pilots. An ignitor that glows but doesn’t light the burners is a flame sensor issue, not an ignitor issue.
Relative humidity above 60% promotes mold and feels muggy even at comfortable temperatures; below 30% causes static electricity and dry skin in winter. Smart thermostats with humidity sensors can trigger dehumidifier equipment or adjust cooling to manage moisture.
Latent heat is the energy absorbed or released when moisture changes state (evaporates or condenses) without changing temperature. In HVAC, latent capacity is the portion of cooling that removes humidity from air—important in humid climates.
A Manual J load calculation determines how many BTUs per hour your home needs to stay comfortable at design conditions—the basis for properly sizing HVAC equipment. Skipping it and guessing by square footage usually results in oversized equipment that short cycles.
Sensible heat changes air temperature without changing moisture content—what a thermometer measures. Your AC’s sensible capacity is what cools the air; the rest goes to latent heat (dehumidification).
Thermal comfort (ASHRAE 55) depends on air temperature, mean radiant temperature, air speed, humidity, clothing, and activity level—not temperature alone. A thermostat set to 72°F in a room with cold windows and no air movement can feel much colder than 72°F in a well-sealed interior space.
The setpoint is the temperature you’ve dialed in on your thermostat—the target the system tries to maintain. Heating and cooling stages engage and disengage based on how far the measured temperature has drifted from the setpoint plus the differential.
A two-stage system has a low-output first stage (typically 60–70% capacity) and a high-output second stage (100%). The first stage runs most of the time, improving efficiency and humidity control; the second stage engages only on the coldest or hottest days.
Variable-speed (or modulating) systems continuously adjust output—compressor speed, gas valve rate, or blower speed—to match the load exactly. They run longer at lower capacity, delivering better humidity control, quieter operation, and higher efficiency than single- or two-stage equipment.
Ventilation rate is the volume of outdoor air introduced per hour, often expressed as ACH (air changes per hour) or CFM. ASHRAE 62.2 sets minimum residential ventilation at roughly 0.15 ACH. Smart thermostats with ERV/HRV integration can schedule ventilation cycles.
Terminal W calls for first-stage heat (gas valve, electric heat strip, or heat pump auxiliary). W2 adds a second stage—a two-stage gas valve, backup heat strip, or a second-stage compressor call.
Terminal Y calls for the compressor (cooling, or heat pump compressor in heat mode). Y2 adds a second-stage compressor call on two-stage or variable-speed systems.
Zoning divides your home into independently controlled areas, each with its own thermostat and motorized dampers in the ductwork. A zone board opens and closes dampers based on each thermostat’s call, so the bedroom runs cooler than the living room without separate systems.
