What it is

Almost all residential HVAC controls run on a 24-volt AC low-voltage circuit. A control transformer steps 120V (or 240V) line voltage down to 24V, and that 24V is what powers the thermostat, the contactor coil, the gas valve, relays, and so on. The thermostat is basically a set of switches: it takes 24V power in on one terminal and routes it back out to whichever load needs to turn on.

Understanding the terminals is the foundation of all control work — wiring a stat, troubleshooting a no-heat/no-cool, or adding a C wire.

How it works

The transformer has two low-voltage legs. One leg is the hot/power side; the other is the common side. The convention:

  • R = 24V hot (power) coming from the transformer to the thermostat. This is the source the stat switches.
  • C = common — the other side of the transformer. The C wire completes the circuit back to the transformer. The thermostat doesn't switch C; loads need C to make a complete loop, and modern Wi-Fi/smart stats need C to power themselves continuously.

The thermostat takes R (24V) and, when it wants something on, connects R through to that load's terminal:

  • Y = cooling. Closing R→Y energizes the compressor contactor (and usually the condenser/outdoor unit). On two-stage cooling you'll see Y1 and Y2.
  • G = indoor blower/fan. R→G energizes the indoor fan relay. This is what runs the fan in "ON" and in cooling.
  • W = heat. R→W energizes the heat — gas valve on a furnace, or the electric heat/aux on an air handler. Second-stage or aux heat is W2 (sometimes labeled AUX or E for emergency).
  • O / B = reversing valve on a heat pump.
  • O energizes the reversing valve for cooling (most common residential).
  • B energizes it for heating (some brands).
  • A given system uses one or the other; the stat is configured to match.
  • Rh / Rc = on stats with separate heating and cooling power, Rh is the heat-side power and Rc is the cool-side power. With a single transformer (most residential), you put a jumper between Rh and Rc (or use the stat's single R terminal) so one R feeds both. Two transformers (some systems) → remove the jumper so heat and cool each have their own power.

So a cooling call on a heat pump might be: R→Y (compressor), R→G (indoor fan), R→O (reversing valve to cool). A heat call on a gas furnace: R→W (and the furnace board runs its own blower on the heat profile, so you may not even send G).

In the field

Standard color convention (helpful, but NEVER trust color over function):

  • R = red
  • C = blue (sometimes black)
  • Y = yellow
  • G = green
  • W = white
  • O/B = orange (O) / dark blue or black (B)
  • W2/Aux/E = often brown or other
  • Y2 = often blue/brown variants

Why you can't trust color: thermostat wire (commonly 18-gauge multi-conductor "thermostat cable") just comes with a bundle of colors. Whoever ran it may have used whatever conductor was handy. The only truth is what each wire is landed on at both ends. Always trace/verify terminal-to-terminal; the colors are a starting hint, not gospel.

Adding a C wire: smart thermostats need 24V common. If there's an unused conductor in the cable, land it on C at the stat and C at the equipment/board. If there's no spare wire, options are a C-wire adapter/add-a-wire kit or running new cable.

Normal values & targets

  • Control voltage: 24V AC nominal (you'll typically read ~24–28V AC R-to-C on a healthy transformer; often around 26–28V open).
  • Measure R-to-C to confirm you have control power at the stat. ~0V means no power getting there (transformer, fuse, wiring).
  • At a load: R-to-(load terminal) should read ~24V when the stat is NOT calling (switch open, full voltage across the open switch) and drop toward ~0V across the closed switch when it IS calling and the load is energized — basic series-circuit behavior.
  • Common control fuse: boards usually have a 3A or 5A low-voltage fuse protecting the 24V circuit.

Common faults & what they mean

  • No 24V at the stat (R-to-C ≈ 0) — dead transformer, blown low-voltage fuse, tripped furnace door switch, or broken wire. Work back toward the transformer.
  • Fan won't run in ON — G circuit: stat, G wire, fan relay/board.
  • No cooling but fan runs — Y circuit: stat to Y, Y wire, contactor coil, or a tripped high-pressure/float switch in the Y path.
  • No heat — W circuit: stat to W, W wire, and the heat source (gas valve / sequencer / aux).
  • Heat pump runs backwards (heats in cool / cools in heat) — O vs B mismatch in the stat config or wiring.
  • Smart stat keeps rebooting / won't hold settings — missing or bad C wire; it's trying to "steal" power through the loads and browning out.
  • Both R's not jumpered on a single-transformer system — only heat OR only cool works depending on where power landed. Add the Rh/Rc jumper.

Tech tips & gotchas

  • Function over color, every time. A "red" wire on G is still the fan wire. Verify landings at both ends before you believe any color.
  • R is power, C is the return. The thermostat switches R out to loads; it never switches C. That mental model explains every terminal.
  • One transformer = jumper Rh/Rc. Two transformers = no jumper. Get this wrong and you'll smoke a transformer or only get half the system.
  • O and B are mutually exclusive per system. Don't land both; configure the stat for whichever the equipment uses.
  • Don't cross 24V hot to ground/common through the wrong terminal — a pinched wire shorting R to C pops the low-voltage fuse instantly. A repeatedly blowing control fuse usually means a shorted thermostat wire (often a chafed cable at the cabinet or a staple through it).
  • On a furnace, the board often controls the blower itself on a heat call — you don't always send G for heat. For cooling and fan-on, you do send G.

Safety / code notes

  • The 24V circuit is low-voltage Class 2, but the transformer's line side is full 120/240V — kill power at the disconnect before working in the cabinet.
  • A shorted low-voltage circuit can repeatedly blow the board fuse; find and fix the short rather than upsizing the fuse — the fuse protects the transformer and wiring.
  • Low-voltage control wiring still must be installed and protected per the applicable electrical code provisions for Class 2 circuits; don't run it where it can chafe against sheet metal or moving parts.