Reference · R · C · W · Y · G · O · B · Rh · Rc · W2 · Y2 · X · L · K · P

Terminal reference — signaling letters

Every letter that shows up on a thermostat sub-base or equipment block, what it means, when it's energized, and the wiring pattern for the systems that need it.

Residential HVAC has been adding signaling letters for forty years. The basic six (R, C, W, Y, G, and ground) handle any single-stage gas furnace with a cooling coil. Beyond that, every additional stage, heat-pump mode, or safety signal gets another letter. This page is the map.

The complete terminal letter map

Every letter you'll encounter

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R 24 VAC hot (single-xfmr systems) From transformer secondary; common in all US residential
Rh 24 VAC hot — HEAT transformer only Present in dual-xfmr systems (boiler + AC)
Rc 24 VAC hot — COOL transformer only Present in dual-xfmr systems (boiler + AC)
C 24 VAC common (return) Closes the 24V loop; powers smart stats
W / W1 1st stage heat call Energizes gas valve / oil relay / aquastat
W2 2nd stage heat call High-fire gas, 2nd element, or auxiliary strip
W3 / Aux / E 3rd stage / emergency heat Heat-pump backup strips or emergency-only element
Y / Y1 1st stage cool / compressor call Energizes outdoor contactor coil
Y2 2nd stage compressor call Two-stage or two-compressor condensers
G Indoor blower fan call Direct fan-on; separate from heat/cool calls
O Reversing valve energized in COOL Carrier, Trane, Lennox, ICP, Goodman — majority
B Reversing valve energized in HEAT Rheem, Ruud, some older Trane
L System status / lockout indicator Drives 'Check System' LED on thermostat
X / X2 Auxiliary indication or second stage Varies; check the equipment diagram
K Combined Y + G (Honeywell combiner) Used on some smart stats to save a conductor
P Pulse input / outdoor temp sensor Rare in retrofits; some ecobee/Honeywell apps
Ground Chassis / cabinet ground Not a signal; bonding only

A real integrated furnace control block

Where the letters actually live: a typical residential IFC board with its three separate terminal strips — low-voltage signals, line-voltage supply, and line-voltage motor/accessory outputs. The LED that flashes error codes is almost always on the same board.

Integrated furnace control board — terminal layout CNT06711 REV C · JOHNSON CONTROLS IGN: HSI · FAN: multispeed · HUM/EAC: dry contact LOW-VOLTAGE TERMINALS R W Y G C W2 Y2 LINE VOLTAGE · 120 VAC L1 N GND pressure sw INDUCER + BLOWER IND HEAT COOL FAN ACCESSORIES · 120 V EAC EAC-N HUM BLOWER SPEED TAPS HI ○ ← red wire MED ○ ← blue/yellow LO ● ← black (in use) unused taps capped on PARK FUSE 3A automotive ATC blade LED1 · fault code blinks DIP switches heat rise, fan delay READING THE BOARD Three terminal strips: low-voltage thermostat (R/W/Y/G/C/W2/Y2), line-voltage supply + pressure switch interlock, and line-voltage motor/accessory outputs. The LED flashes a fault code — count the flashes, consult the label on the cabinet door (or the error-codes reference). DIP switches configure behavior; changing them requires a power cycle. Layouts vary by OEM but the element families (signal, line, motor, accessory, diagnostic) are universal.
A representative integrated furnace control (IFC). Brands vary — the layout families are universal: signal terminals, line-voltage inputs, line-voltage outputs, accessory terminals, fuse, LED, and DIP switches.

The dual-transformer install — where Rh/Rc matter

A gas furnace with a cooling coil uses one transformer. A hydronic boiler paired with a split AC system uses two — one in the boiler for the zone calls, one in the air handler for the compressor and fan calls. The thermostat sees both 24 V supplies on separate terminals (Rh, Rc), and the factory jumper between them must be removed or the two transformers fight.

Dual-transformer system — separate Rh and Rc supplies Thermostat dual-transformer mode Rh Rc W Y G C ⚠ Rh–Rc jumper REMOVED Boiler (heat supply) Xfmr #1 · heat only Xfmr 1 · 120→24 VAC Rh Aquastat / zone relay panel W input Rh out W in C 120V L Air handler / AC Xfmr #2 · cool + fan Xfmr 2 · 120→24 VAC Rc Cool/fan control board Y, G inputs Rc out Y in G in C Rh W C (from boiler) Rc Y G ▲ REMOVE THE Rh–Rc JUMPER Single-transformer stats ship with a factory jumper tying Rh to Rc (so one R supply powers both calls). On a dual-transformer install, that jumper ties two separate 24V supplies together — they'll be out of phase, and you'll smoke one or both transformers at power-up. Always pull the jumper (or set the stat to dual-xfmr mode) BEFORE applying power. Verify 0 VAC Rh-to-Rc before energizing.
Dual-transformer topology. Boiler feeds Rh; air handler feeds Rc. Same thermostat sees both, but the Rh–Rc jumper is removed so the two supplies never touch each other.

Wiring patterns by system type

Which terminals get wired for which system

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Single-stage AC + gas furnace R · C · W · Y · G Most common residential
Single-stage heat pump R · C · Y · G · O or B O/B selects reversing valve mode
2-stage AC + 2-stage gas R · C · W · W2 · Y · Y2 · G Two-stage thermostat required
Heat pump with aux heat strips R · C · Y · G · O/B · W2 W2 drives backup strips
Dual-fuel (HP + gas furnace aux) R · C · Y · G · O/B · W2 + outdoor sensor Outdoor temp sensor usually via P or on stat
Hydronic boiler + split AC Rh · Rc · W · Y · G · C (no jumper) Dual-transformer — see Rh/Rc note above
Electric baseboard + ductless cool Separate low-V systems Usually two stats, no dual-xfmr
Communicating (Carrier Infinity, Lennox iComfort, Trane Link) R · C + serial bus (A/B) Replaces discrete letter signaling

O vs B — the reversing valve in one table

Which terminal energizes the reversing valve?

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Valve energized in COOL mode Terminal O Carrier, Bryant, Payne, Heil, Tempstar, ICP, Goodman, Amana, Lennox, modern Trane
Valve energized in HEAT mode Terminal B Rheem, Ruud, Weatherking; some older Trane
Valve logic inverted via stat config Configurable on most smart stats Nest, Ecobee, Honeywell T6, etc., all have O/B selection in setup
Proprietary reversing (no O/B) Carrier Infinity, Lennox iComfort, some Daikin/Mitsubishi Bus-commanded; not user-wireable
Verify in the field by Jumper R→O or R→B at equipment Watch for refrigerant direction change; avoid repeated cycling

G, E, K — fan and combined-signal edge cases

Less-common letters and when they matter

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G always energized during W/Y Equipment board does this internally You rarely need to call G from stat during heat/cool
Fan-only call (G without W or Y) Stat 'fan on' setting Useful for circulation; most stats support this
E (Emergency heat) Heat pumps only Energizes backup strips directly, bypasses compressor
K (combined fan+compressor) Honeywell combiner accessory Reduces conductor count when installing smart stat on 4-wire
L (service lamp) Modern stats show fault codes instead Legacy feature; some heat pumps still drive it
P (pulse / outdoor sensor) Rare retrofit terminal Usually an outdoor temp sensor for dual-fuel lockout