HVACopediaThermostat & board terminals
Terminal letters, check voltages and abbreviations on ignition boards.
Thermostat terminal letters
| Letter | Meaning |
|---|---|
| R, Rc, Rh | 24 V power. Rc for cooling, Rh for heating, R when there is only one |
| C | 24 V common |
| Y, Y1, Y2 | Compressor contactor, stage 1 and 2 |
| W, W1, W2 | Heat, stage 1 and 2 (on a heat pump, auxiliary heat) |
| G | Indoor fan |
| O / B | Reversing valve. O energizes in cooling, B energizes in heating |
| E | Emergency heat (heat pump) |
| Aux | Auxiliary heat (heat pump) |
| L | Service or fault light (heat pump) |
| HMD, DHD or H, DH | Humidify, dehumidify |
| Out, OD, OS | Outdoor sensor |
| In, ID | Indoor sensor |
| CR | Cooling relay |
| M1 (and similar) | Zone damper |
Colours are not standard between brands. Label wires by terminal letter, not by colour.
Terminal check with the fan on AUTO
| Mode | R to W | R to Y | R to G |
|---|---|---|---|
| Heating call | Closed, 0 V | Open, full voltage | Open, full voltage |
| Cooling call | Open, full voltage | Closed, 0 V | Closed, 0 V |
A closed circuit reads 0 V. An open one reads the source voltage (about 25 to 28 V on low-voltage control, 120 or 240 V on line-voltage).
Low-voltage circuit
- Secondary amps = VA ÷ 24. A 40 VA transformer carries 1.67 A.
- A healthy 24 V transformer reads about 27 to 28 V in the circuit. 24 V or less means it is ageing or the circuit is overloaded.
- Add up the current of all the loads (contactor, gas valve, relays, zone valves, humidifier, ignition control, thermostat). The total must stay below the transformer VA ÷ 24, or fit a bigger one.
- Clamp-meter trick for small currents: wrap the wire around the jaws 10 times and divide the reading by 10.
- Fuse the secondary. Thermostat wire: 18 AWG up to about 50 ft, 16 AWG beyond.
- Power-stealing thermostats can false-trigger relays and boards. A bleed resistor (for example 270 Ω, 3 W) between the stat leg and common can cure it.
Zone damper voltages
| Damper type | Zone not calling |
|---|---|
| Normally open, 2 wires | More than 24 V across common and control |
| Normally closed, 2 wires | 0 V |
| Power-open / power-close, 3 to 5 wires | 24 V common to close, 0 V common to open |
On a zone panel, a cooling call shows 24 V from Y to C, a heating call 24 V from W to C. Zoned systems still need 400 cfm per ton across the coil, so use a bypass damper.
Abbreviations on ignition control boards
| Mark | Meaning |
|---|---|
| GV, MV, V1 | Main gas valve |
| PV | Pilot valve |
| MV/PV | Main and pilot valve together |
| GND, BRN GND | Burner ground |
| 24V GND | 24 V common (unswitched secondary) |
| 24V | 24 V switched secondary |
| TH, TH-W | Thermostat heat call |
| SENSE, REMOTE SENSE, FS | Flame sensor |
| SPARK, IGN COIL | Spark igniter |
| IGN | Hot-surface igniter |
| VD | Vent damper |
| PSW | Pressure switch |
| IND | Inducer |
Take it with you
All of these tables, plus the calculators, are in the HVACopedia app, offline.
Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.