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What do thermostat wire labels in 24V~ HVAC systems mean?

1. Common thermostat wires.

Label Description
R

The R, Rc, and Rh wires are the power wires for your heating and cooling system. You should have at least one of them.
Rc
Rh
W or W1 The W or W1 wire controls your heating equipment.
W2 Stage-2 heating or stage-2 auxiliary heating.
Y or Y1

The Y or Y1 wire controls your compressor. This component is responsible for:

1. Air conditioner: cooling.

2. Heat pump system: cooling and heating.
Y2

Stage-2 compressor.
C

The C wire is also known as the common wire. This wire connects your HVAC system to the common ground and can help provide power to your thermostat.

The C wire is required for most smart thermostats. If you do not have one, you can:

1. Run a new C wire from your HVAC control board.

2. Install a Meross C Wire Adapter to your HVAC control board.

3. Install a 24VAC transformer directly to the thermostat's C and R terminals.
O

Heat pump systems use a reversing valve controlled by the O/B wire. Your reversing valve tells your system when to switch between heating and cooling.

Note: In some systems, the common wire may be labeled as B. If you do not have a heat pump system and are missing a C wire, the B wire might be the common wire.
B
O/B
G The G wire controls your fan. Your fan pushes the warm or cool air through your vents into the rooms of your home.
E Some thermostats have an E terminal. The E wire triggers your emergency heating, which is typically provided by the auxiliary heating equipment.
AUX The AUX wire controls your auxiliary heating equipment. Auxiliary heating sources can be either electric or fossil fuel-based.
W3 Stage-3 heating or stage-3 auxiliary heating.
Hum The Hum wire controls your humidifier.
Dehum The Dehum wire controls your humidifier.
S or S1 Some traditional thermostats require S wires to connect an outdoor temperature sensor. The Meross thermostat eliminates that need by obtaining outdoor temperature data for your area directly over the network.
Install the Meross thermostat without these wires.
S1
G2 2nd-speed fan.
G3 3rd-speed fan.

 

 

 

2. Other thermostat wires.

 

You may have wires with the following labels, which can be configured as follows:

(1) Gl, Gm, Gh: These wires represent fan speeds (low, medium, high). Select G, G2, and G3 respectively.

(2) H/Hum1, H2/Hum2: These wires are for humidifiers. H/Hum1 corresponds to Hum. If you have two "Hum" wires, your humidifier requires a 2-wire installation with a 24VAC relay. And we recommend using the 90-290Q model or a similar relay. Refer to the wiring diagram below to install it.

 

 

(3) D/DH, DH2/Dehum2: These wires are for dehumidifiers. D/DH corresponds to Dehum. If you have two "Dehum" wires, your dehumidifier requires a 2-wire installation with a 24VAC relay. And we recommend using the 90-290Q model or a similar relay. Refer to the wiring diagram above to install it.

(4) L: Usually used in some systems to indicate Emergency Heat status. The Meross thermostat doesn't require this wire, you can leave it unconnected.

(5) PEK: This wire typically appears when a C Wire Adapter from certain brands is installed. Please contact our customer support for further investigation.

(6) L1, L2: These are Live wires used in 120V/240V high-voltage electric heating systems. If you have these wires, our device is not compatible with your system.

 

 

 

 
 

 

 

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