Conventional Thermostat

Heat Pump Thermostat

R or RH thermostat terminal - For heating power (24 Volts a.c.) if an RC terminal is present. For heating and cooling if no other R terminals.

R or RH thermostat terminal - For heating power (24 Volts a.c.) if an RC terminal is present. For heating and cooling if no other R terminals.

RC thermostat terminal - for cooling power (24 volts a.c.)

RC thermostat terminal - for cooling power (24 volts a.c.)

W thermostat terminal - for the first stage of heating. Most systems only have one stage of heating so this will be the most commonly used terminal.

W thermostat terminal - for the first stage of heating. Heat pumps systems can have many different stages for back-up heat (unless you have a geothermal system)

W2 thermostat terminal - this will be for the second stage of heating. Some gas furnaces have high fire and low fire and rely on this terminal for high fire staging. It not uncommon for electric furnaces to have two stages for heating.*

W2 thermostat terminal - this will be for the second stage of heating. Some gas furnaces have high fire and low fire and rely on this terminal for high fire staging. It not uncommon for electric furnaces to have two stages for heating.*

Y thermostat terminal - this thermostat terminal will engage the cooling. Usually this is wired directly to the compressor contactor located in the condensing unit. The wiring for this usually passes through the air handling unit on a split system.  

Y thermostat terminal - this thermostat terminal will engage the cooling. Usually this is wired directly to the compressor contactor located in the condensing unit. The wiring for this usually passes through the air handling unit on a split system.  

Y2 thermostat terminal - this thermostat terminal is for the second stage cooling. Very few residential units have two stages of cooling. There are exceptions. Older two-stage systems rely on a secondary control other than the thermostat to engage the second stage. Consult manufacturer's paperwork or a professional HVAC technician if you have a two-stage compressor or two compressors.

Y2 thermostat terminal - this thermostat terminal is for the second stage cooling. Very few residential units have two stages of cooling. There are exceptions. Older two-stage systems rely on a secondary control other than the thermostat to engage the second stage. Consult manufacturer's paperwork or a professional HVAC technician if you have a two-stage compressor or two compressors.

G thermostat terminal - this thermostat terminal is for indoor blower fan control.

G thermostat terminal - this thermostat terminal is for indoor blower fan control.

C thermostat terminal - this is the common side of the 24 volts step down transformer. If the thermostat pulls power from the transformer (usually located in the air handler) for power then this thermostat terminal is necessary for the thermostat to function properly. Older mercury type thermostats usually have indicator lights and this terminal is necessary so that these indicator lights will function.

C thermostat terminal - this is the common side of the 24 volts step down transformer. If the thermostat pulls power from the transformer (usually located in the air handler) for power then this thermostat terminal is necessary for the thermostat to function properly. Older mercury type thermostats usually have indicator lights and this terminal is necessary so that these indicator lights will function.

S1 thermostat terminal - this terminal is in newer thermostats and is a bell and whistle for a high tech digital thermostat. This terminal works in conjunction with the S2 thermostat terminal (noted below) for reading outside air temperatures for display purposes only in (conventional) non-heat pump thermostats.

S1 thermostat terminal - this terminal is in newer thermostats and is a bell and whistle for a high tech digital thermostat. This terminal works in conjunction with the S2 thermostat terminal (noted below) for reading outside air temperatures for display purposes only in (conventional) non-heat pump thermostats. For heat pump thermostats this terminal can be an energy saver by cutting out the outdoor unit off when the outside air temperature drops below a temperature where the outside unit becomes inefficient. This temperature varies depending on geographical location.

S2 thermostat terminal - this thermostat terminal works in conjunction with S1 terminal noted above.

S2 thermostat terminal - this thermostat terminal works in conjunction with S1 terminal noted above.  

O or B Thermostat Terminal - this thermostat terminal controls the reversing valve in the heat pump condensing unit. This switches over the unit from heating to cooling. The O terminal in the thermostat will control the reversing to activate in cooling mode. The O terminal is default heating. Using the B terminal the reversing valve will activate in the heating mode. The default is cooling in this type of unit. Usually, the B terminal is only used in Rheem and Ruud Heat Pump Systems. 

If you are unsure of the terminals when wiring a thermostat please consult an HVAC technician.