The B thermostat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. O or B - These thermostat terminals are for heat pumps. There is no universal color used for this terminal although black is often used. Many digital thermostats require 24 volts for power so the common wire is necessary. On a split system the blower fan is in the air handler while with a package unit the blower fan is in the outdoor package unit.Ĭ (common) - This is the thermostat terminal which originates from the transformer and is necessary to complete the 24 volts power circuit in the thermostat but only if the thermostat consumes electricity for power. G - This is the thermostat terminal used for the fan relay to energize the indoor blower fan. Heat Pumps use staging for auxiliary heat and need a W2 terminal. There are gas furnaces with low fire and high fire and some depend on control from a two-stage heating thermostat with a W2 terminal.
W2 - This is the thermostat terminal used for second stage heat.
This wire should go directly to the heating source whether it be a gas or oil furnace, electric furnace, or boiler. W - This is the thermostat terminal for heating. Many systems only have a single compressor but if you have two compressors which should only operate off of one thermostat then you need the Y2 thermostat terminal for second stage cooling. Y2 - This is the thermostat terminal for cooling second stage if your system is so equipped. Some manufacturers put a terminal board strip near the control board in the air handler so a splice is not needed. Typically a thermostat wire pull is made to the air handler on split systems and then this wire is spliced for the separate wire pull which is made to the condenser. Y - This is the terminal for cooling or air conditioning and goes to the compressor relay. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer. RH - The RH terminal is designated for the power for heating. In this case the power from the transformer in the air conditioning system would go to the thermostat terminal. A transformer for cooling and a transformer for heating. RC - The RC terminal is designated for the power for cooling. If you have a package unit then the transformer is in the package unit. For this reason, it is a good idea to kill the power at the condenser and the air handler before changing or working on the wiring at the thermostat. This comes from the transformer usually located in the air handler for split systems but you may find the transformer in the condensing unit. R - The R terminal is the power for the thermostat. Installation Basics - HVAC Air Conditioning & Heating Systems
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