Replacing a Thermostat: Wiring Colors

Replacing thermostat by yourself without a Denver HVAC technician can be pretty harrowing if you do not hook up the wiring correctly. Inside of the thermostat there are several terminals which must hook up with the right wires. The R-terminal is the thermostat's power. R means Red, the Colorado heating repairman should have red wires going to this terminal. This power comes from the transformer. The transformer is usually in the condensing unit or the air handler in a split system. Because they are supplying power, it is a good idea to kill the power to both the condenser and the air handler before working on the thermostat.

The RC terminal is the power for cooling. Some large HVAC systems have two transformers, one for cooling and one for heat. In such a case, the power from the ac system goes to the thermostat terminal. A jumper can be installed between RC and RH for a single transformer heating and cooling system. The RC terminal is also red.

The RH terminal is the heating power-in. This is also red and can be jumped to the RC.

The Y terminal is yellow and goes to the compressor relay. Some Denver HVAC specialists use a terminal board strip on the air handler control board which makes splices not needed. Y2 is usually light blue and is the code for a second stage cooler. If you do have two compressors, both should work off the same thermostat. Most Denver heating and air companies do not need the cooling of a two stage company.

W is the heating terminal. W is for white terminal. This should go directly to your heating source, whether it be a furnace or boiler. W2 is for second stage heat and is usually brown. This is for gas furnaces with low and high settings. Heat pumps use W2 for auxiliary heat.

G is for green and goes to the indoor blower fan relay.

C is for Common, and does not have a specific color although black is seen frequently. For digital thermostats that consume power, the common wire is necessary to complete the 24 volt circuit.

Orange "O" and "B" Blue are for heat pumps. Rheem and Ruud heat pumps use the B terminal for the cooling reversing valve. Other HVAC manufacturers use the O terminal. Trane, Lennox, Goodman, Ducane, Heil, Carrier, Fedders, Amana, and Janitrol are normally hooked up to the O terminal by HVAC contractors in Denver.

E is for emergency heating. This has no standard color but should be wired to the heating relay or E terminal strip board.

S1 and S2 are for outdoor air sensors. This has no standard color, but should use a shielded wire to reduce interference and the possibility of water damage from outside.

Warning: These colors are the defaults. The person wiring the thermostat could have used unconventional color coding.