Wired or wireless thermostats must be installed with correct wire connections.
Most thermostats are powered by and switch low voltage control power. Usually 24VAC. The HVAC system is equipped with one or more transformers that reduce the line voltage to safer control voltage. Some thermostats switch 115VAC or line voltage. If you don’t understand the difference and if you don’t have a multimeter to test for voltage, you should not attempt a DIY replacement.
Electricity is here to stay and if you plan DIY projects you should get some safety and technical training in its use.
Before you begin your replacement project, gather some information. What type system are you replacing the thermostat for? Following are the most common:
- Single stage heat only. (Could be line voltage!!!)
- Single stage cooling only
- Single stage cooling, single stage gas heat
- Single stage cooling, two stage heat
- Heat pump with emergency heat
Select a universal replacement with all the features you want or require.
Remove your existing thermostat from the wall without disconnecting the wires. Use your multimeter to test for voltage. Write down the wire colors connected (and not connected) to your thermostat. Also write down the terminal designation on the thermostat or its sub base that the wire is connected to. As an example the red wire is connected to the R terminal. Note any jumper wires or jumper connectors connecting two or more terminals. An example might be Rc to Rh.
Most systems have one source of 24VAC control power or one control transformer. Some systems have a control transformer for cooling Rc, and another transformer for heating Rh. A single transformer system might require that RC and Rh be jumpered.
- R terminal is the 24VAC power supply from the control transformer.
- C terminal is the common terminal that completes the 24VAC control circuit. The black wire is recommended. Digital thermostats need a common for operation. This wire may not exist at the thermostat if you are replacing a non-digital thermostat.
- Y terminal is usually connected to the first stage cooling contactor. Your thermostat will switch power from R to Y to turn on the compressor. There is probably a time delay relay built into the circuit that will delay this action. The wire color for the Y terminal is usually yellow.
- Y2 terminal is the second stage of cooling if your system has 2 compressors. The wire color is usually blue.
- W terminal is used for first stage of heat. This terminal uses the white wire. This wire will be run to the furnace air handler.
- W2 terminal is second stage heat for multistage furnaces and boilers or back-up heat for heat pumps. The brown wire is normally used for stage 2 heat.
- G terminal is for the indoor blower. The green wire will energize the indoor blower relay and will switch line power on/off to the indoor blower motor.
- O or B terminal. Heat pumps must switch the reversing valve from cooling to heating. This terminal performs that function. The wire may be orange.
- E terminal is for emergency heat. Wire color varies.
- S1 and S2 terminal are for outdoor air temperature sensors.
Disconnect power to the outdoor unit. Remove the access panel or panels to the electrical section. This section contains high voltage and can cause death. Check for the presence of voltage with your multimeter before proceeding. Write down the terminal designation and the wire color attached. Do the same thing at the indoor unit and verify terminal designation and wire color.
If you are satisfied with the wiring for your system, simply disconnect you old thermostat and reconnect the correct wire to the correct terminal on your new thermostat.
Contact us if you have any questions.