What should the voltage be at the thermostat?

The most common type of voltage from a central heating and/or cooling system is 24 volts (24Vac). The old thermostat may say somewhere on the thermostat that it is for 24 volt (24Vac) or maximum 30 volts (30Vac). If you see 120 Vac or 240 Vac on the thermostat, a low voltage thermostat will NOT work with your system.

How does a 24 volt thermostat work?

They usually operate at 24V ac power, and the source of this control power comes from a control transformer mounted either just outside, or inside the furnace body. This function allows you to circulate the air in the house by using the fan in the furnace without heating or cooling the air moving through the system.

How do thermostats get power?

The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).

Where does the 24 volt power come from on a thermostat?

So the 24 volt power would come from the transformer and passed to the G terminal which controls the fan relay. If your thermostat only has an R terminal (versus separate RH and RC terminals) then the R terminal will pass power to all the other terminals except the C terminal.

What is the difference between RC and RH on a thermostat?

If your thermostat only has an R terminal (versus separate RH and RC terminals) then the R terminal will pass power to all the other terminals except the C terminal. The RC terminal is the terminal for 24-volt hot and passes power to the cooling circuit for air conditioners. If you have an RC terminal then you also have an RH terminal.

Does my old thermostat have a ‘C’ terminal?

In some cases, your old thermostat may not have a ‘C’ terminal. In such cases, there are chances that a ‘C wire’ is already in place but left unconnected. Look for any wire that is tucked away behind your thermostat. If there is any such wire, verify it as the common wire by examining the circuit board inside your furnace.

How do I know which wire is C on my thermostat?

If you see a wire of different color, say Yellow in the ‘C’ terminal of your current thermostat, most probably it should be your ‘C’ wire. You can verify this by opening the circuit board of your furnace, and make sure the same color wire is connected to the ‘C’ terminal on the furnace circuit board. Take a look at the image given below.