To know at what voltage your thermostat is working, you need to turn off the power and check the wiring. One way is to look into the gang box on your current thermostat. If you have white and black wires, then it is probably 120V.
MODEL/SERIAL NUMBER – This describes your thermostat type. For most thermostats, the Model Number is located on the back of the detachable face plate.
Connect the Multimeter to the Thermostat
Place the multimeter's red probe onto the thermostat terminal marked R for red; you can place the black probe onto any of the other terminals. The multimeter reading should be at or close to the thermostat's voltage level.
Line voltage thermostats can handle up to 120V or 240V. Line voltage is common in electric baseboard heaters, radiant heaters, and convection heaters.
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.
To know at what voltage your thermostat is working, you need to turn off the power and check the wiring. One way is to look into the gang box on your current thermostat. If you have white and black wires, then it is probably 120V.
If you're upgrading to a new smart thermostat, a c-wire is a necessary addition, too. Short for “common wire,” the c-wire is needed to deliver continuous 24-volt electricity to the new thermostat.
The black wire is the "hot" wire, it carries the electricity from the breaker panel into the switch or light source. The white wire is the "neutral" wire, it takes any unused electricity and current and sends it back to the breaker panel.
Red, R wire: This carries the 24V power supply to the thermostat, thus allowing it to communicate with your HVAC system. Usually connected to the R or Rh terminal. White, W wire: This signals the heating to switch on or off as needed. Green, G wire: This wire operates the blower fan in your HVAC system.
Faulty Wiring or Equipment: Bad wiring can prevent power from reaching your thermostat. It can also increase the risk of a short circuit or fire. Check whether the thermostat's wiring is frayed, broken, or installed incorrectly (the wires/terminals are usually color-coded).
This is the most typical thermostat wiring style, and it applies to systems that regulate both heat and air conditioning. The wires are typically arranged as follows: red for 24-volt hot, white for heat, yellow for cooling, green for the fan, and blue for common (although the common wire may be a different color).
Use a voltmeter to measure 24 VAC between the C and R wires. If you don't get anything then either the fuse inside the 24 volt control transformer is blown or there is no power to the AC. If you can see 24 VAC on those two wires, then the C wire is good. The C wire is the common return wire to the control transformer.
Before installing a new thermostat, you must check your home's compatibility. Houses are wired in two ways, line voltage, and low voltage, and you'll want to buy a thermostat that will operate with your particular heating and cooling system. Therefore, choosing the right digital thermostats is important.
Turn off the power to your thermostat before checking the wiring as high voltages can be dangerous and even fatal. You can also look into the gang box of your existing thermostat. If you have black and white wires you likely have 120V. If you have black and red wires you likely have 240V.
Thermostats come in two basic types: low-voltage and line-voltage. The low-voltage system uses gas, electricity or oil – best suited to controlling the current of air. These thermostats are ideal for unitary power systems and water heating systems.
Take note of the different colored wires – white for the neutral wire, black for the hot wire, and green for the ground wire. Connect the white wire to the silver screw on the outlet, the black wire to the brass screw on the outlet, and the green wire to the green screw on the outlet.
Many types of insulated wire and cable have labels on the sides. The numbers and letters indicate such topics as insulation type, maximum voltage rating and the wire size and material.
Extra connections between ground and circuit neutral may result in circulating current in the ground path, stray current introduced in the earth or in a structure, and stray voltage. Extra ground connections on a neutral conductor may bypass the protection provided by a ground-fault circuit interrupter.
Look for connectors that have labels that are the same as your thermostat. For instance, you might find labels like R, W, G, Y, C or O/B. You should find wires attached to these connectors that are the same color as the wires that connect to your thermostat.
Don't panic. A C-wire provides continuous power and Wi-Fi connectivity to a smart thermostat. C-wires can sometimes be hidden away in the wall behind the thermostat. Older HVAC systems may not have C-wires, but there are workarounds, like using a G-wire or adapter.
The C, or common wire, provides smart thermostats with continual power. It's usually blue, but it may also be black, brown or purple. The O or orange wire connects to your heat pump (if applicable).
But sometimes, you may need to connect a common wire (C wire) or power accessory, like the Nest Power Connector, to make your system compatible. Unlike other wires connected to your thermostat, a C wire doesn't control heating or cooling functions. It only provides constant power to your thermostat.