The thermostat is the command center for an HVAC system. The thermostat is what will switch the equipment on and off. Most people are familiar with the thermostat on the wall for their heating and cooling, but they’re in almost everything from water heaters to freezers and kitchen equipment.

Modern thermostats sense temperature using a thermistor. The thermistors resistance value changes depending on the surrounding temperature. A small microcontroller converts the measured resistance to temperature and decides based on desired settings if heating or cooling should be switched on.

Modern thermostats have the benefit of being programmable.

Visual of electrical flow

How the C Wire Works

The C wire is also known as the common wire. The C wire runs from your heating or cooling unit 24v common to the thermostat. The C wire connection most often used are the designated wire connections from the furnace to the thermostat on the control board at the furnace or air handler. On older units without a control board, the common side of the transformer is used.

Most Wi-Fi thermostats require a common wire to provide constant power to the thermostat. Thermostats without a C wire will only have a complete circuit during a call for heating or cooling. An example of this being R+W closing to heat a space. Without a complete circuit, a thermostat needs to rely solely on batteries to power the display.

C wire powering thermostat in standby

What if I don’t have a common wire?

Generally, a smart thermostat will need five wires:

R – 24v from the furnace or air handler transformer

W – Heating

Y – Cooling

G – Fan

C – common

It’s very common to run into thermostats with only four wires. When you only have R, W, Y, and G you can still make it work. It’s best just to run the new wire, but that’s not always practical or cost-efficient.

Power Stealing

Nest is the most popular smart thermostat that takes advantage of power stealing and does not require a c wire.

These thermostats “steal” power by pulsing a call for heat. The thermostat very quickly creates and ends a call for heat to close the circuit in an attempt to charge the battery inside the thermostat. Most homeowners would never know this is happening, but on older style furnaces, the homeowner might notice the furnace does startup.

Power Extender Kits or Wire Saver Modules

Ecobee includes what they refer to as the PEK (power extender kit) and Honeywell has what they call the wire saver kit. Venstar makes the Add-a-Wire.

Essentially these kits allow a homeowner to run a smart thermostat with only four wires (R, W, Y, G) and the kit will split a signal removing the need for the 5th wire (C wire).

Fan G for C

If you have wires R, W, Y, and G at your thermostat, you can remove G and move it over to C at your thermostat and on your control board at the HVAC unit. The thermostat will now have a constant power source, but the homeowner will lose the ability to turn the fan on manually if they’d like to.

Run New Wire

Running new wire is ultimately the best option, but not always practical given time and material cost to the customer. Some residences can seem almost impossible to run a new wire from the unit to the thermostat, but when this is an option, it’s the best option.

Millivolt furnace with WiFi thermostat

There’s still a lot of millivolt furnaces out there. You’ll find a lot of the owners of them won’t give them up because “they don’t make these furnaces like they used to.” However, many of these owners want the fancy Wi-Fi thermostats. If you’re not sure how to get the 24v out of a millivolt system, this tip is for you.

Let’s go over a couple of basics.

You’ll be using an isolation relay which is just a SPST (Single Pole, Single Throw) NO (normally open) relay to separate the different voltages in a system.

Millivolt furnaces operate using a thermopile to power the system. A thermopile is merely multiple thermocouples. Some millivolt systems will also use a thermocouple for flame safety. Under load, a thermopile should operate at no less than 190mV at the absolute minimum.

Please refer to the wiring diagram while reading the information below.

You’re going to feed 120v to the transformer which is stepped-down to 24v for the thermostat.

Wire your hot side of the 24v transformer to R on the Wi-Fi thermostat.

Connect W on the thermostat to one side of the relay coil. On the other side of the coil, you’ll add C from the thermostat, and common wire of the transformer.

You’re going to take the thermostat wires off your old millivolt thermostat and add them to the NO terminals on the relay.

Upon a call for heat, the relay coil will be powered closing the SPST relay powering the millivolt system (NO terminals) for heat.

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