Hvac - How can I add a &quotC&quot wire to my thermostat? - Home Improvement Stack Exchange

Hvac – How can I add a &quotC&quot wire to my thermostat? – Home Improvement Stack Exchange

Hvac – How can I add a &quotC&quot wire to my thermostat? – Home Improvement Stack Exchange

How to connect a c wire

Let’s start by explaining what the c-wire is and why it’s needed.

In ancient times, thermostats were simple switching devices, using mercury switches to complete the circuit and turn on the heating/air conditioning.

Mercury switches were commonly used in bimetallic thermostats. the weight of the moving mercury droplet provided some hysteresis by moving the bimetallic spring a bit past the point it would normally assume, thus keeping the thermostat off a bit longer before going to the on state, and then keeping the thermostat a bit longer before returning to the on state. out of state the mercury also provided a very positive on/off action and could withstand millions of cycles without contact degradation. source

Because of this, there was no reason to run a return wire to the thermostat. For example, a thermostat that only controls heat would only require 2 wires.

Thermostat wiring without a C wire

Thermostat wiring without a C wire showing electricity

newer thermostats that offer clocks, backlit displays, wifi, etc. they are controlled by circuit boards and integrated circuits. these new circuits require a path for the electricity to return to the source and therefore require additional wire. this new cable is known as the c cable or common cable.

Thermostat wiring with a C wire

Thermostat wiring with a C wire showing electricity

If you’re lucky when you upgrade to a newer thermostat that requires this connection, there will be an extra (unused) wire in the thermostat cable. if it isn’t, you’ll need to run a new wire to the thermostat.

if you have heating and air conditioning, you will have to pull an 18/5 cable. if you’re just hot you can get away with pulling 18/3 cord, but you might want to pull 18/5 anyway to make it easier to add ac in the future.

enter image description here

There are no standards for wire color, so any wire can be used for any purpose. the most common color code would be (note: this is for forced air furnaces, heat pumps and other systems may be different).

  • red – r – 24 vac
  • red – rh – 24 vac (dedicated to heat call)
  • red – rc – 24 vac (dedicated to heat call) cooling)
  • green – g – fan on
  • white – w – hot warning
  • yellow – y – cold warning
  • blue or black – c – common

this solution is illustrated in this honeywell video. With this solution, you lose the ability to manually turn on the fan, but the fan will still work properly in the automatic position.

Warning: This procedure involves rewiring to the heater and may not be approved by all manufacturers. check with the oven manufacturer and all local codes before attempting this procedure. make sure the oven switch is off before you start.

  1. make sure the heater switch is off.
  2. start by removing the heater access panel and locating the thermostat wires.
  3. remove the wire from the terminal g and connect to terminal c.
  4. Using a short piece of 18 awg wire, make a jumper and connect it between terminals y and g (this is only necessary if you have both and center air).
  5. replace access panel.
  6. remove thermostat from wall to access wiring.
  7. remove wire from g and connect to terminal c.
  8. replace thermostat.
  9. turn oven switch back on.

A transformer uses coils of wire, magnetism, and a bit of magic to transfer power from the primary side of the transformer to the secondary side of the transformer. usually during the transfer, the voltage increases or decreases. In the case of our oven, we’re probably talking about taking 120 Vac and turning it into 24 Vac. once the voltage has been reduced, we can use the lower voltage and a thermostat to control the oven.

Now that you know even less about transformers than before, let’s look at a diagram.

Highlighted Furnace Wiring Diagram

This is an actual oven wiring diagram, but you’ll notice I’ve highlighted a few things. first, in red I have highlighted the 120v primary side of the transformer. I’ve also highlighted the secondary side of the transformer in a couple shades of blue. this was done to illustrate that one side of the transformer’s secondary winding (light blue) is connected to the r or power terminal. while the other side of the secondary winding (dark blue), is attached to the c or “neutral” terminal.

location transformers

in an outline

On a schematic or wiring diagram, a transformer will look like this.

You will often see a number written on each side, indicating the expected voltages on each side of the transformer. notice in the schematic above that the top side shows 120v (120 volts), while the bottom side shows 24v (24 volts).

in the real world

when you actually dig through hvac equipment, a transformer will look like this.

Transformer Transformer

note the rectangular center section, flanked by a bulge on each side. these are the typical physical characteristics of a transformer.


Transformers typically have a volt-ampere (VA) rating, which can be used to determine the amount of current that can safely flow through the transformer’s coil leads. To determine the maximum current, simply divide the VA value by the voltage.

for example, a 120v/24v 40va transformer would be capable of generating 1.66667 amps on the secondary.

40va/24v = 1.66667a

and 0.3333 amps in the primary

40va/120v = .3333a

Normally this is not a problem, since all the transformer supplies are switches and relays. if you install a thermostat that draws more current than the transformer can carry, you will have problems. so in this case you will need to upgrade the transformer and the fuses that protect it (since fuses are sized based on va rating).

On some systems, there will be separate transformers for the heating and cooling systems. In these situations, you will need to check with the thermostat manufacturer to determine which system should provide the c-wire. In the case of nest and honeywell (and probably others), their thermostats expect the c-wire to come from the cooling system.

When connecting the wires to the thermostat on these systems, you will need to remove the jumpers between the r terminals. connect the r wire from the heating system to r or rh, and the r wire from the cooling system to rc. then you will need to connect wire c from the cooling system to terminal c on the thermostat.

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