Everything you need to know about thermostat C wire |
Posted: November 25, 2020 |
Modern thermostats using backlit display and Wi-Fi require a supply of power. This is what the C wire does. The R (red) wire supplies power but not continuously. To ensure a continuous supply, a common wire is required to complete the circuit. If you are planning to install a smart thermostat on your own, make sure that the system already has a C-wire. If it has one, it might be tucked behind the current thermostat if not in use. You need to use a new cable if there is no C wire. Thermostat cable is a bundle of color-coded wires. You can buy thermostat cables in different weights. You need 18/3 for heat and 18/5 for heat and AC. Identifying a Thermostat CableYou can identify the cable by looking behind the thermostat. Remove it from the wall and see all the wired connections. Check the terminal. If you can find a wire with the label “C”, you can install a smart thermostat. There you will also find screws with labels R, C, W, W2, G, Y/Y2. What if a furnace comes without a C-wire hooked up? You need to do the following if the HVAC system is not compatible with the thermostat:
What are the standards for wire color? You can use any wire for any purpose. When you look behind the thermostat, you may find that all are not the same.
Do you need a C wire If you want to upgrade your thermostat, you need a C wire. It does not matter whether your thermostat is smart or not, if it is a modern one, it requires a source of power. There are some manufacturers who claim that their thermostats don't need a C-wire to work. However, power comes from the cooling or heating system. Keep in mind that the cooling or heating system must be running. When the cooling or heating system is not running, their thermostats pulse the heat wire. This turns on the furnace and it pulls a bit of power. Some manufacturers even warn that their products may not be compatible with some single cycle systems with no C-wire. However, outdated wiring becomes a headache. Future Proofing
The “mercury blob” thermostats are not that common now. Even no-frills thermostats require a power source. You can use an adapter or new wiring to add a C-wire and ensure compatibility with the latest technology. Fan Wire TrickThe fan wire can be repurposed as a power wire. However, you will not be able to turn the fan on manually. You can buy thermostat cable, adapter and other required supplies online. Make sure that you are using quality wires only.
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