Why the Common Wire (C-Wire) May Come With a Cost

Why the Common Wire (C-Wire) May Come With a Cost

by ENA Team

Perhaps you’ve heard of the C Wire (aka the common wire), a power source frequently found in thermostat installations. This wire provides a continuous power flow of 24 V to the thermostat. In the past, this power source wasn’t necessary because thermostats were simple devices without WiFi radios, backlighting, displays and other onboard technology which draw energy. And if you don’t have a C Wire, most smart thermostats won’t be able to operate without a costly retrofit. If you’re considering ENASTAT, however, worry not; it was engineered to operate without one, saving you time and money. More on that later.  

First things first, let’s take a look at a couple of ways by which you can identify if your system has a C Wire. 

Option One: Check the Wires Behind Your Thermostat

After shutting off power at the breaker, detach your thermostat from the wall so you can examine the wiring behind it. If there’s a wire connected to the terminal labeled “C” on the back of your existing thermostat, you should be good to go. But if you don’t see one initially, check to see if the prior installer tucked the wire into the wall. 

Option Two: Check the Wires Inside Your Furnace

As a reminder, make sure power to the unit is disconnected. Once you remove the furnace cover (check your manual if this step’s process is unclear), and look for a row of screws with labels such as W, G, R, C, etc. If you don’t see the C port, you won’t be able to run most smart thermostats (again with the notable exception of ENASTAT) without running a new cable from your furnace to your thermostat.  

Wiring Tips

Something of note as you’re considering a thermostat installation: there’s no standardization of wire color, unfortunately, so you’ll have to examine the labels closely. Below are some of the common color applications for thermostat wiring: 

  • Blue or Black – C – Common wire
  • Red – R – 24VAC power from the furnace’s transformer
  • Red – Rc – 24VAC (dedicated to heating)
  • Red – Rc – 24VAC (dedicated to cooling)
  • Green – G – Fan
  • White – W – Heat
  • Yellow – Y – Air conditioner

What happens without a C Wire? 

Without the presence of a common wire, a thermostat may “pulse” the heat wire to get power when the HVAC system is not running, prompting your furnace to kick on. In some systems this may not create an issue, but in others it may cause the on/off cycling of your unit which induces wear and tear and higher energy bills. 

What can be done? 

If you don’t have a C-Wire and you’re considering a smart thermostat, one option is to hire a professional to install and run the wires between your unit and your thermostat. In larger commercial and industrial applications, this can potentially cost thousands of dollars. There are some adapters on the market which can make certain models compatible without a C-Wire, but we recommend checking out ENASTAT and chatting with a member of our team about how it can work in your application. 

The ENASTAT Difference 

We designed ENASTAT from the ground-up to avoid the “pulsing” phenomenon previously mentioned. ENASTAT draws a minimal amount of power from the system when it’s running to keep the internal battery charged, avoiding the need to turn on the unit unnecessarily to cooperate. ENASTAT will determine the optimum current draw by monitoring the HVAC system’s power supply, then communicating that data to its cloud-based analytics system so it can form an “energy budget.” Overall, the ENASTAT design is power efficient and based on a multiprocessor structure that can be turned on/off depending on the operational mode. Additionally, there’s no power-hungry visual display both for purposes of durability and security. This intelligent ”inside-out” design means it doesn’t draw more power than is needed at any given time, providing an easy install and no expensive retrofit costs.




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