No C-Wire? Check Venstar Add-A-Wire!

Wi-Fi thermostats are power-hungry beasts, and they need to be fed extra juice if they’re going to do everything they’re advertised to. This is why you’ve probably heard that you need a C-wire to power your new thermostat.

Venstar ACC0410 Add-A-Wire Accessory for All 24 VAC Thermostats (4 to 5 Wires), White

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If you don’t have an extra wire going into the back of your thermostat, then you might have some serious issues installing your new wireless smart thermostat.

The Venstar Add-A-Wire Adapter promises to fix the problem without putting extra holes in your wall or necessitating a call to an HVAC technician. We took a closer look at this gadget to see just how well it can replace a real C-wire.

Problems & Solutions

A common wire, also called a C-wire, is an extra line going into the back of a thermostat to provide additional power. Older thermostats either had dials or simple LCD screens that didn’t need much voltage. Smart thermostats have sophisticated schedulers and wireless Internet connectivity, so they need more power.

If you own even a slightly older home, then the chances are that you don’t have an extra wire and can’t readily use a modern wireless thermostat.

The Venstar Add-A-Wire claims to be able to hook up to your existing HVAC system and provide this additional juice without having to run an additional wire. It’s basically a converter that turns a four wire system into a five wire one.

What is the Venstar Add-a-Wire?

At first glance, the Add-A-Wire is nothing more than a small electrical box with mounting holes and a few wires coming out of the bottom of it. The box can attach to your HVAC system and connect to all four wires at the source. You’ll then be able to run the five wires from the bottom of the unit to your new thermostat.

Circuit boards inside of the box can split the voltage that’s fed through your existing lines. This allows it to power another line that’s free to act as a C-wire. It does this completely by itself, so you won’t have anything to calibrate or work on.

While you might know Venstar as a manufacturer of wireless thermostats, the Add-A-Wire is designed to work with all different kinds of models. Since it just splices some wires and doesn’t do anything else, it’s compatible with nearly every thermostat on the market that needs a C-wire.

What Are the Alternatives?

The simplest way to get around the lack of a C-wire is to select a thermostat that doesn’t require one. Both the ecobee with a Power Extender Kit and the Emerson Sensi with a Common Wire Kit work without a C-wire.

Unsupported hacks also provide an out for users willing to try them out. Power stealing, as the term implies, involves letting your thermostat suck a little bit of electricity from your HVAC system to recharge an onboard battery.

Some prefer to instead wire a 24VAC transformer to their thermostat to give it juice. While you can also use the so-called fan wire hack to turn your G-wire into a C-wire, this would leave you without manual fan control.

Ease of Installation

Unlike any of the hacks, the Add-A-Wire doesn’t cost you any sort of functionality and it’s fairly easy to install. When you first open the box you should find:

  • Venstar’s patented converter box with the wires already attached
  • Some extra wires
  • Printed paper manual
  • Four wire nuts

It shouldn’t take much more than 45-50 minutes to install the converter box. A pair of pliers, a screwdriver, and some electrical tape is all you’ll need beyond what’s already in the box. Turn off the power for safety’s sake before your start. Trace the wires coming into the back of your old thermostat so you know where they come from.

Once you’ve found the point of origin for these wires, disconnect them and attach them to the screw terminals on the Add-A-Wire box. Find a safe place to mount the box and firmly secure it in place.

A couple of pre-drilled holes on the side of the unit make this easy. The adapter’s design is sturdy enough that it shouldn’t move after it’s mounted. If you ever needed to move it in the future, then it’s easy enough to unscrew it and disconnect it.

You’ll then be able to route the five wires coming out of the bottom of the Venstar electrical box through to your thermostat. The included manual offers a few different wiring diagrams, so you can be sure that you won’t use the ability to control the house fan or do anything else you could with your original thermostat.

While many users find they don’t have any problems with the installation, Venstar did put together a short video that shows the process in detail:

A couple of people have mentioned that the physical size of the adapter makes it hard to find the right place to put it. Otherwise, homeowners who have fixed a light switch or an electrical outlet shouldn’t have any other trouble with getting it hooked up.

Final Verdict

Venstar’s Add-A-Wire Adapter is easy enough for most DIY-inclined homeowners to install, and it’s far more effective than using a trick like the fan wire hack.

If you’ve bristled at the thought of paying a professional HVAC technician to install a C-wire for your new thermostat, then you might find it suits your needs perfectly.

Emerson Sensi and ecobee users who have access to the kits available for those thermostats probably won’t need an Add-A-Wire since they do the same job of splicing a four wire system for you.

Those who’d rather not have to mess with a transformer nor an Add-A-Wire splicer might look at these as good alternatives. You can get these thermostats with the kits included, so you wouldn’t have to worry about ordering any accessories separately.

Users looking to get everything in a single package might prefer this route. Otherwise, Venstar’s solution is ideal for many situations. The fact that it’s not tied to any single brand makes it an attractive option no matter what sort of smart thermostat you plan to install.

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