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Like a lot of smart home devices, smart thermostats usually need power even when the system is not activated. This is typically achieved through the use of a C-wire which completes the power circuit.

If your system doesn’t have one, you have options, read on!

Option 1 : Avoid rewiring
9.3 Venstar ACC0410 Add-A-Wire Accessory for All 24 VAC Thermostats (4 to 5 Wires), White
Top PickA hassle-free solution to the C-wire problem.
9.2 C-Wire Power Adapter
Runner upPerfect for all Honeywell smart thermostats.
8.6 24 Volt C-Wire Power Adapter/Transformer for Ecobee Nest Honeywell Smart Wifi Thermostat and Ring Doorbell by Fyve Global | 25 ft Cable
The easiest option for smart thermostats without adding a C-wire.
Option 2: Buy a Thermostat that does not require a C-wire
9.3 Honeywell Home RTH7560E 7-Day Flexible Programmable Thermostat-Extra-Large Backlit Display, White
Top PickOne of the best user-friendly thermostats.
9.2 Google Nest Learning Thermostat - Programmable Smart Thermostat for Home - 3rd Generation Nest Thermostat - Works with Alexa - Stainless Steel
Runner UpA robust energy-saving thermostat with top features.
9.0 Emerson Sensi Wi-Fi Smart Thermostat for Smart Home, DIY, Works With Alexa, Energy Star Certified, ST55
Smart home-compatible thermostat.
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What is a C-Wire on a thermostat?

There's a common misconception that C-wires help power the thermostat, and it's not entirely accurate. The wires that help power your thermostat are referred to as hot wires, and they provide a 24V source of energy to help your HVAC system run smoothly. 

A C-wire, on the other hand, helps provide a return path for the 24V power lighting up your thermostat. This lets the thermostat run efficiently without causing a disruption in any of the other switches. C-wires also come in handy when dealing with wireless thermostats because they provide a continuous power source instead of relying on batteries. That means you won't have to worry about battery drainage for reasons like Wi-Fi connections, etc. 

Thankfully, there are ways to get around the C-wire requirement for buyers looking to upgrade to smart thermostats. You have the option of adding a C-wire by rewiring your whole system – which will require the services of an HVAC technician.

The other two less-troublesome options are investing in a high-quality transformer (to convert your 4-wire system to a 5-wire system) or buying a thermostat that doesn't require a C-wire at all. 

At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.

Option 1 : Avoid rewiring

9.3
Top Pick
A hassle-free solution to the C-wire problem.

The Venstar Add-A-Wire accessory is a tailor-made solution for older homes without a C-wire connection. The ACC0410 can help you install your smart thermostat without having to resort to costly measures like getting in touch with an HVAC specialist.

Top Features
  • Extremely easy to install.
  • Suitable for 4-wire and 5-wire systems.
  • Compatible with almost all thermostats on the market.

The Venstar ACC0410 is designed to provide you additional wiring whenever the need arises – making it perfect for thermostat applications. That means this converter can help you turn your 4-wire system into a 5-wire one. Or turn your 5-wire system into a 6-wire one. 

Installation

What we liked best about the Venstar converter is its ease of installation. After turning the power supply to your old thermostat off, you'll need to find and disconnect the wires and re-attach them to the Venstar converter through the screw terminals. You have the option of mounting the Venstar converter by drilling holes or using some double-sided tape. 

Finally, the Venstar ACC0410 is compatible with almost all thermostats on the market, thanks to the converter's built-in circuit board that helps divide the voltage running through the existing lines, allowing another wire to act as the C-wire.

9.2
Runner up
Perfect for all Honeywell smart thermostats.

If you're a fan of Honeywell thermostats and are looking for a way to add one to your household without adding a c-wire, then say hello to the Honeywell C-Wire Power Adapter.

Top Features
  • Compatible with all Honeywell thermostats.
  • Compact enough to fit in tight spaces.
  • Easy to set up and install.

This Honeywell converter is custom-built to work with the brand's thermostats to make them functional for homeowners without a C-wire. The converter's overall dimensions measure 2.6 x 2.6 x 1 inches, and its weight is approximately 5 ounces – meaning you don't have to worry about the product sticking out like a sore thumb. 

Setup

Speaking of sticking out, the Honeywell Power Adapter is incredibly easy to set up and can be mounted via push terminals or adhesive tape. 

Apart from being economical, the Honeywell adapter includes a 4-wire input and a K terminal that provides for multiplexed signals. While installation does require wiring, there's a detailed manual to help you through the process, and customers find the process easy to handle – even for beginners.

8.6
The easiest option for smart thermostats without adding a C-wire.

The C-wire AC transformer is perfect for folks looking for an option for their ecobee, Nest, or Honeywell thermostats without the hassle of adding a C-wire.

Top Features
  • Works with ecobee, Nest, and Honeywell thermostats.
  • 20 feet of cable for ease of use.
  • Minimal wiring required.

Plus, the adapter comes with 20 feet of cable to ensure there's no trouble setting up even if your outlet is nowhere near your device. All you need to do is shut off the power at your breaker box and install one of the wires from the adapter to the RC and C terminal. Then plug in the power adapter to the nearest outlet, turn the power back on, re-attach the faceplate, and you're done!

The C-wire AC transformer also comes equipped with a jumper wire (for HVAC systems with cooling) and a user manual to make sure the installation process is trouble-free.

Option 2: Buy a Thermostat that does not require a C-wire

9.3
Top Pick
One of the best user-friendly thermostats.

An easy-to-install thermostat with advanced capabilities and no C-wire required!

Top Features
  • Doesn't require a common wire.
  • 7-day scheduling.
  • Smart response learning.

If you're looking to get benefits like 7-day scheduling, smart response learning, or a sizable backlit display – without dealing with the battery drains brought on by Wi-Fi thermostats – you need to check out the Honeywell Home RTH7560E. 

Another advantage the product offers is that it can run without a C-wire. That means you can upgrade from your old thermostat without any complications like requiring the services of an HVAC technician or complicated wiring procedures.

Batteries

The Honeywell Home RTH7560E thermostat relies on two AA batteries that can run the device for up to 7 months without requiring a change. And, in the event that your batteries do need changing, you'll be alerted ahead of time, with automatic alerts. 

The Honeywell Home thermostat offers features like 7-day scheduling so you can map out your cooling or heating needs for the entire week in advance and tweak the setup if changes occur. 

Features

The thermostat can also pick up on your cooling or heating needs and adjust the temperature according to your usage. So, if the weather turns from warm to cool at a certain point, you can expect the Honeywell thermostat to adjust accordingly – without you having to move a muscle. 

There are also benefits like auto-alerts for filters and low batteries, a clock that'll automatically adjust for daylight savings time, and a backlit display that can display temperature in Celsius or Fahrenheit. 

Finally, the Honeywell Home thermostat is compatible with most types of cooling, heating, and heat pump systems.

What we liked
  • Auto-reminders for filter change and low battery.
  • Runs on AA batteries.
  • Large backlit display with reader-friendly numbers.
What we didn't like
  • Some buyers have complained about the product's triangular wall-plate.
9.2
Runner Up
A robust energy-saving thermostat with top features.

One of the best-looking thermostats available commercially, that's Energy Star-certified, and easy to install.

Top Features
  • Doesn't require a C-wire to function.
  • Energy-certified.
  • Can be paired with Nest's Power Connector should batteries not be enough

The Google Nest Learning thermostat doesn't need much introduction because it's a pretty big hit among buyers. And it's easy to see why. 

Let's start with the fact that the Nest Learning Thermostat doesn't require a C-wire to function. That means you can upgrade your old thermostat with this product without any problems because it runs on CR2 batteries. 

Power Connector

However, on the odd chance that your HVAC system doesn't do too well with a battery-operated thermostat – you can choose to invest in Nest's Power Connector to give your thermostat a continuous power source. The other option is to add another C-wire to your existing system – which is costly and time-consuming. Furthermore, since both the thermostat and the power connector are of the same brand – compatibility won’t be an issue. 

Now that we've covered how you can tackle any C-wire-related troubles with the Nest Thermostat – let's move on to discussing some of the product's best features. 

Savings

With energy costs that seem to keep rising, the Nest Learning thermostat brings you an Energy-certified way to cut 10% on heating and 15% on cooling bills. What's best is, you don't need to do much to save on energy – the thermostat studies your temperature schedule and builds a power-saving version around your preferences. 

Plus, the thermostat uses your phone's location and sensors to monitor your location, and if you've left home – it will automatically set the Eco-Temperature setting to stop wasting energy. 

Installation

The product is also easy to install. You can have your Nest Thermostat set up and ready to use in thirty minutes or less. And, you can gain remote access to the thermostat with Amazon Alexa and the app.

What we liked
  • Energy-saving with features like Eco-friendly and energy history usage.
  • Offers remote access via voice assistant.
  • Includes smart scheduling function.
What we didn't like
  • Slightly pricier than other options on the market.
9.0
Smart home-compatible thermostat.

A power-saving, smart home- friendly thermostat that has no C-wire requirements.

Top Features
  • Doesn't require a C-wire for most HVAC systems.
  • Energy-certified.
  • Compatible with SmartThings and HomeKit platforms.

We've saved the Sensi Smart Thermostat for last because it doesn't require a C-wire for conventional heating and cooling systems, such as electric/gas furnace, air conditioner, or boilers. 

However, the thermostat does require common wiring for heat or cool only systems, along with heat pumps. 

Converter

In that case, you'll have to purchase an adapter or converter separately to ensure the product's batteries don't drain out too quickly. Try scrolling through the section above to learn your choices for a compatible adapter for the Sensi thermostat. 

You'll be glad to know that this thermostat is Energy-certified, like the Nest Learning thermostat. And it can help you cut back on your energy bill by almost 23%. The thermostat also offers features like 7-day scheduling, geofencing, and remote access via the app. 

Platforms

The Sensi Thermostat is also smart home-compatible and will sync with Samsung SmartThings and Apple HomeKit platforms. However, you should note that for HomeKit compatibility, you will need a C-wire connection. 

Like all top-tier thermostats, the Sensi ST55 is easy to install with a step-by-step tutorial on the app. The product also offers smart alerts for extreme humidity or temperature readings for safety.

What we liked
  • Easy installation in up to 30 minutes.
  • Offers necessary features like 7-day scheduling, geofencing, etc.
  • Ease of access through in-app and voice assistant compatibility.
What we didn't like
  • May require the use of a separately purchased adapter for heat or cool only systems and heat pumps.

The Verdict

Homeowners settled in old-fashioned homes don't need to dread the lack of a C-wire when it comes to thermostats anymore. You can always upgrade your older thermostat with a new and improved one by either opting to purchase an adapter/ converter to switch your 4-wire set up to a 5-wire set up. 

Or, you can buy a smart thermostat that runs on batteries and does not need a common wire.

9.3
Top Pick
A hassle-free solution to the C-wire problem.

The Venstar Add-A-Wire accessory is a tailor-made solution for older homes without a C-wire connection. The ACC0410 can help you install your smart thermostat without having to resort to costly measures like getting in touch with an HVAC specialist.

Top Features
  • Extremely easy to install.
  • Suitable for 4-wire and 5-wire systems.
  • Compatible with almost all thermostats on the market.
Need Help with a Smart Thermostat?
Get a Free Estimate from a Trusted Local Pro
Installation · Setup · Maintenance · Repairs · Replacements

Sources

https://sensi.emerson.com

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