How to Upgrade to a Smart Thermostat with Four Wires

This video is part of This Old House: Live—an initiative that provided daily programming and opportunities to connect with our experts during the pandemic.

Upgrading to a smart thermostat is an excellent way to modernize your home’s heating and cooling system and potentially save on energy costs. These devices offer precise temperature control, smartphone apps, and technology that can learn your routines.

Most smart thermostats require five wire connections, but some older thermostats only have four. If your home falls into this category, you can often use a workaround to finish your upgrade.

In this guide, we’ll walk you through the process of installing a smart thermostat with four wires, using insights from This Old House expert Ross Trethewey. In the video above, Trethewey demonstrates the installation of a Google Nest Thermostat E, offering valuable tips and tricks along the way.

Understanding Smart Thermostats

Smart thermostats are advanced temperature control devices that connect to your home’s Wi-Fi network. No matter the manufacturer, here’s what smart thermostats typically offer.

• Remote temperature control via smartphone apps
• Learning algorithms that adapt to your schedule
• Energy usage reports
• Integration with other smart home devices

These features can improve comfort and potentially save energy. However, before purchasing a smart thermostat, make sure it’s compatible with your existing HVAC system and wiring. 

One critical compatibility factor is the C wire (common wire). Smart thermostats require a constant 24-volt power supply to run their processors and Wi-Fi radios, and the C wire provides exactly that. Without it, you may be stuck relying on batteries, which can drain quickly and cause reliability issues. If your existing thermostat wiring includes a blue wire, that’s typically your C wire — and it’s the most important wire for making a smart thermostat work.

Smart thermostats also represent a major leap over older programmable models, which many homeowners never fully utilized. Thanks to occupancy sensors and geofencing features that detect your comings and goings, smart thermostats reset temperatures automatically — no programming required. Motion sensing enables auto-scheduling: when the thermostat detects activity, it knows you’re home, and when it doesn’t sense motion over a period of time, it knows you’re away and adjusts accordingly. Additional features like smart fan circulation can even balance temperatures between rooms by running the fan to mix air between hot and cold zones.

Preparing to Install a Smart Thermostat

When possible, we recommend installing a new thermostat when the weather is fairly mild and you don’t have urgent heating and cooling needs. In case something goes wrong, you won’t be at the mercy of the elements. 

Checking Compatibility

First, familiarize yourself with your current thermostat’s wiring. Most systems use a standard color-coding system for wires, but it’s always best to remove the faceplate and double-check.

Smart thermostats typically require a C-wire (common wire) for constant power. This is typically the fifth wire in a five-wire system. However, not all homes have this wire installed. In the video, Ross demonstrates a workaround for four-wire systems lacking a C-wire. We’ll walk you step-by-step through the process below.

When Ross encountered a four-wire system lacking a C-wire on Ask This Old House, he explored several potential solutions: finding the splice where wires were disconnected (which he described as “like a needle in a haystack”), running an entirely new wire from the thermostat to the attic, or repurposing the fan wire to serve as the common wire. As Ross explained, running new wire can be “really challenging because we’re getting through two wall cavities” — through the first-floor wall cavity, through the second-floor wall cavity, and then up to the attic.

Gathering Tools and Materials

Before you begin the installation process, gather the necessary tools and materials:

• Screwdriver(s): In this case, Trethewey needed both a Phillips head and a small flathead screwdriver
• Smart thermostat kit: Make sure you have the baseplate and mounting screws.
• Smartphone or tablet: You’ll need this to set up the new system.
• Multimeter or voltmeter: This tool is optional, but handy to ensure your safety.

Before starting any work on your thermostat, always turn off the power to your HVAC system at the circuit breaker. This step is crucial for your safety and to prevent damage to your system. Use a multimeter or voltmeter to check that the power is indeed off.

Removing the Old Thermostat

The first step in installing your new smart thermostat is removing the old one. Here’s how to do it safely.

1. Turn off the power to your HVAC system at the circuit breaker.
2. Remove the cover of your old thermostat.
3. Take a clear photo of the current wiring configuration for reference.
4. Remove the backup batteries.
5. Unscrew the mounting screws holding the thermostat to the wall.
6. Gently tug the wires to give yourself some slack.
7. Carefully disconnect the wires from the terminals, labeling them if necessary.

Pro tip: Remove the mounting screws before disconnecting the wires to prevent them from falling into the wall.

At this point, you’ll see whether your old thermostat has four or five wires. If it has five wires, the process to connect the new smart thermostat is pretty straightforward. With four wires, you’ll need the workaround.

Either way, don’t throw out your old thermostat yet. If the new one has connection problems, you can temporarily reinstall the old one so you can still control your HVAC system while you troubleshoot.

Installing the New Smart Thermostat

With the old thermostat removed, you’re ready to install your new smart thermostat. 

Installing the Baseplate

Depending on the brand, your new thermostat may look a little different, but here are the steps Trethewey used.

1. Mount the baseplate of the smart thermostat to the wall using the provided screws.
2. Connect the wires to the appropriate terminals on the baseplate.
3. Attach the thermostat to the baseplate.
4. Turn the power back on at the circuit breaker.
5. Follow the manufacturer’s instructions to set up and configure the thermostat.

If the old thermostat left a visible paint outline on the wall, many smart thermostat manufacturers include a trim plate that covers the mark. However, if you have matching paint on hand, you may get a cleaner look by skipping the trim plate entirely and touching up the wall before mounting the new baseplate.

Wiring the Smart Thermostat

Proper wiring is crucial for the functionality of your smart thermostat. In general, wire connections are as follows:

• W (white wire): Heating
• Y (yellow wire): Cooling
• R (red wire): Power from HVAC system
• G (green wire): Fan
• C (blue wire): Common wire (constant power)

If your system lacks a C-wire, Trethewey recommends using the G wire (typically for fan control) as the C-wire, connecting it to the C terminal on both the thermostat and the furnace. 

While this method sacrifices independent fan control, it provides the constant power needed for the smart thermostat to function properly. Otherwise, you’ll need to replace the thermostat’s internal battery frequently, and most people don’t use the “fan only” mode anyway.

Before repurposing any wires, verify continuity between the thermostat and the furnace or air handler. Using a multimeter set to continuity mode, test the wire you plan to repurpose by touching probes across it at both ends. You should hear a beep confirming a complete circuit. If you get an open loop reading — indicated by “OL” on the display and no beep — it means the wire is spliced or broken somewhere in the wall, and you’ll need to choose a different spare wire or consider running new cable.

Finishing Installation

The Google Nest thermostat snaps back onto the baseplate by means of a pin connector; it doesn’t require tools. Other brands may require screws or other connectors. Once the thermostat is attached, you can turn the power back on and follow the manufacturer’s setup instructions. Note that smart thermostats are essentially computers, so they require time to boot up.

Configuring Your Smart Thermostat

Once your smart thermostat is installed and powered on, you’ll need to configure it. This process typically involves:

1. Connecting the thermostat to your home’s Wi-Fi network
2. Setting up an account with the manufacturer’s app
3. Inputting information about your HVAC system
4. Setting your preferred temperature schedules

Trethewey recommends initially turning off all advanced features and deleting pre-programmed schedules. This approach allows you to familiarize yourself with the basic functions before gradually enabling more advanced features. If you have difficulty at this step, most manufacturers provide a helpline for thermostat setup.

Troubleshooting Common Smart Thermostat Issues

While installing a smart thermostat is generally straightforward, you may encounter some challenges. Here are some common issues and their solutions:

• Thermostat not powering on: Double-check all wire connections and ensure the C-wire (or its substitute) is properly connected.
• Wiring issues: If your wiring is degraded or the connections still aren’t working, contact a licensed electrician or HVAC technician.
• Wi-Fi connection problems: Ensure your home’s Wi-Fi signal is strong at the thermostat’s location. You may need a Wi-Fi extender if the signal is weak.
• Incorrect temperature readings: Allow the thermostat time to calibrate. If issues persist, consult the manufacturer’s troubleshooting guide.

Maximizing Your Smart Thermostat’s Potential

To get the most out of your new smart thermostat, consider these tips:

• Maintain a consistent heating or cooling schedule for the first week to allow the thermostat’s smart features to learn your schedule.
• Use geofencing capabilities to automate temperature changes when you leave or return home.
• Regularly review energy reports to identify potential savings opportunities.
• Integrate your thermostat with other smart home devices for enhanced automation.

Our Conclusion

Installing a smart thermostat is a relatively simple DIY project that can lead to improved comfort and potential energy savings. By following the steps outlined in this guide, most homeowners can successfully upgrade to a smart thermostat in about an hour. Remember to prioritize safety, double-check wire connections, and take your time to ensure a proper installation.