How to Upgrade to a Smart Thermostat without Changing the Wiring

Steps:

1. Start by turning off the service switch at the furnace.
2. Remove the old thermostat.
a. Pull off the front cover.
b. Unscrew the screws inside.
c. Disconnect the wires.
d. Pull off the back cover.
3. Mount the hub in a convenient location near the furnace using a drill and a few screws.
4. Disconnect the wiring from the thermostat to the furnace and run that wiring to the hub. There’s an obvious place on the hub to screw the wires in properly.
5. Run the thermostat wire from the furnace to the hub. The red wire goes to R, the white wire goes to W, and the blue wire (common wire) goes to C.
6. Mount the new thermostat back cover at the location of the old one. This might require drilling for new screws.
7. Using the old two wires, wire the new thermostat and connect it to the back cover.
8. Turn the service switch back on at the furnace.
9. Download the smart app and program it to the thermostat following the manufacturer’s instructions.

Pro Tip: When removing the old thermostat, you may find a paint outline where the previous unit was mounted. If the new thermostat is a different shape, the manufacturer often includes a trim plate that can cover the old marks — but if you have matching paint on hand, you can skip the trim plate for a cleaner look.

One of the biggest advantages of upgrading to a smart thermostat is the GPS functionality built into your smartphone. As demonstrated on This Old House, the thermostat uses your phone’s GPS to detect when the last person leaves the house, automatically lowering the temperature. It also works in reverse — as you approach home, the system senses you’re on your way and begins adjusting the temperature so the house is comfortable when you arrive.

Resources:

Richard installed the InvitaTM Wifi Thermostat 564, which is manufactured by Tekmar Controls.

For this project, Richard used a wireless approach: a receiver wired at the heating unit communicates wirelessly with the thermostat, which can be hung on any wall. He also connected a device called a gateway to a local router, enabling any web-based device — a computer, smartphone, or tablet — to control the heating system remotely. This setup avoids the need to run new thermostat wires through finished walls.

Pro Tip: When upgrading to a smart thermostat, the C wire (common wire) is the most critical connection. As explained during a This Old House project, the common wire provides a 24-volt power supply to the thermostat’s brain, allowing it to actually function. Without it, you’re stuck relying on batteries — which, as Richard has noted, is “a pain in the butt for a thermostat.” That dedicated power supply is what makes smart features like Wi-Fi connectivity and remote control possible.

Shopping List:

Smart thermostat
Thermostat wire

Tools: