CNET published a DIY guide to installing a smart thermostat, and most of its value sits in the checks you do before you buy anything. We went through it with an installer’s eye. Here is what it says and what we would do with it.
What the guide says
The author checked with the nonprofit Energy Trust of Oregon, which stressed that the thermostat has to be compatible with your heating and cooling system. That matters most if you have a multistage system, or one that lets you set airflow to low, medium and high.
The guide then lists wiring checks. The new thermostat goes in the same spot as the old one, and the wiring there varies from house to house:
- C-wire: the common wire is a low-voltage wire that can power a thermostat. Some models require it. The guide says the Nest Learning Thermostat does not. Adapters exist, and Ecobee offers a kit, but the guide calls that a higher level of electrical work.
- High-voltage wiring: the average smart thermostat cannot handle high-voltage current. If you have large electric baseboards or an electric furnace, your home may not be ready. Look for a 120 volt or higher label, or L1/L2 contact points.
- Stranded wire: some thermostats, Nest among them, do not support stranded wire connections. The guide says these are typically rare.
It adds that companies offer online compatibility checkers, but you will probably need to take the old unit off the wall to know for certain.
The install steps, briefly
The guide’s order of work is sensible. Cut power at the breaker, then change the setpoint to force the system to call and confirm it does not start. Photograph the wiring and its labels (Y1, G, W1 and so on) before disconnecting anything, and tag wires if colors repeat. Pull the old unit, feed the wires through the new mounting plate, level it, reconnect each wire, snap on the thermostat face, restore power and finish setup in the app over Wi-Fi. Expect firmware updates right away. The new unit may be a different size, so you may need paint or a trim kit to cover the old marks.
On professional installs, the guide says that can cost up to $200 on average depending on location, and that some brands, Nest named, have offered free installation with a new model.
What it means for an installed system
- Look before you shop. Pop the old cover and photograph the wiring first. Whether you have a C-wire and what the terminals say will narrow your choices faster than any review.
- Line voltage means stop. If the existing unit is labeled 120 volts or has L1/L2, a typical smart thermostat is the wrong product.
- Multistage systems deserve a second look. Confirm the specific model supports your stages before you order.
- Plan the integration separately. The guide covers hardware and the manufacturer’s app, not Home Assistant or other platforms. Check platform support for the exact model before buying.
The guide is aimed at homeowners, and its message is fair: the wiring is usually manageable if you document it. The catch is in the compatibility checks. Do those first, and the rest is an afternoon.