The wiring may also be switched for your fan settings. If you're comfortable doing so, you can try to switch the G wire, which controls the HVAC fan, with the Y wire, which often controls cooling.

Alternatively, you can screenshot your wiring diagram before you disconnect the Nest from the wall. Open the Nest app, tap the Settings cog in the upper-right corner, then select Equipment > Wiring.

2. Pop Off the Faceplate You’ll need to remove the faceplate of your old thermostat to get at the low-voltage wiring and backplate. It should pop right off with a little bit of effort.

Wiring adapter included: No; sold separately for $25 at Amazon, Best Buy, and Google. The fourth-generation Google Nest Learning Thermostat earns one of the top spots in our smart thermostat ratings.

“A heat pump thermostat is designed to have multiple options for heating your home,” Sherman says. “These are often referred to as ‘two heat, one cool’ thermostats, or similar verbiage.” Because heat ...