Why is Your Nest Thermostat Not Heating? (How to Fix Your Nest Thermostat). In today’s video, we will be talking about why your Nest thermostat is not heating. • Battery issues A low or malfunctioning battery in your thermostat can lead to a poor internet connection and may interfere with the thermostat's HVAC system. This results in a thermostat that won't heat. If your Nest thermostat has battery issues, you may notice a low battery message or a blinking red light. • Poor internet connection Poor WiFi signal strength interferes with the standard functionality of your nest thermostat, including not heating. • Blocked AC filters The AC filters trap dirt particles; however, air fails to pass through when clogged, leading to a defective thermostat. • Malfunctioning sensor A malfunctioning sensor in your thermostat is a recipe for a defective thermostat, which can be repaired by replacing it.