The answer to 'why is my furnace not heating' is that there are several common causes, including a malfunctioning thermostat, clogged air filters, issues with the pilot light or ignition, and problems with the furnace's blower motor or other components. Symptoms can include the furnace not turning on at all, blowing cold air, or making strange noises.
Some issues can be fixed through basic DIY troubleshooting and maintenance, but more complex problems usually require the expertise of a professional HVAC technician. It's important to address furnace issues promptly to ensure your home stays warm and safe throughout the winter.