The answer to 'why is my HVAC system not heating' is that there are several common causes, including a thermostat issue, a problem with the heating unit itself, or a lack of airflow. Some common symptoms are the system not turning on, the house not getting warm enough, or the system cycling on and off frequently.
If your HVAC system is not heating properly, first check the thermostat to ensure it is set to 'heat' and the temperature is set higher than the current room temperature. You should also check the air filters and replace them if they are dirty. If those simple fixes don't work, the issue may be with the heating unit, such as a problem with the furnace, boiler, or heat pump. In those cases, it's best to call in a professional HVAC technician to diagnose and repair the problem.