The answer to 'HVAC system not heating' is that there could be several common causes, such as a malfunctioning thermostat, a problem with the heating unit itself, or issues with the ductwork or airflow. Symptoms may include the system not turning on, the house not reaching the desired temperature, or uneven heating throughout the home.
Some potential solutions include checking the thermostat settings, replacing the air filter, and inspecting the heating unit for any issues. However, if the problem persists or you are unsure of the cause, it's best to call a professional HVAC technician to diagnose and repair the issue properly.