The answer to 'why is my HVAC system not heating' is that there are several common causes, including a malfunctioning thermostat, dirty air filters, issues with the furnace or boiler, or problems with the ductwork. Symptoms may include the system not turning on, poor airflow, or the house not reaching the desired temperature.
Some potential solutions include checking and replacing the air filter, resetting the thermostat, or inspecting the furnace or boiler for any issues. If the problem persists, it's best to call a professional HVAC technician to diagnose and repair the issue, as improper maintenance or repairs can be dangerous.