Your AC died on a 95-degree July afternoon. Your heat stopped working the morning after Christmas. HVAC problems always pick the worst possible time. The DMV area has brutal summers and cold winters, so a good HVAC tech is worth knowing before you actually need one.
A solid HVAC company does more than swap the broken part. They check refrigerant levels, test airflow, inspect the heat exchanger for cracks (a cracked exchanger leaks carbon monoxide - not a "we'll fix it next time" situation), and tell you honestly whether the system is worth repairing or getting close to replacement age.
For repairs, expect $150-$500 for most common fixes. New systems run $5,000-$12,000 installed for most homes. Red flags: anyone who insists on a full replacement without diagnostics, or quotes a price significantly below everyone else. Ask if they're NATE-certified and licensed in Maryland/Virginia/DC.