HVAC Systems
System types and sizing
Know the main system types: forced-air furnaces, boilers, heat pumps (air-source and ground-source), and ductless mini-splits. Understand that sizing must follow a load calculation (Manual J or equivalent), not rules of thumb. Oversizing causes short cycling, poor humidity control, and comfort complaints. Undersizing fails to meet load on design days.
Distribution and efficiency
Duct design matters: return paths, static pressure, and leakage. A system can have a high-efficiency unit and still perform poorly because of duct leakage or restricted airflow. Know AFUE (furnace), SEER2/HSPF2 (heat pumps and AC), and that these rated efficiencies are not delivered efficiency. Airflow should generally be about 400 CFM per ton of cooling; low airflow causes coil freezing and high static pressure.
Traps
Questions often imply that replacing equipment alone fixes comfort. In reality, envelope and duct work usually deliver more. Another trap: confusing AFUE with combustion efficiency. Also watch for refrigerant charge and airflow being ignored; a system with correct charge but low airflow still underperforms.
Worked example
A 3-ton air conditioner should move roughly 1,200 CFM. If measured airflow is 900 CFM, that is 300 CFM per ton, which is low and may cause coil icing and reduced capacity. Check filter, duct sizing, and blower settings before assuming the equipment is faulty.
Controls and commissioning
Understand thermostats, zoning, and commissioning steps: verify charge, airflow, and temperature rise (for furnaces, typically 25-55°F depending on the unit).