Supply register (vent)Conditioned air blows into the room here.
Return grille (return register)Room air is drawn back into the system through it.
FilterIt catches dust before the air reaches the blower and coil.
Return ductIt carries room air back to the air handler.
Trunk duct (main supply duct)The main supply duct; smaller ducts branch off it.
Branch ductIt takes part of the air to one room's outlet.
Supply plenumIt passes air from the unit into the supply ducts.
Equipment in a house system 6
ThermostatIt switches the system on and off to hold a set temperature.
Outdoor unit (condenser)It holds the compressor, the condenser coil and a fan.
Air handlerThe indoor unit whose blower moves the air through the system.
Evaporator coilCold refrigerant inside it takes heat out of the passing air.
Refrigerant linesTwo copper lines join the indoor coil to the outdoor unit.
Condensate drainIt carries away water that condenses on the cold coil.
Rooftop unit 8
Roof curbA raised frame on the roof that the unit sits on.
CompressorIt pumps refrigerant between the two coils.
Condenser fanIt pulls outside air through the coil at the end.
Condenser coilIt gives the building's heat off to the outside air.
Supply fan (evaporator fan)It blows the conditioned air down the supply duct.
Outdoor-air hood (fresh air hood)Outside air enters the unit under this cover.
EconomizerIts dampers bring in cool outside air to cool the building.
Gas heating section (gas burner section)Gas burns here to heat the air in winter.
How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it. House system: University of Florida IFAS Extension, Energy Efficient Homes: The Duct System (https://edis.ifas.ufl.edu/publication/FY1024); US Department of Energy, Energy Saver pages Central Air Conditioning, Minimizing Energy Losses in Ducts and Air Conditioner Maintenance (Internet Archive copies: https://web.archive.org/web/2025/https://www.energy.gov/energysaver/central-air-conditioning, https://web.archive.org/web/2025/https://www.energy.gov/energysaver/minimizing-energy-losses-ducts, https://web.archive.org/web/2025/https://www.energy.gov/energysaver/air-conditioner-maintenance); Pacific Northwest National Laboratory, Building America Solution Center, Traditional Split Heat Pumps (https://basc.pnnl.gov/resource-guides/traditional-split-heat-pumps); Wikipedia, Duct (flow), Air handler and Heating, ventilation, and air conditioning (https://en.wikipedia.org/wiki/Duct_(flow), https://en.wikipedia.org/wiki/Air_handler, https://en.wikipedia.org/wiki/Heating,_ventilation,_and_air_conditioning); US Navy, Utilitiesman 3 and 2 (1973), for the plenum (https://archive.org/details/ERIC_ED097442). Rooftop unit: two unrelated makers' installation manuals for packaged rooftop units (https://archive.org/details/manualsbase-id-636446 and https://archive.org/details/manualsbase-id-7604); Wikipedia, Air handler and Economizer (https://en.wikipedia.org/wiki/Economizer). Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; a schematic of no maker's product, nothing traced. Licence: own work
How it works
The parts of a residential HVAC system are an air handler, a coil, an outdoor unit and the ducts that carry the air. The first drawing cuts a house through to show them. The second opens a commercial rooftop unit.
Read the full guide
Parts of a central air conditioning system, in the order the air goes
Room air is drawn in at the return grille and down the return duct. It passes the filter and enters the air handler, where the blower pushes it over the evaporator coil. Cooled air fills the supply plenum, runs along the trunk duct and up each branch duct to a supply register. Refrigerant lines join the coil to the outdoor unit.
Parts of a commercial HVAC system on the roof
A rooftop unit holds everything in one cabinet on a roof curb. Outside air enters at the hood and the economizer. The supply fan sends air down the supply duct, past the gas heating section. The compressor, condenser coil and condenser fan are in the far end.
In winter a furnace or a heat pump supplies the heat, and the same ducts and registers carry it.
Questions and answers
What are the main parts of an HVAC system in a house?
A thermostat, an air handler or furnace with a blower and filter, an evaporator coil, an outdoor unit, refrigerant lines between them, and ducts: a return side that brings room air back and a supply side that ends at the registers.
What are the parts of a condenser?
The condenser is the outdoor unit. Its cabinet holds the compressor, the condenser coil and a fan. The air conditioner page on this site opens one up and names each part.
What is the difference between a supply register and a return grille?
Conditioned air blows out of a supply register into the room. A return grille takes room air back in, and it goes down the return duct to the filter and the air handler.
What is a rooftop unit?
A packaged unit: the coil, blower, compressor, condenser and often a gas heating section sit in one cabinet on the roof. Ducts pass down through the roof curb into the building.