HVAC System Architecture
How a complete commercial HVAC system fits together — from the central plant (cooling tower, chiller, boiler, VRF) through the air handling unit and air distribution to the zone terminals, hydronic pumps, condensate drainage, and the building management system. Hover, tap, or focus any component or connection for its description and code/standard reference.
Hover, tap, or focus any component on the drawing (or a circuit below it) for details. Click to pin; move away or click again to clear.
Component Reference
Every component in the drawing above, with its function and the code/standard reference that governs it.
Central Plant
Cooling Tower
A cooling tower rejects heat from the condenser-water loop to the outdoor air by evaporation, with make-up water replacing what is lost to evaporation, drift, and blowdown.
📘 ASHRAE 90.1 / CTIChiller
The chiller produces chilled water for the building cooling coils by removing heat through the refrigeration cycle; an air-cooled chiller rejects that heat directly to outdoor air.
📘 AHRI 550/590Boiler
The boiler heats hot water for the heating coils and zone reheat, distributing it through the hot-water supply and return loop.
📘 ASHRAE 90.1 / ASME BPVCExpansion Tank
The expansion tank absorbs the volume change of water as it heats and cools, holding the hydronic loop at a stable pressure and preventing relief-valve discharge.
📘 ASHRAE Systems & Equip.VRF Outdoor Unit
The VRF condensing unit modulates compressor capacity and routes refrigerant to multiple indoor units, allowing simultaneous heating and cooling with high part-load efficiency.
📘 AHRI 1230 / ASHRAE 15Air Handling Unit (AHU)
Mixing Box & Dampers
The mixing box blends return air with outdoor air through modulating dampers, enabling economizer “free cooling” when outdoor conditions allow and holding the minimum ventilation rate.
📘 ASHRAE 62.1 / 90.1Filters
Air filters remove particulates from the airstream to protect equipment and improve indoor air quality; filtration efficiency is rated by MERV per ASHRAE 52.2.
📘 ASHRAE 52.2Cooling Coil (CHW)
A chilled-water (or direct-expansion) cooling coil cools and dehumidifies the supply air; the moisture it removes drips into a pan and leaves through the condensate drain.
📘 AHRI 410Heating Coil (HW)
A hot-water (or electric) heating coil warms the supply air during heating mode and morning warm-up, fed from the boiler’s hot-water loop.
📘 AHRI 410Humidifier
The humidifier adds moisture to the supply air to maintain space humidity setpoints during dry winter conditions.
📘 ASHRAE 62.1Supply Fan
The supply fan moves conditioned air through the ductwork to the zones; on a VAV system it rides a variable frequency drive to track duct static pressure and save energy at part load.
📘 AMCA 210 / ASHRAE 90.1Air Distribution
Supply & Return Ductwork
Ductwork distributes conditioned air to and from the zones. It is sized by the Manual D / equal-friction method and sealed and insulated to SMACNA standards.
📘 SMACNA / ASHRAE 90.1VAV Terminal Unit
A variable-air-volume terminal box modulates the airflow delivered to a zone to match its cooling load, typically adding a reheat coil for heating and low-load conditions.
📘 ASHRAE 90.1 / Gl. 36Reheat Coil
The reheat coil warms the cool central supply air at the zone terminal when a space calls for heating, after the central system has cooled and dehumidified the air.
📘 ASHRAE 90.1 §6.5.2CAV Supply Diffuser
A constant-air-volume supply diffuser delivers a fixed airflow into the occupied space, distributing supply air for even temperature and comfort.
📘 ASHRAE 55Exhaust Air Grille
Exhaust grilles remove stale or contaminated air from the space and route it outdoors, balancing the outdoor-air intake.
📘 ASHRAE 62.1 / IMCReturn Air Grille
Return grilles collect room air and route it back through the return ductwork to the AHU to be re-conditioned or relieved.
📘 ASHRAE 62.1Terminal / Zone Equipment
Fan Coil Unit (FCU)
A fan coil unit is a local terminal with a fan and a chilled/hot-water coil that conditions a single zone, without requiring central ducted supply air.
📘 AHRI 440VRF Indoor Unit (Ducted)
A ducted VRF indoor unit is served by refrigerant from the VRF outdoor unit and provides independent zoned heating or cooling.
📘 AHRI 1230Pumps & Drainage
Chilled Water Pumps
Chilled-water pumps circulate water between the chiller and the cooling coils. Variable-speed (primary-secondary or variable-primary) pumping cuts energy use at part load.
📘 ASHRAE 90.1 §6.5.4Hot Water Pumps
Hot-water pumps circulate heated water between the boiler and the heating and reheat coils throughout the building.
📘 ASHRAE 90.1 §6.5.4Condensate Drain System
The condensate system carries water from the cooling-coil drain pan through trapped piping to a floor drain, preventing overflow and microbial growth.
📘 IMC §307Controls & BMS
Building Management System (BMS)
The BMS is the supervisory controller that schedules equipment, trends data, manages alarms, and optimizes energy across the HVAC system from a central interface.
📘 ASHRAE Gl. 13 / 36Field Controllers
Field controllers are distributed DDC controllers that run the local control loops for AHUs, VAV boxes, and plant equipment, reporting to the BMS over a protocol such as BACnet.
📘 BACnet (ASHRAE 135)Sensors
Temperature, humidity, differential-pressure, and CO₂ sensors supply the feedback the controllers use to hold setpoints, reset supply conditions, and modulate ventilation.
📘 ASHRAE Gl. 36Connections & Flows
The air, refrigerant, and hydronic flows that tie the system together — each shown as a colored line in the diagram above.
Supply Air
Conditioned air delivered from the air handling unit through the supply ductwork to the zones.
📘 ASHRAE 55 / 62.1Return Air
Air drawn back from the zones to the AHU to be re-conditioned, relieved, or exhausted.
📘 ASHRAE 62.1Outdoor Air
Fresh ventilation air brought in to meet ASHRAE 62.1 minimum outdoor-air rates and to provide economizer cooling.
📘 ASHRAE 62.1Exhaust Air
Air discharged from the building to remove heat, humidity, odors, and contaminants.
📘 ASHRAE 62.1 / IMCRefrigerant (Cooling)
Refrigerant piping carrying the working fluid in cooling mode between DX and VRF equipment.
📘 ASHRAE 15 / AHRIRefrigerant (Heating)
Refrigerant piping in heating mode, where a heat pump or VRF system reverses the cycle to deliver heat indoors.
📘 ASHRAE 15Condensate Drain
Gravity drainage of cooling-coil condensate through trapped piping to a floor drain.
📘 IMC §307Hot Water
The hydronic hot-water supply/return loop circulating heat from the boiler to the heating and reheat coils.
📘 ASHRAE 90.1Chilled Water
The hydronic chilled-water supply/return loop circulating cooling from the chiller to the cooling coils.
📘 ASHRAE 90.1