Commercial HVAC Systems Flashcards

What is a 'Variable Air Volume' (VAV) system and how does it differ from a 'Constant Volume' (CAV) system?

Answer: VAV varies supply air volume to each zone while maintaining constant supply temperature; CAV delivers constant air volume at varying temperatures

A VAV system delivers conditioned air at constant supply temperature but modulates the volume delivered to each zone using VAV terminal boxes, matching supply to actual demand.

What is a 'chilled water system' in commercial HVAC?

Answer: A central system where a chiller produces chilled water distributed to air handling units throughout the building

A chilled water system uses a chiller to cool water to 44-48°F, which is pumped to air handling units throughout the building where it absorbs heat from building air.

In commercial HVAC, what is a 'cooling tower' and what process does it use to reject heat?

Answer: A heat rejection device that cools condenser water by evaporating a portion of the water into the air stream

A cooling tower rejects heat from the chiller's condenser water to the atmosphere through evaporative cooling—a small percentage of the circulating water evaporates, removing heat at approximately 1,000 BTU per pound of water evaporated.

What is a 'packaged rooftop unit' (RTU)?

Answer: A self-contained HVAC unit on the roof containing all heating and cooling components, serving commercial spaces through ductwork

A packaged RTU contains all components (compressor, condenser coil, evaporator coil, heating section, and air handling components) in one factory-assembled package, mounted on the roof.

What is an 'air handling unit' (AHU)?

Answer: A large central air treatment and distribution unit that conditions and circulates air throughout the building via ductwork

An AHU is a large centralized unit that conditions air through heating/cooling coils, filters, and humidification before distributing it through ductwork.

What is a 'building automation system' (BAS)?

Answer: An integrated control system that monitors, controls, and optimizes all building mechanical systems including HVAC, lighting, and elevators

A BAS centralizes monitoring and control of all building systems, enabling optimized operation, energy management, fault detection, and integration of HVAC, lighting, fire/life safety, and other systems.

What is 'economizer' mode in a commercial air handling system?

Answer: Using outdoor air for free cooling when outdoor conditions are cool enough, reducing or eliminating mechanical cooling requirements

An economizer maximizes outdoor air use for cooling when outside conditions are favorable—typically below about 55°F dry bulb or below the system's changeover enthalpy setpoint.

What is 'commissioning' of a commercial HVAC system?

Answer: A systematic quality-assurance process verifying that HVAC systems are installed and operating as designed to meet the owner's project requirements

Commissioning (Cx) is a structured quality assurance process from design through installation, startup, and operation that verifies all HVAC systems perform as specified.

In commercial HVAC, what does the term 'tons of refrigeration' mean?

Answer: A unit of cooling capacity equal to 12,000 BTU/hr (the heat needed to melt one ton of ice per day)

One ton of refrigeration equals 12,000 BTU/hr of cooling capacity—derived from the heat required to melt one ton (2,000 lbs) of ice in 24 hours.

What is 'heat recovery' in commercial HVAC and provide an example?

Answer: Recovering heat from the condenser to preheat domestic hot water instead of rejecting all heat to a cooling tower

Heat recovery captures heat that would otherwise be wasted—the most common commercial application is using chiller condenser heat to preheat domestic hot water, reducing boiler energy use.

What is a 'variable frequency drive' (VFD) and how does it save energy?

Answer: An electronic device that controls motor speed by varying electrical frequency, saving energy because power decreases with the cube of speed reduction

A VFD controls AC motor speed by varying electrical frequency. Because power follows the cube law (50% speed = 12.5% power), even small speed reductions yield dramatic energy savings.

What is 'coefficient of performance' (COP) as it applies to commercial chillers?

Answer: The ratio of cooling output energy to total electrical energy input

COP is the ratio of useful cooling output to total energy input—a chiller with COP 5 produces 5 units of cooling for every unit of electrical energy consumed.

What is the purpose of a 'cooling coil condensate drain pan' in an air handling unit?

Answer: To collect and drain condensation from the cooling coil to prevent water damage and biological growth

As warm humid air contacts the cold cooling coil, moisture condenses and drips into the drain pan below. The pan must be properly pitched, drained, and maintained to prevent mold and bacterial growth.

What is the 'oil return' consideration in a commercial DX system with long pipe runs?

Answer: Refrigerant velocity in suction lines must be maintained above minimum velocity to carry oil back to the compressor, especially in vertical risers

Compressor oil circulates with refrigerant throughout the system. Suction line velocity must be maintained above minimum (typically 700-1000 FPM in vertical risers) to entrain and carry oil droplets back to the compressor.

What is 'ASHRAE Standard 90.1' and why is it important for commercial HVAC?

Answer: Energy efficiency standard for commercial buildings specifying minimum equipment efficiencies and system design requirements

ASHRAE Standard 90.1 establishes minimum energy efficiency requirements for commercial buildings—forming the basis for most state and local energy codes.

What is a 'fan coil unit' (FCU) in commercial HVAC?

Answer: A small decentralized terminal unit with a fan, coil, and filter installed in individual rooms or zones

A fan coil unit is a compact terminal device containing a fan, heating/cooling coil(s), and filter, installed in individual spaces—common in hotels, apartments, hospitals, and offices served by central chilled and hot water systems.

What is 'total heat of rejection' in a chiller system?

Answer: The total heat rejected to the cooling tower, equal to the sum of cooling capacity plus compressor heat of compression

Total heat of rejection is the heat that must be removed from the condenser—it equals the cooling capacity plus the compressor work. For a 100-ton chiller, rejection is approximately 120 tons.

What is a 'dedicated outdoor air system' (DOAS) and what are its advantages?

Answer: A system that handles only 100% outdoor air ventilation separately from the space conditioning system, often with energy recovery

A DOAS handles 100% outdoor air ventilation with energy recovery and pre-conditioning, delivered to zone equipment—this separation improves humidity control, energy efficiency, and ventilation accuracy.

What does 'IPLV' (Integrated Part Load Value) measure for commercial chiller performance?

Answer: A weighted average efficiency across multiple part-load conditions that better represents real-world chiller operation

IPLV weights chiller efficiency at four operating conditions (100%, 75%, 50%, 25% load) to represent typical building operation, since chillers rarely operate at full design load.

What is the purpose of 'static pressure control' in a VAV system?

Answer: Controlling duct static pressure to ensure adequate pressure is available for all VAV boxes to meet their setpoints

Static pressure control modulates supply fan speed (via VFD) to maintain a duct static pressure setpoint—providing enough pressure for all open VAV boxes while minimizing fan energy at part load.