HVAC SYSTEMS

Criteria to be considered

• Temperature• Humidity• Air motion• Air purity or quality• Air changes per hour• Air and/or water velocity requirements• Local climate• Space pressure requirements• Capacity requirements, from a load calculation analysis• Redundancy

Criteria to be considered

• Spatial requirements• Security concerns• First cost• Operating cost, including energy and power costs• Maintenance cost• Reliability• Flexibility• Life-cycle analysis• Sustainability of design• Acoustics and vibration• Mild dew prevention

Additional Criteria

• Supporting a process, such as operation of computer equipment• Promoting a germ-free environment• Increasing sales• Increasing net rental income• Increasing property salability

System Constraints

• Performance limitations (e.g., temperature, humidity, space pressure)• Available capacity• Available space• Available infrastructure• Building architecture

Constructability Constraints

• Existing conditions• Maintaining existing building occupancy and operation• Construction budget• Construction schedule• Ability to phase HVAC system installation• Equipment availability (i.e., delivery lead times)

Selection Report

• The goal• Criteria for selection• Important factors, including advantages and disadvantages• Other goals• Security concerns• Basis of design• HVAC system analysis and selection matrix• System narratives• Budget costs• Recommendation

System Characteristics (Decentralized/Centralized)

• Temperature, Humidity, and Space Pressure Requirements.• Capacity Requirements.• Redundancy. • Facility Management.• Spatial Requirements. • First Cost.• Operating Cost.• Maintenance Cost.• Reliability• Flexibility. • Sound and Vibration

Decentralized Systems

• Decentralized systems can be one or more individual HVAC units, each with an integral refrigeration cycle, heating source, and direct or indirect outside air ventilation.

• Components are factory designed and assembled into a package that includes fans, filters, heating source, cooling coil, refrigerant compressor(s), controls, and condenser.

• Equipment is manufactured in various configurations to meet a wide range of applications.

Decentralized Systems

• Window air conditioners• Through-the-wall room HVAC units• Air-cooled heat pump systems• Water-cooled heat pump systems• Multiple-unit systems• Residential and light commercial split systems• Self-contained (floor-by-floor) systems• Packaged, special-procedure units (e.g., for computer rooms)

Advantages

• Heating and cooling can be provided at all times, independent of the mode of operation of other building spaces.• Manufacturer-matched components have certified ratings andperformance data.• Assembly by a manufacturer helps ensure better quality control and reliability.• Manufacturer instructions and multiple-unit arrangements simplify installation through repetition of tasks.• Only one zone of temperature control is affected if equipmentmalfunctions.• The system is readily available.

• One manufacturer is responsible for the final equipment package.

Advantages

• For improved energy control, equipment serving vacant spacescan be turned off locally or from a central point, without affectingoccupied spaces.• System operation is simple. Trained operators are not usuallyrequired.• Less mechanical and electrical room space is required than withcentral systems.• Initial cost is usually low.• Equipment can be installed to condition one space at a time as abuilding is completed, remodeled, or as individual areas are occupied,with favorable initial investment.• Energy can be metered directly to each tenant.

Disadvantages

• Performance options may be limited because airflow, cooling coilsize, and condenser size are fixed.• Larger total building installed cooling capacity is usually requiredbecause diversity factors used for moving cooling needs do notapply to dedicated packages.• Temperature and humidity control may be less stable, especiallywith mechanical cooling at very low loads.• Standard commercial units are not generally suited for large percentages of outside air or for close humidity control. Custom orspecial-purpose equipment, such as packaged units for computerrooms, or large custom units, may be required.• Energy use is usually greater than for central systems if efficiencyof the unitary equipment is less than that of the combined centralsystem components.

Disadvantages

• Air distribution control may be limited.• Operating sound levels can be high, and noise-producingmachinery is often closer to building occupants than with centralsystems.• Ventilation capabilities are fixed by equipment design.• Equipment’s effect on building appearance can be unappealing.• Air filtration options may be limited.• Discharge temperature varies because of on/off or step control.• Condensate drain is required with each air-conditioning unit.• Maintenance may be difficult or costly because of multiple piecesof equipment and their location.

Unitary equipment

• Single-zone, constant-volume• Multizone, constant-volume • Single-zone, variable-volume• Multisplit

Centralized Systems (ADV) • Primary cooling and heating can be provided at all times, independent of the operation mode of equipment and systems outside the central plant.• Using larger but fewer pieces of equipment generally reduces thefacility’s overall operation and maintenance cost. It also allowswider operating ranges and more flexible operating sequences.• A centralized location minimizes restrictions on servicing accessibility.• Energy-efficient design strategies, energy recovery, thermalstorage, and energy management can be simpler and more costeffective to implement.• Equipment operation can be staged to match load profile andtaken offline for maintenance.• Distinct cooling and heating can be provided.

• A central plant and its distribution can be economically expandedto accommodate future growth (e.g., adding new buildings to theservice group).• Load diversity can substantially reduce the total equipment capacityrequirement.• Submetering secondary distribution can allow individual billingof cooling and heating users outside the central plant.• Major vibration and noise-producing equipment can be groupedaway from occupied spaces, making acoustic and vibration controlssimpler. Acoustical treatment can be applied in a single locationinstead of many separate locations.• Issues such as cooling tower plume and plant emissions are centralized, allowing a more economic solution.

Disadvantages

• Equipment may not be readily available, resulting in long leadtimefor production and delivery.• Equipment may be more complicated than decentralizedequipment, and thus require a more knowledgeable equipmentoperator.• A central location within or adjacent to the building is needed.• Additional equipment room height may be needed.• Depending on the fuel source, large underground or surface storagetanks may be required on site. If coal is used, space for storagebunker(s) will be needed.

Disadvantages

• Access may be needed for large deliveries of fuel (oil or coal).• Heating plants require a chimney and possibly emission permits,monitoring, and treatments.• Multiple equipment manufacturers are required when combiningprimary and ancillary equipment.• System control logic may be complex.• First costs can be higher.• Special permitting may be required.• Safety requirements are increased.• A large pipe distribution system may be necessary (which mayactually be an advantage for some applications).

ALL-AIR HVAC SYSTEMS

• All-air systems provide sensible and latent cooling capacity solely through cold supply air delivered to the conditioned space.

• No supplemental cooling is provided by refrigeration sources within the zones and no chilled water is supplied to the zones.

• Heating may be accomplished by the same supply airstream, with the heat source located either in the central system equipment or in a terminal device serving a zone.

Single-duct, single-zone system

• Simple VAV.• VAV Reheat or VAV Dual Duct• VAV with Independent Perimeter System• VAV with Constant Zone Airflow Volume• VAV with Economizer Cycle

VAV system

Reheat system

DUAL-DUCT SYSTEMS

Dual-duct two-fan blow-through cycle

Single-fan dual-duct draw-through cycle with hot duct

reheat.

OTHER SYSTEMS

• MULTIZONE SYSTEMS• SIMPLE ROOFTOP SYSTEMS

Advantages• Major equipment is centrally located in dedicated servicespaces, which allows maintenance to take place in unoccupiedareas.• Major noise-generating equipment is centrally located in aspace that can be acoustically isolated, allowing for reasonablenoise control opportunities.• There is no condensate drain piping or HVAC power wiring inoccupied areas (as opposed to unitary or fan-coil systems).Among the specific advantages of all-air systems are:• Such systems are well suited to air-side economizer use, heatrecovery, winter humidification, and large-volume outdoor airrequirements.

• They are the best choice for close control of zone temperatureand humidity.• They are generally a good choice for applications where indoorair quality is a key concern.• They are amenable to use in smoke control systems.• There is simple seasonal changeover.• Such systems generally permit simultaneous heating and coolingin different zones.

Disadvantages

• All-air systems use significant amounts of energy to move air(approximately 40% of all-air system energy use is fanenergy).• Ductwork space requirements may add to building height.• Air balancing may be difficult.• It is difficult to provide comfort in locations with low outdoortemperatures and typical building envelope performance whenwarm air is used for perimeter heating.• Providing ready maintenance accessibility to terminal devicesrequires close coordination between mechanical, architectural,and structural designers.