5.9 HVAC Systems and Components
Perimeter Outside Air Ventilation Systems. Perimeter ventilation units shall be self-contained DX package units or air-handling units with fan section having variable speed drive, chilled water cooling coil, hot water heating coil, enthalpy heat recovery wheel, or desiccant wheel and supply air filtration. The perimeter ventilation units shall provide 100-percent outside air. Reheat shall be hot gas bypass, a heat pipe or a run around coil. Chilled water shall be generated by an air-cooled chiller or a 24-hour chiller. If a desiccant wheel is used for controlling the specific humidity discharge at the wheel, condenser reheat shall be used for regeneration of the desiccant, along with minimum electric backup. Supply air dew point leaving the unit shall be maintained at 10°C (50°F) and the supply air dry bulb temperature leaving the air-handling unit shall be a minimum of 21.1°C (70° F) and not greater than 25.6°C (78° F) during occupied hours. During occupied hours, this unit shall operate to deliver conditioned ventilation air and maintain positive pressure in the perimeter zone with respect to outside air pressure.
During unoccupied hours, the unit shall run at 40 percent of its capacity to provide conditioned air at 10°C (50° F) dew point and at least 21.1°C (70°F) to help maintain positive pressure in the perimeter zone with respect to outside air. In both the occupied and unoccupied modes the system shall operate to adjust the airflow as required to maintain a differential positive pressure in the perimeter zone relative to the prevailing pressure outside the building. When the outside air dew point drops below 2.8°C (37°F), the unit shall have the capacity to maintain neutral pressure with respect to the outside by exhausting relief air from the return duct system. The ventilation unit shall have self-contained microprocessor controls capable of connecting to and interoperating with a BACnet or LONWORKS direct digital control (DDC) Building Automation System. It shall also be equipped with dampers to set the design airflow through the unit, and also an analog or digital display which measures and displays the amount of air flowing through the unit continuously.
Interior Outside Air Ventilation Systems. Interior ventilation units shall be self-contained DX packaged units or air-handling units with chilled water-cooling coil, hot water heating coil, and supply air filtration. Interior ventilation units shall incorporate enthalpy heat recovery wheel or desiccant wheel, heating coil, and a cooling coil. Heat recovery shall include use of building relief and exhaust air. Utilize condenser waste heat for desiccant regeneration. The supply air from the ventilation units shall be ducted to the return plenum section of the air handling unit(s) serving the interior zones. Supply air dew point leaving the unit shall be maintained at 10°C (50°F) and the supply air dry bulb temperature shall be a minimum of 21.1°C (70° F) and not greater than 25.6°C (78° F). During occupied hours, this unit shall operate to provide conditioned ventilation air. The unit shall be inoperative during unoccupied hours. The unit shall have air-monitoring devices to indicate that the supply air is always 10 percent greater than the exhaust/relief air. The dedicated ventilation unit shall have self-contained microprocessor controls capable of connecting to and interoperating with a BACnet or LONWORKS Direct Digital Control (DDC) Building Automation System. It shall also be equipped with dampers to set the design airflow through the unit, and also an analog or digital display which measures and displays the amount of air flowing through the unit continuously.
Fan Coil System. For perimeter spaces, provide four-pipe fan coil units with cooling coil, heating coil, 35 percent efficiency filters, internal condensate drain, and overflow drain. Unit shall have self-contained microprocessor controls and shall be capable of connecting to and interoperating with a BACnet or LONWORKS Direct Digital Control (DDC) Building Automation System. Fan coil units shall be capable of operating with unit mounted or remote mounted temperature sensor.
Fin Tube Heating Systems. When fin-tube radiation is used, reheat should not be featured with perimeter air distribution systems. Fin-tube radiation shall have individual zone thermostatic control capable of connecting to a self-contained microprocessor that can interface with a BACnet or LONWORKS Direct Digital Control (DCC) Building Automation System.
Variable Volume System with Shutoff Boxes. Variable Air Volume (VAV) systems with full shutoff VAV boxes shall be used for perimeter zone applications only. VAV shutoff boxes shall be used only with the perimeter air distribution systems in order to eliminate the need for reheat. The air-handling unit and associated VAV boxes shall have self-contained microprocessor controls capable of connecting to and interoperating with a Direct Digital Control (DDC) Building Automation System.
Variable Volume System with Fan-Powered Boxes. Variable air volume (VAV) systems with fan-powered VAV boxes may be used for both perimeter and interior zone applications. The air-handling unit and associated VAV boxes shall have self-contained microprocessor controls capable of connecting to and interoperating with a BACnet or LONWORKS Direct Digital Control (DDC) Building Automated System. Fan powered boxes shall be equipped with a ducted return, featuring a filter/filter rack assembly and covered on all external exposed sides with two-inches of insulation. The return plenum box shall be a minimum of 61 mm (24 inches) in length and shall be double wall with insulation in-between or contain at least one elbow where space allows. Fan-powered boxes may have hot water heating coils used for maintaining temperature conditions in the space under partial load conditions. Fan powered boxes located on the perimeter zones and on the top floor of the building shall contain hot water coils for heating.
Underfloor Air Distribution System. Underfloor air distribution systems shall incorporate variable air volume (VAV) units designed to distribute the supply air from under the floor using variable volume boxes or variable volume dampers running out from underfloor, ducted, main trunk lines. Air shall be distributed into the space through floor-mounted supply registers that shall be factory fabricated with manual volume control dampers. Supply air temperature for underfloor systems shall be between 10°C (50°F) dew point and 18°C (64°F) dry bulb. For perimeter underfloor systems, provide fan coil units or fin tube radiators located beneath the floor with supply air grilles or registers mounted in the floor. The air-handling unit, VAV boxes, and variable volume dampers shall have self-contained microprocessor controls capable of connecting to and interoperating with a BACnet or LONWORKS direct digital control (DDC) Building Automation System. The maximum zone size of an underfloor air distribution system shall not exceed 2,360 l/s (5,000 CFM).
Underfloor Air Displacement System. Underfloor air displacement systems shall incorporate variable air volume (VAV) units designed to distribute the supply air from under the floor using variable volume boxes or variable volume dampers running out from underfloor, ducted, main trunk lines. The VAV boxes or control dampers shall be hard ducted or connected directly to the main trunk lines. Air shall be distributed into the occupied space through floor-mounted, low-turbulence, displacement flow, swirl diffusers and shall contain a dust collection basket situated below the floor. Supply air temperature for underfloor systems shall be 10°C (50°F) dew point and 18°C (64°F) Dry Bulb. For perimeter underfloor systems, provide fan coil units or fin tube radiators located beneath the floor with supply air grilles or registers mounted in the floor. The air-handling unit, VAV boxes, and variable volume dampers shall have self-contained microprocessor controls capable of connecting to and interoperating with a BACnet or LONWORKS Direct Digital Control (DDC) Building Automation System. The maximum capacity of an underfloor air distribution system shall not exceed 2,360 l/s (5,000 cfm).
Heat Pump Systems. Console perimeter heat pump system(s) may be considered for the perimeter zone. For the interior zone either a packaged heat pump variable volume system or a central station air handling unit with cooling-heating coil with VAV boxes shall be considered. Condenser water loop temperatures shall be maintained between 15°C (60°F) and 27°C (80°F) year round, either by injecting heat from a gas fired, modular boiler if the temperature drops below 15°C (60°F) or by rejecting the heat through a cooling tower if the temperature of the loop rises above 35°C (95°F) dry bulb. Outside air shall be ducted to the return plenum section of the heat pump unit. Heat pumps shall be provided with filter/filter rack assemblies upstream of the return plenum section of the air-handling unit.
HVAC System Components
Air-Handling Units (AHU’s). Air-handling units shall be sized to not exceed 11,800 l/s (25,000 cfm). Smaller units are encouraged to facilitate flexible zone control, particularly for spaces that involve off-hour or high-load operating conditions. To the extent possible, “plug-n-play” AHU configurations should be considered, facilitating easy future adaptations to space-load changes. Psychrometric analyses (complete with chart diagrams) shall be prepared for each air-handling unit application, characterizing full and part load operating conditions. Air-handling unit/coil designs shall assure that conditioned space temperatures and humidity levels are within an acceptable range, per programmed requirements, and ASHRAE Standards 55 and 62.
Depending on sensible heat ratio characteristics, effective moisture control may require cooling coil air discharge dew point temperatures as low as 10°C (50°F). As required, provide face-by-pass or heat recovery features to re-heat cooling coil discharge temperatures for acceptable space entry. Provide a direct form of re-heat and/or humidification only if space conditions require tight environmental control, or if recurring day-long periods of unacceptable humidity levels would otherwise result.
Supply, Return and Relief Air Fans: Centrifugal double width double-inlet forward curved and airfoil fans are preferable for VAV systems. All fans shall bear the AMCA seal and performance shall be based on tests made in accordance with AMCA Standard 210. Fans should be selected on the basis of required horsepower as well as sound power level ratings at full load and at part load conditions. Fan motors shall be sized so they do not run at overload anywhere on their operating curve. Fan operating characteristics must be checked for the entire
range of flow conditions, particularly for forward curved fans. Fan drives shall be selected for a 1.5 service factor and fan shafts should be selected to operate below the first critical speed. Thrust arresters should be designed for horizontal discharge fans operating at high static pressure.
Coils: Individual finned tube coils should generally be between six and eight rows with at least 2.1 mm between fins (12 fins per inch) to ensure that the coils can be effectively and efficiently cleaned. Dehumidifying coils shall be selected for no more than negligible water droplet carryover beyond the drain pan at design conditions. All hot water heating and chilled water cooling coils shall be copper tube and copper finned materials. Equipment and other obstructions in the air stream shall be located sufficiently downstream of the coil so that it will not come in contact with the water droplet carryover. Cooling coils shall be selected at or below 2.5 m/s face velocity (500 fpm) to minimize moisture carryover. Heating coils shall be selected at or below 3.8 m/s face velocity (750 fpm).
Drains and Drain Pans: Drain pans shall be made of stainless steel, insulated and adequately sloped and trapped to assure drainage. Drains in draw-through configurations shall have traps with a depth and height differential between inlet and outlet equal to the design static pressure plus 2.54 mm (1 inch) minimum.
Filter Sections: Air filtration shall be provided in every air handling system. Air-handling units shall have a disposable pre-filter and a final filter. The filter media shall be rated in accordance with ASHRAE Standard 52. Pre-filters shall be 30 percent to 35 percent efficient. Final filters shall be filters with 85 percent efficiency capable of filtering down to 3.0 microns per ASHRAE 52. Filter racks shall be designed to minimize the bypass of air around the filter media with a maximum bypass leakage of 0.5 percent.
Filters shall be sized at 2.5 m/s (500 FPM) maximum face velocity. Filter media shall be fabricated so that fibrous shedding does not exceed levels prescribed by ASHRAE 52. The filter housing and all air-handling components downstream shall not be internally lined with fibrous insulation. Double-wall construction or an externally insulated sheet metal housing is acceptable. The filter change-out pressure drop, not the initial clean filter rating, must be used in determining fan pressure requirements. Differential pressure gauges and sensors shall be placed across each filter bank to allow quick and accurate assessment of filter dust loading as reflected by air-pressure loss through the filter and sensors shall be connected to building automation system.
UVC Emitters/Lamps: Ultraviolet light (C band) emitters/lamps shall be incorporated downstream of all cooling coils and above all drain pans to control airborne and surface microbial growth and transfer. Applied fixtures/ lamps must be specifically manufactured for this purpose. Safety interlocks/features shall be provided to limit hazard to operating staff.
Access Doors: Access Doors shall be provided at air handling units downstream of each coil, upstream of each filter section and adjacent to each drain pan and fan section. Access doors shall be of sufficient size to allow personnel to enter the unit to inspect and service all portions of the equipment components.
Plenum Boxes: Air-handling units shall be provided with plenum boxes where relief air is discharged from the air handling unit. Plenum boxes may also be used on the return side of the unit in lieu of a mixing box. Air-flow control dampers shall be mounted on the ductwork connecting to the plenum box.
Mixing Boxes: Air-handling units shall be provided with mixing boxes where relief air is discharged from the air handling unit.Mixing boxes may also be used on the return side of the unit in lieu of a plenum box. Air flow control dampers shall be mounted within the mixing box or on the ductwork connecting to the mixing box.
Terminals. VAV terminals shall be certified under the ARI Standard 880 Certification Program and shall carry the ARI Seal. If fan-powered, the terminals shall be designed, built, and tested as a single unit including motor and fan assembly, primary air damper assembly and any accessories.
VAV terminals shall be pressure-independent type units.
Units shall have BACnet or LONWORKS self-contained controls.
Fan-powered terminals: Fan-powered terminals shall utilize speed control to allow for continuous fan speed adjustment from maximum to minimum, as a means of setting the fan airflow. The speed control shall incorporate a minimum voltage stop to ensure the motor cannot operate in the stall mode.
All terminals shall be provided with factory-mounted direct digital controls compatible and suitable for operation with the BAS.
Air Delivery Devices. Terminal ceiling diffusers or booted-plenum slots should be specifically designed for VAV air distribution. Booted plenum slots should not exceed 1.2 meters (4 feet) in length unless more than one source of supply is provided. “Dumping” action at reduced air volume and sound power levels at maximum m3/s (cfm) delivery should be minimized. For VAV systems, the diffuser spacing selection should not be based on the maximum or design air volumes but rather on the air volume range where the system is expected to operate most of the time. The designer should consider the expected variation in range in the outlet air volume to ensure the air diffusion performance index (ADPI) values remain above a specified minimum. This is achieved by low temperature variation, good air mixing, and no objectionable drafts in the occupied space, typically 150 mm (6 inch) to 1830 mm (6 feet) above the floor. Adequate ventilation requires that the selected diffusers effectively mix the total air in the room with the supplied conditioned air, which is assumed to contain adequate ventilation air.
Motors. All motors shall have premium efficiency as per ASHRAE 90.1. 1/2 HP and larger shall be polyphase. Motors smaller than 1/2 HP shall be single phase. For motors operated with variable speed drives, provide insulation cooling characteristics as per NEC and NFPA.
Boilers. Boilers for hydronic hot water heating applications shall be low pressure, with a working pressure and maximum temperature limitation as previously stated, and shall be installed in a dedicated mechanical room with all provisions made for breeching, flue stack and combustion air. For northern climates, a minimum of three equally sized units shall be provided. Each of the three units shall have equal capacities such that the combined capacity of the three boilers shall satisfy 120 percent of the total peak load of heating and humidification requirements. For southern climates, a minimum of two equally sized units at 67 percent of the peak capacity (each) shall be provided. The units shall be packaged, with all components and controls factory preassembled. Controls and relief valves to limit pressure and temperature must be specified separately. Burner control shall be return water temperature actuated and control sequences, such as modulating burner control and outside air reset, shall be utilized to maximum efficiency and performance.Multiple closet type condensing boilers shall be utilized, if possible. Boilers shall have self-contained microprocessor controls capable of connecting to and interoperating with a BACnet or LONWORKS Direct Digital Control (DDC) Building Automated System. Boilers shall have a minimum efficiency of 80 percent as per ASHRAE 90.1.
Individual boilers with ratings higher than 29 MW (100 million Btu/hour) or boiler plants with ratings higher than 75 MW (250 million Btu/hour) are subject to review by the Environmental Protection Agency.
Boilers shall be piped to a common heating water header with provisions to sequence boilers on-line to match the load requirements. All units shall have adequate valving to provide isolation of off-line units without interruption of service. All required auxiliaries for the boiler systems shall be provided with expansion tanks, heat exchangers, water treatment and air separators, as required.
Gas Trains: Boiler gas trains shall be in accordance with International Risk Insurance (IRI) standards.
Automatic Valve Actuators: Gas valve actuators shall not contain NaK (sodium-potassium) elements since these pose a danger to maintenance personnel.
Venting: Products of combustion from fuel-fired appliances and equipment shall be delivered outside of the building through the use of breeching, vent, stack and chimney systems. Breeching connecting fuel-fired equipment to vents, stacks and chimneys shall generally be horizontal and shall comply with NFPA 54. Vents, stacks and chimneys shall generally be vertical and shall comply with NFPA 54 and 211. Breeching, vent, stack, and chimney systems may operate under negative, neutral, or positive pressure and shall be designed relative to the flue gas temperature and dew point, length and configuration of the system, and the value of the insulation techniques applied to the vent. Venting materials may be factory fabricated and assembled in the field and may be double or single wall systems depending on the distance from adjacent combustible or noncombustible materials. Material types, ratings and distances to adjacent building materials shall comply with NFPA 54 and 211.
Heat Exchangers. Steam-to-water heat exchangers shall be used in situations where district steam is supplied and a hot water space heating and domestic hot water heating system have been selected. Double-wall heat exchangers shall be used in domestic hot water heating applications. Plate heat exchangers shall be used for waterside economizer applications.
Chillers. Chillers shall be specified in accordance with the latest Air-conditioning and Refrigeration Institute (ARI) ratings procedures and latest edition of the ASHRAE Standard 90.1. As a part of the life cycle cost analysis, the use of high-efficiency chillers with COP and IPLV ratings that exceed 6.4 (0.55 kW/ton) should be analyzed. Likewise, the feasibility of gas-engine driven chillers, ice storage chillers, and absorption chillers should be considered for demand shedding and thermal balancing of the total system.
BACnet or LONWORKS Microprocessor-based controls shall be used. The control panel shall have self-diagnostic capability, integral safety control and set point display, such as run time, operating parameters, electrical low voltage and loss of phase protection, current and demand limiting, and output/input-COP [input/output (kW/ton)] information.
Chilled water machines: When the peak cooling load is 1760 kw (500 tons) or more, a minimum of three chilled water machines shall be provided. The three units shall have a combined capacity of 120 percent of the total peak cooling load with load split percentages 40-40-40 or 50-50-20. If the peak cooling load is less than 1760 kW (500 tons), a minimum of two equally sized machines at 67 percent of the peak capacity (each) shall be provided. All units shall have adequate valving to provide isolation of the off-line unit without interruption of service. Cooling systems with a capacity less than 50 tons shall use air cooled chillers.
Chillers shall be piped to a common chilled water header with provisions to sequence chillers on-line to match the load requirements. All required auxiliaries for the chiller systems shall be provided with expansion tanks, heat exchangers, water treatment and air separators, as required. If multiple chillers are used, automatic shutoff valves shall be provided for each chiller.
Chiller condenser bundles shall be equipped with automatic reversing brush-type tube cleaning systems.
Chiller condenser piping shall be equipped with recirculation/bypass control valves to maintain incoming condenser water temperature within chiller manufacturer’s minimum. Part load efficiency must be specified in accordance with ARI Standard 550/590.
The design of refrigeration machines must comply with Clean Air Act amendment Title VI: Stratospheric Ozone Protection and Code of Federal Regulations (CFR) 40, Part 82: Protection of Stratospheric Ozone.
Chlorofluorocarbon (CFC) refrigerants are not permitted in new chillers. Acceptable non-CFC refrigerants are listed in EPA regulations implementing Section 612 (Significant New Alternatives Policy (SNAP) of the Clean Air Act, Title VI: Stratospheric Ozone Protection. (Note: GSA accepts these criteria in documenting certification of LEED ratings. )
Refrigeration machines must be equipped with isolation valves, fittings and service apertures as appropriate for refrigerant recovery during servicing and repair, as required by Section 608 of the Clean Air Act, Title VI. Chillers must also be easily accessible for internal inspections and cleaning.
Ice Storage Equipment. Ice-on-coil systems shall be considered in locations where the demand costs of electricity are greater than $15.00 per kW (demand costs for peak generation, transmission, and delivery costs), including prefabricated tanks with glycol coils and water inside the tank. The tank shall be insulated and its capacity and performance shall be guaranteed by the vendor. Self-contained, fabricated ice storage system shall have self-contained BACnet LONWORKS microprocessor controls for charging and discharging the ice storage system and capable of being connected to a central building automation system. Other types of ice storage systems are not permitted.
Cooling Towers. Multiple cell towers and isolated basins are required to facilitate operations, maintenance and redundancy. The number of cells shall match the number of chillers. Supply piping shall be connected to a manifold to allow for any combination of equipment use. Multiple towers shall have equalization piping between cell basins. Equalization piping shall include isolation valves and automatic shutoff valves between each cell. Cooling towers shall have ladders and platforms for ease of inspections and replacement of components. Variable speed pumps for multiple cooling towers shall not operate below 30 percent of rated capacity.
Induced draft cooling towers with multiple-speed or variable-speed condenser fan controls shall be considered. Induced draft towers shall have a clear distance equal to the height of the tower on the air intake side(s) to keep the air velocity low. Consideration shall be given to piping arrangement and strainer or filter placement such that accumulated solids are readily removed from the system. Clean-outs for sediment removal and flushing from basin and piping shall be provided.
Forced draft towers shall have inlet screens. Forced draft towers shall have directional discharge plenums where required for space or directional considerations. Consideration shall be given to piping arrangement and strainer or filter placement such that accumulated solids are readily removed from the system. Clean-outs for sediment removal and flushing from basin and piping shall be provided. The cooling tower’s foundation, structural elements and connections shall be designed for a 44 m/s (100 MPH) wind design load. Cooling tower basins and housing shall be constructed of stainless steel. If the cooling tower is located on the building structure, vibration and sound isolation must be provided. Cooling towers shall be elevated to maintain required net positive suction head on condenser water pumps and to provide a 4-foot minimum clear space beneath the bottom of the lowest structural member, piping or sump, to allow reroofing beneath the tower.
Special consideration should be given to de-icing cooling tower fills if they are to operate in sub-freezing weather, such as chilled water systems designed with a water-side economizer. A manual shutdown for the fan shall be provided. If cooling towers operate intermittently during sub-freezing weather, provisions shall be made for draining all piping during periods of shutdown. For this purpose indoor drain down basins are preferred to heated wet basins at the cooling tower. Cooling towers with waterside economizers and designed for year-round operation shall be equipped with basin heaters. Condenser water piping located above-grade and down to 3 feet below grade shall have heat tracing. Cooling towers shall be provided with BACnet LONWORKS microprocessor controls, capable of connecting to central building automation systems.
Chilled Water,Hot Water, and Condenser Water Pumps. Pumps shall be of a centrifugal type and shall generally be selected to operate at 1750 RPM. Both partial load and full load must fall on the pump curve. The number of primary chilled water and condenser water pumps shall correspond to the number of chillers, and a separate pump shall be designed for each condenser water circuit. Variable volume pumping systems should be considered for all secondary piping systems with pump horsepower greater than 10 kW (15 HP). The specified pump motors shall not overload throughout the entire range of the pump curve. Each pump system shall have a standby capability for chilled, hot water, and condenser water pumps.
Each boiler cooling tower and chiller group pumps shall be arranged with piping, valves, and controls to allow each chiller-tower group to operate independently of the other chiller and cooling tower groups.
See Chapter 7, “Fire Protection Engineering,” for fire protection provisions for cooling towers.