Chilled-Water VAV Systems - HVAC.Amickracing

Primary System Componentsunit are thermally similar, potentially allowing for reduced energy use. Adrawback is that, depending on the shape of the building, it may necessitatethe use of multiple, vertical air shafts, which can result in less usable floorspace. This approach may be combined with the use of floor-by-floor airhandlingunits to serve the interior zones.Indoor versus outdoor air-handling unitsThe VAV air-handling unit can be located either outdoors or indoors. Anoutdoor unit is typically installed on the roof of the building. An indoor unit istypically installed in a penthouse, the basement, or a mechanical equipmentroom located on one of the occupied floors of the building.Table 2 describes the advantages and drawbacks of each location.Table 2. Indoor versus outdoor location of air-handling unitsIndoor AHUAdvantages:• Preventive maintenance on the AHU is performed indoors, awayfrom harsh weather.• Thermal performance of the AHU casing is less critical since indoortemperatures are milder, resulting in less heat loss/gain.Outdoor AHUAdvantages:• Frees indoor floor space that can be used by the building owner orleased to a tenant.• Less concern about condensation (sweating) on the exteriorsurfaces of the AHU, since any condensation drains onto the roofsurface.Disadvantages:• Requires indoor floor space that could have otherwise been used bythe building owner or leased to a tenant.• May require the equipment room to be conditioned to preventcondensation (sweating) on the exterior surfaces of the AHU.Disadvantages:• Preventive maintenance on the AHU may need to be performedduring harsh weather or require the construction of an outdoorservice corridor.• May require more space for vertical air shafts, reducing the amountof usable floor space.• Roof structure may need to be strengthened to support the addedweight of the AHUs.• Thermal performance of the AHU casing is more critical sinceextreme outdoor temperatures result in more heat loss/gain.Installing the air-handling units indoors requires floor space that could haveotherwise been used by the building owner or leased to a tenant. Followingare several strategies used to minimize the floor space required by an indoorair-handling unit:• Stacked configurationsIf the frame of the air-handling equipment is sturdy enough, componentscan be stacked on top of one another to minimize the footprint, or floorspace required.The top unit in Figure 6 depicts an example VAV air-handling unit, withboth a supply fan and relief fan, configured on one level. The overalllength is 23.2 ft (7.1 m) and the width is 7.8 ft (2.4 m).12 Chilled-Water VAV Systems SYS-APM008-EN