The following article was published in ASHRAE Journal January 2005 Copyright 2005 American Society of Heating Refrigerating and Air Conditioning Engineers Inc It is presented for educational purposes only This article may not be copied and or distributed electronically or in paper form without permission of ASHRAE Standard 62 2001 Addendum 62n Single Path Multiple Zone System Design By Dennis Stanke Member ASHRAE A NSI ASHRAE Standard 62 2001 Ventilation for Acceptable Indoor Air Quality 1 as modi ed by Addendum 62n 2 prescribes new minimum breathing zone ventilation rates and new calculation procedures to nd intake air ow for different ventilation systems Previous articles3 4 discussed the design of simple ventilation systems singlezone 100 outdoor air and changeover bypass VAV in compliance with Addendum 62n requirements Here we examine the design of a more complex set of ventilation systems namely single path multiplezone recirculating systems Although the Ventilation Rate Procedure in Standard 62 has required speci c calculations Equation 6 1 for multiple zone systems since 1989 the calculation procedure was sketchy at best consequently it was widely misunderstood and largely ignored by designers Addendum 62n includes a detailed calculation procedure for multiple zone system design Use of this procedure is expected to increase consistency among designers and reduce the tendency to de28 ASHRAE Journal sign multiple zone systems especially VAV systems that provide inadequate ventilation for some fully occupied zones Addendum 62n also includes operational control options that can be used to modulate ventilation capacity as ventilation load and or ef ciency varies but these options are left to a future article The following discussion covers only design calculations Many HVAC systems are con gured as single supply or single path multipleashrae org zone recirculating ventilation systems For instance constant volume systems with terminal reheat traditional constantvolume multizone systems single duct VAV systems and single fan dual duct VAV systems all provide ventilation from a single source or path A single fan dual duct system supplies air to each space using two different ducts but the air in each duct contains the same fraction of outdoor air because one fan a single source delivers the same air mixture to each duct Other systems have multiple ventilation paths including dual fan dual duct VAV systems and VAV systems with fan powered or induction terminal units Single duct VAV systems with series fan powered boxes are always dual path ventilation systems but those with parallel fan powered boxes are single path with the local fan off and dual path with it on Although any of these HVAC systems may be used in variAbout the Author Dennis Stanke is a staff applications engineer with Trane La Crosse Wis He is vice chair of SSPC 62 1 January 2005 ous building types we narrow our discussion to a single duct VAV system with throttling VAV boxes for interior zones and reheat VAV boxes in perimeter zones applied in an example of ce building Demonstrating Compliance by Example Our example system Figure 1 includes a central air handler with a modulating outdoor air damper that may be controlled as an economizer a variable volume supply fan to deliver primary air cooling only throttling VAV boxes in the interior zones throttling VAV boxes with electric reheat in the perimeter zones a central return fan and a central relief damper for building pressure control Although we won t discuss system control details here it s important that we share the same mental picture of the VAV system we re designing Intake air ow is sensed and maintained by adjusting the intake damper position Often the return and outdoor air dampers are linked such that closing the outdoor air damper opens the return air damper proportionately Alternately these dampers can be controlled separately to reduce fan energy while maintaining proper intake air ow but this has no impact on ventilation requirements at design conditions Primary air temperature is sensed and maintained by sequentially adjusting the heating coil control valve economizer dampers and coolingcoil control valve Duct pressure is sensed and maintained at setpoint by adjusting the primary fan capacity via fan speed for instance or inlet guide vane position Zone temperature is sensed and maintained at the cooling setpoint by adjusting the setpoint for VAV box primary air ow VAV box air ow is sensed and maintained at setpoint by adjusting the position of the VAV box damper For zones that need reheat zone temperature is sensed and maintained at the heating setpoint by adjusting reheat capacity electric reheat or a hot water valve and thereby discharge air temperature Return air plenum pressure at the central air handler is sensed and maintained by adjusting return fan capacity Building pressure is sensed and maintained between set limits by adjusting the relief central exhaust damper position Since multiple zone systems provide the same primary air mixture to all zones the fraction of outdoor air in the primary airstream must be suf cient to deliver the outdoor air ow needed by the critical zone the zone needing the greatest fraction of January 2005 outdoor air in its primary airstream In the past many designers simply added the zone outdoor air ow requirements and set the intake air ow to match this sum which resulted in a very low outdoor air fraction and many underventilated zones Some designers went to the other extreme nding the highest fraction of outdoor air needed by any zone in the system and setting the intake air ow to provide this fraction at all times This approach considers only rst pass outdoor air giving no credit for unused recirculated outdoor air and results in a very high outdoor air fraction and overventilation in all zones Proper design in compliance with Addendum 62n calculation procedures strikes a balance between these extremes appropriately accounting for both critical zone needs and unused recirculated outdoor air Let s look at an example of ce building Figure 2 We assumed that thermal comfort can be achieved using only eight VAV thermostats with each thermostat controlling one or more VAV boxes We considered each of these comfort zones or HVAC zones per ASHRAE Standard 90 1 2001 as a separate ventilation zone According to Addendum 62n a ventilation zone is one occupied space or several occupied spaces with similar occupancy category
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