Copyright 2004 American Society of Heating Refrigerating and Air Conditioning Engineers Inc This posting is by permission of ASHRAE Journal This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission Contact ASHRAE at www ashrae org Standard 62 2001 Addendum 62n Ventilation for Changeover Bypass VAV Systems By Dennis Stanke Member ASHRAE A NSI ASHRAE Standard 62 2001 Ventilation for Acceptable Indoor Air Quality 1 prescribes ventilation rates for commercial and institutional buildings Historically Standard 62 required both zone and system level calculations for the design of multiple zone ventilation systems such as single path constant volume and VAV systems Unfortunately without clear calculation procedures system level calculations frequently were misinterpreted or ignored by designers As a result many multiple zone systems were underventilated In an effort to avoid underventilation and increase calculation consistency among designers Addendum 62n2 updates prescribed ventilation rates and the calculation procedure for zone ventilation air ow and for system intake airflow for different ventilation systems A previous article3 introduced Addendum 62n rates and described the design of single zone and 100 outdoor air ventilation systems This article discusses the 62n compliant design of a very speci c multiple zone ventilation system changeover bypass VAV also called variable volume and temperature VVT A changeover bypass VAV system Figure 1 uses a constant volume air handler often a packaged rooftop unit or split DX system to ventilate and cool or heat a large area within a building The area served comprises many comfort zones or HVAC zones def ined by ANSI ASHRAE IESNA Although Addendum 62n shows ventilation rates in both IP and SI units this paper uses IP except in selected speci c calculations This is because 62n uses rational conversions not mathematical 22 ASHRAE Journal ashrae org Standard 90 14 as areas with heating and cooling requirements that are suf ciently similar so that desired conditions can be maintained throughout using a single sensor Each comfort zone has its own thermostatically controlled VAV damper and ideally all comfort zones associated with an air handler have similar thermal loads that is all zones need some level of heating or some level of cooling like interior spaces or southern perimeter spaces In practice however many systems include comfort zones with dissimilar loads some zones need heating at the same time that other zones need cooling In either case each comfort zone usually is considered as a ventilation zone de ned by Addendum 62n as an area with similar occupancy category occupant density zone air distribution effectiveness and zone primary air ow per unit area About the Author Dennis Stanke is a staff applications engineer with Trane La Crosse Wis He is vice chair of Standing Standards Project Committee 62 1 November 2004 The changeover bypass VAV system includes a central bypass damper that modulates open as the zone VAV dampers modulate closed The air handler delivers approximately constant primary air ow while each comfort zone receives a variable air volume to match its thermal load Air handler mode heating or cooling is usually based on zone by zone polling to determine the number of zones calling for heating or cooling for instance and the strength of the calls Air handler capacity may be controlled by the largest load or by simply maintaining primary air temperature at a heating or cooling setpoint As the zone dampers close and zone air ow drops the bypass damper opens to maintain a constant differential pressure between the primary duct and the return plenum and consequently relatively constant air ow through the fan and coils This results in relatively constant outdoor air intake ow ignoring wind effects Note Depending on building construction and the operation of relief exhaust fans and or dampers mixed air pressure may actually rise relative to outdoor pressure as the bypass damper opens If the minimum outdoor air damper position is xed it should be set to ensure at least minimum outdoor air intake ow with the bypass damper at both extremes open to maximum and fully closed A single duct delivers air for thermal comfort and ventilation to all comfort zones When the system is occupied the outdoor air damper at the air handler opens to provide minimum outdoor air intake ow This outdoor air ow helps determine design cooling and heating capacity Designing to Comply with Addendum 62n Changeover bypass VAV systems can be designed and operated to comply with Addendum 62n thereby ensuring proper ventilation in each zone To comply the system must deliver suf cient outdoor air intake ow at the air handler as well as suf cient outdoor air to the breathing zone within each ventilation zone These ows which are inextricably linked must be determined for design cooling and design heating conditions Because this is a multiple zone recirculating system wherein the air handler supplies a mixture of outdoor air and recirculated return air to more than one ventilation zone Equations 6 5 through 6 8 of Addendum 62n must be used In the following paragraphs we consider two approaches to ventilation system design for an area within an example of ce building Figure 2 In the rst approach we work from the zones to the air handler selecting minimum zone primary air ow settings and then calculating the outdoor air intake ow needed for proper ventilation of all zones In the second approach we work from the air handler to the zones selecting Along with many other Web published addenda Addendum 62n will be incorporated into an updated version of Standard 62 2001 and published as ASHRAE Standard 62 1 2004 later this year November 2004 the outdoor air intake ow and then calculating the required settings for minimum zone primary air ow Based on thermal loads our example building area is divided into eight comfort zones each with its own thermostat and VAV damper We consider each of these comfort zones as a separate ventilation zone The following zone level calculations must be performed regardless of which approach to system design is used Figure 3 1 Look up the minimum people outdoor air rate R p and the minimum area outdoor air rate R a in Addendum 62n Table 6 1 for each zone Using peak zone population P z and zone oor area A z nd the design breathing zone outdoor air ow using Equation 6 1 Vbz Rp Pz Ra Az We could
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