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16 AS HRAE Journa l ash rae .o rg November 2006

About the Author

Steven M. Miller, P.E., owns HVAC Design Solu-

tions in San Diego.

Calculation Tools for the

Ventilation Rate Procedure

Standard 62.1

Screenshot from the 62MZCalc spreadsheet included with the Standard 62.1-2004 Users Manual.

ANSI/ASHRAE Standard 62.1-2004 has made significant ad-

vancements in calculations and directives to help improve

outdoor air delivery to the occupants. These changes and additional

calculations, especially for multizone systems, have made it moredifficult for engineers to determine the required outdoor air quanti-

ties. In the past, it was easy for the engineer to apply the 5 cfm

(2.4 L/s), 10 cfm (4.8 L/s), or 15 cfm (7.1 L/s) times the number

of occupants to arrive at the recommended outdoor air quantity.

However, it has become evident that this simplistic approach is not

adequate to provide good indoor air quality.

To improve ventilation air delivery,

HVAC engineers should incorporate

the calculations and directives of the

standard into the normal design pro-

cess. Several tools exist to make the

ventilation rate procedure calculations

less difficult.

Ventilation Air Delivery ProblemDuring the evolution of the current

standard, the Standing Standards Project

Committee (SSPC) 62.1 and researchers

quantified several factors that added to

the degradation of indoor air quality.

As indicated inFigure 1, the delivery

of outdoor air (OA) from the air-handling

By Steven M. Miller, P.E., Member ASHRAE

2006, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Journal Vol. 48, Nov. 2006. For personal use

only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.


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November 2006 AS HRAE Journa l 17

unit to the breathing zone can be reduced significantly, due to

a combination of operating conditions:

The reduction of primary airflow by the variable air vol-ume terminal unit decreases the supply air quantity to the

zone and consequently reduces the amount of outdoor air

delivered to the zone. The energy saving with VAV systems

has often been at the cost of indoor air quality.1

Delivery of the primary air to the breathing zone is de-

pendent on the performance of the air diffusers. If the

diffusers are not selected to drive the primary air down to

the breathing zone at a sufficient velocity, less OA will be

available at the occupied level.

Even if the diffusers are selected to deliver the recommend-

ed 150 fpm (0.8 m/s) velocity at the breathing level, based

on cooling air quantity, the delivery can be significantly

changed when the supply air system switches to heating

mode. This can happen when a heating coil at the VAV

terminal is activated, which creates a buoyancy effect and

short-circuits the supply air back to the return register at

the ceiling level. (ASHRAE HandbookFundamentals,

Chapter 33, recommends limiting the heating discharge

temperature differential of ceiling mounted diffusers to

approximately 15F (8C). At higher temperatures, the

ANSI/ASHRAE Standard 55-2004,Thermal Environ-

mental Conditions for Human Occupancy, maximum

temperature gradient of 5F [3C] in the occupied zone

could be exceeded.)

A short-term occupancy increase in a zone, such as anextended meeting in an office or in conference rooms,

reduces the per-person OA quantity for that zone.

SSPC 62.1 and ASHRAE researchers have addressed the

problems associated with delivery of ventilation air to the

breathing zone, and have revised the standard to account for

these problems. The results of the calculations included in

Standard 62.1-2004 will provide a minimum value for outdoor

air quantity that we can use as the required outdoor air quantity

for each air-handling system. In the past, we did not have a

good tool to quantify the recommended outdoor air volume. As

a result, we have seen several instances of building occupant

complaints and sick building syndrome. By complying with the

directives in the standard, we can start with reliable baseline

values for ventilation air quantities. From this baseline, we can(optionally) make changes to our system designs that improve

the ventilation air delivery efficiency and consequently improve

the indoor air quality even further.

A common complaint from engineers is that Standard 62.1-

2004 is too time consuming and difficult to apply. However, new

tools are available, including the new 62.1 Users Manual with

its associated practice spreadsheet, and a third-party compliance

software package (both available at www.ashrae.org/bookstore).

These tools help with application of the standards procedures,

and make the process less time consuming. The engineer can use

these tools to optimize the ventilation air quantity. Optimization

can help avoid the potentially excessive air quantities that can

occur using other direct calculation methods.

Outdoor Air Calculation Procedures

Standard 62.1-2004 includes two alternatives for determining

the recommended OA quantity: the ventilation rate procedure

and the IAQ procedure.2

The ventilation rate procedure is a prescriptive procedure

that determines OA intake rates based on the occupancy

category, number of occupants, and floor area. The floor area

OA requirement is based on contaminant sources and source

strengths typical for the occupancy category. The procedure

also accounts for system ventilation efficiency (including the

fraction of outdoor air discharged to the zone), as well as theventilation effectiveness of the air-distribution devices.

The IAQ procedure is a design procedure that determines OA

intake requirements and other system design parameters based

on analysis of contaminant sources, contamination concentra-

tion targets, and perceived acceptability targets. Compliance

through this procedure is not within this articles scope.

In most cases, the ventilation rate procedure is the more

straightforward method of determining the recommended

OA rate. Typically, the engineer does not have all of the in-

formation needed to perform the IAQ procedure at the early

design stages.

To improve ventilation air delivery, HVAC engineers should incorporate

the calculations and directives of the standard into the normal design

process. Several tools exist to make the ventilation rate procedure

calculations less difficult.


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Outdoor Air Quality

Prior to completing ventilation design using either design

procedure, the engineer is required to investigate the local

outdoor air quality and review the findings with the building

owner or representative. This requires three steps of investiga-

tion and documentation.1. Regional Air Quality.The status of compliance with

national ambient air quality

standards shall be determined

for the geographic area of the

building site for a status of

either attainment or non-at-

tainment for each pollutant

listed in the standard, Table

4-1. Compliance status for the

region can be found at www.

epa.gov.

2. Local Air Quality.An

observational survey of thebuilding site and immedi-

ate surroundings shall be

conducted to identify local contaminants that could enter the

building. The survey should include items listed in the standard

and the 62.1 Users Manual, including date, time, area surveyed,

description of nearby facilities, observation of odors or irritants,

visible plumes or air contaminants, sources of vehicle exhaust,

and prevailing wind.

3. Documentation.The information gathered during outdoor

air quality investigation must be documented in a report and

reviewed with the building owners or their representatives.

The documentation must include a conclusion by the engineerrelative to the acceptability of outdoor air quality.

If the investigation indicates non-attainment for one or more

of the applicable pollutants or if a local source of contamina-

tion exists, the engineer should consider remedial measures

including air-cleaning devices for the specific pollutants and

using appropriate air intake locations.

Ventilation Rate Procedure

The ventilation rate procedure addresses essentially three

types of air-handling systems for the purpose of determining

required OA ventilation.

1. Single-Zone Systems.One air handler supplies a mixture

of outdoor air and recirculated air to only one zone.2. 100% Outdoor Air Systems.One air handler supplies

only outdoor air to one or more zones.

3. Multizone, Single Supply Recirculating Systems.One

air handler or central system supplies a mixture of outdoor air

and recirculated return air to more than one zone from a single

location (e.g., reheat, single-duct VAV, single-fan dual-duct, and

multizone).

Simplified OA Calculation Tasks

Calculation of the Standard 62.1-2004 outdoor air quantity

typically can be reduced to eight to 10 tasks.

1. Establish the ventilation zonesand tabulate the associ-

ated floor areas.Ventilation zones are one or several occupied

spaces that have a similar occupancy category, similar occupant

density, similar air-distribution effectiveness, and similar zone

primary airflow per unit area. Ventilation zones are not neces-

sarily the same as thermal control zones that are used in loadcalculations, but they can be the same in many cases.

2. Select the appropriate

occupancy categoryfor each

of these ventilation zones

from the list in Table 6-1 of

the standard.

3. Determine the occupan-

cy of each ventilation zone.

Standard 62.1-2004 includes

an estimated default value

based on the zone floor area

and the occupant density for

the occupancy category. If thenumber of people expected to

occupy the zone during typi-

cal usage cannot be accurately predicted, these default values

can be used to calculate the required outdoor air quantities. If

the number of people canbe determined (e.g., by using the

architects programming, by counting the number of chairs

or desks in a zone, or other means), the actual number should

be used in the calculations. (If the engineer is using the 2003

International Mechanical Code,which takes its table from the

pre-Addendum nversion of the standard, the engineer must use

the default values as minimums, not defaults.) If the occupancy

is expected to increase at times, such as for extended meetingsin an office, in conference rooms, or other areas within the

zone, the anticipated peak occupancy for the zone should be

included in the calculations, along with a time-related multiplier

to allow for the short-term occupancy.3Engineering judgment

is needed to establish the anticipated number of people as ac-

curately as possible.

4. Calculate the outdoor air required in the breathing zone,

based on the combination of occupant-related requirements and

area-related requirements. These calculations are performed

automatically with the 62.1-2004 Users Manual spreadsheets or

with the available third-party compliance software programs.

5. Determine the correct air distribution effectiveness

value from Table 6-2 of the standard for each ventilationzone. The correct selection should be based on the expected

worst case for each zone. For example, if a zone could have a

supply air temperature more than 15F (8C) above the room

temperature when a terminal reheat coil is energized, then the

corresponding effectiveness number should be selected. This

has a smaller effectiveness number (0.8), which will require a

larger quantity of outdoor air than if ceiling supply of cool air

had been selected.

6. Establish the design primary airflow to the zone from

load calculations.This includes outdoor intake air and recircu-

lated air from the air-handling system, but does not include air

Intended Ventilation Air Delivery

to Occupants

Effective Ventilation Air

Deficit at the BreathingZone Due to Combination

Of Operating Conditions

1. Application of VAV,Reduction to Minimum

Primary Air Quality

2. Less Than Optimum AirDiffusion Effectiveness

3. Air Buoyancy Effect (Warm AirShort-Circuiting to Return at Ceiling)

4. Short-Term IncreaseIn Occupancy (Meeting or

Conference)

Figure 1: Effective ventilation diagram.


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transferred or air recirculated to the zone by other means (such

as locally recirculated airflow).Also, establish the minimum

primary airflow to the zone, which is the minimum expected

primary airflow when a variable volume device is at minimum

design flow rate.

7.If the air-handling system includes secondary recirculationat the zone, it is necessary to perform additional calculations to

determine the outdoor air requirements, as shown in Appendix

A of the standard. This provision is for systems that provide part

of their ventilation by recirculating air from other zones without

directly mixing it with outdoor air (e.g., dual-fan dual-duct,

fan-powered mixing box, fan-powered VAV box, and transfer

fans for conference rooms). The quantity (cfm or L/s) of sec-

ondary recirculation airflow to zoneshould be determined

by the airflow volume of the secondary system fan or blower.

Figure 2 shows a diagram describing the secondary recircula-

tion system.4With secondary recirculation systems, it is also

necessary to determine the estimated portion of secondary

recirculation air from the common return. This air quantitywill be a portion of the previously established quantity of sec-

ondary recirculation airflow to zone. This air quantity should

be based on engineering judgment and depends on whether the

air-handling return system is ducted or is a ceiling plenum type

of return system.

8. Calculate the system ventilation efficiency, based on the

ventilation zone with the minimum value of zone ventilation

efficiency. This will require calculating the zone ventilation

efficiency of several zones (or possibly all zones) to determine

the one with the minimum ventilation efficiency, also described

as the ventilation-critical zone. This ventilation-critical zone

is the zone that requires the largest fraction of outdoor air inthe primary airstream. Since this zone determines the outdoor

air requirement for the associated air-handling system, the

Appendix A calculations should be used, rather than using the

less accurate Table 6-3 values. If the ventilation efficiency of

this ventilation-critical zone is unnecessarily low, the outdoor

air requirement of the air-handling system can be unnecessar-

ily high, which could result in wasted energy and construction

costs. (Appendix A calculations can be simplified by using the

previously mentioned tools.)

9.If the ventilation-critical zone has a zone ventilation ef-

ficiency that is significantly lower than the next lowest zone

ventilation efficiency, the engineer should provide the extra

time and effort to optimize the zone ventilation efficiencies.As noted in Standard 62.1-2004, The designer may increase

the zone supply flows during the design process, particularly to

the critical zones requiring the highest fraction of outdoor air,

and thereby reduce the system outdoor air intake requirements

determined in the calculation, sometimes dramatically.5

Typically during design, the VAV minimum is set by factors

such as minimum controllable operating point of the VAV box,

by office standards that may dictate a minimum air change

rate; by minimum air quantity per unit area; or by air volumes

required for building heating, depending on the system type.

In some instances, the minimum for one zone might be signifi-

cantly smaller, proportionally, than minimums for other zones.

This can create an unnecessarily large outdoor air requirement

at the air-handling unit.

Optimization improves the ventilation efficiency of the

ventilation-critical zone and can significantly reduce the out-

door air requirement of the associated air-handling system.

Analysis of the ventilation-critical zone(s) should include trad-

eoff comparisons between energy saved at the air-handling unit

with lower outdoor air quantity, vs. the potential of increasedheating required at the zone level to avoid overcooling when

the minimum airflow is increased.

See the Optimization Processsidebar for an example.6Op-

timization involves an iterative (multiple-trial) procedure to

arrive at the most economical outdoor air quantity, while still

complying with the requirements of Standard 62.1-2004.

With experience, the optimization process can be completed

in a short time. As can be seen from this small example, im-

proving the ventilation efficiency of the ventilation-critical

zone has reduced the required OA from the original 3,046 cfm

(1438 L/s) to the final required OA of 1,638 cfm (773 L/s).

And, the requirements of the ventilation rate procedure are

still being met.10. Calculate the design outdoor intake quantity based

on the uncorrected outdoor air intake and the final system

ventilation efficiency. This value will be used as the outdoor

air intake quantity for the air-handling system. The previously

mentioned tools perform these calculations automatically.

Ensure Outdoor Air Will Be Provided to Occupants

After completing the ventilation calculations and establish-

ing the outdoor air quantity for each air-handling system, it is

essential that the engineer make provisions to ensure the HVAC

equipment actually provides the required outdoor air quantity.

ExhaustAir

ReliefAir

Outdoor AirIntake(Vot)

FanOr AHU

(Vps)

(Vps Vot)

Fully Mixed Fraction of SecondaryRecirculation (Where Applicable)

(Er) (1 Ep) (Vdz)

(Vpz) = (Ep) (Vdz)

VAV Box(Where

Applicable)(1 Ep) (Vdz)

Secondary Refrigeration(Where Applicable) (Vdz)

Zone

(1 Ez) (Vdz)

(Ez)(Vdz)

(Vbz) = (RpPz+ RaAz)Breathing

Zone

SecondaryRecirculationDirectly FromZone (WhereApplicable)

(1 Er)(1 Ep) (Vdz)

Figure 2: Ventilation system (Figure A.1 in Standard 62.1-2004).


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November 2006 AS HRAE Journa l 21

Original Condition.To begin the

process, the user inputs the designinformation, including design airflow

and minimum airflow to each zone

(six zones in this example). The

row for Zone Primary Outdoor

Air Fraction (Zp) shows the third

zone (third column) highlighted in

yellow. This indicates it is the ventila-

tion-critical zone, determined by the

associated calculations: 0.76(Zp) in

this example, [1,060(Voz)/1,400(Vdz)= 0.76(Zp)]. The other zones (Zp)

Optimization Process

values are significantly lower, indicating that optimizationwould be advisable. To improve the ventilation efficiency of

this ventilation-critical zone, the minimum airflow (Vdz) can

be increased. The original design minimum for this zone is

First Step.The minimum airflow

for the ventilation-critical zone has

been increased to 1,800 cfm (850

L/s). This reduces the zone primary

OA fraction (Zp) to 0.59 and reducesthe required OA intake to 2,173 cfm

(1026 L/s).

Second Step. The minimum has


L/s) for the ventilation-critical zone.

This reduces the (Zp) to 0.49 and re-duces the required OA to 1,868 cfm

(882 L/s). Notice also that the color

of the Minimum VAV airflow cell for

the third zone has changed to orange

(from the original red). This indicates

that the zone is not as critical for op-

timization as it was with the original

minimum VAV airflow values.Note: the two values in the rows labeled For optimization, add to keyminimum VAV airflow to zone (cfm) and OA quantity is optimized when reqd OA intake change

is less than are guideline optimization quantities. They are based on tradeoff calculations between decreasedOA intake heating load at the air-handling unit vs. zone-level

reheat penalty, when zone minimums are increased.

1,400 cfm (661 L/s). The results of the original input values(before optimization) indicate a Required OA Intake Flow

(cfm) (Vot) of 3,046 cfm (1438 L/s), shown in the column

labeled Calc in this example.


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Third Step.The minimum has


L/s) for the third zone. The Zp is

now 0.43 and the required OA is

reduced to 1,717 cfm (810 L/s). TheRequired OA Intake Flow (cfm)

(Vot) reduction from the previous

step is 151 cfm (71 L/s), which is still

above the calculated 37.7 cfm (17.8

L/s) shown in the row, OA Quantity

is Optimized When Reqd. OA Intake

Change is Less Than. Therefore,

another step of optimization should

be completed.

Fourth Step.The minimum for

the third zone has been increased

to 2,700 cfm (1274 L/s). This re-

duces the associatedZpto 0.38 and

reduces the required OA to 1,638

cfm (773 L/s). Note also that the

venti lation-critical zone indication

(yellow highlight) has shifted to the

fifth zone (0.40 in this example). The

Required OA reduction from the

previous step is now only 79 cfm

(37.3 L/s), so further optimization is

not necessary. (If further optimiza-

tion had been indicated, the userwould now switch to the fifth zone

and increase the minimum airflow

[Vdz] to that zone.)

The mechanical ventilation systems are to be designed and

controlled to maintain at least the required minimum outdoor

airflow under any load condition. Standard 62.1-2004 states,

VAV systems with fixed outdoor air damper positions must

comply with this requirement at minimum supply airflow.7

In the interest of energy conservation, this requirement would

indicate that OA dampers on VAV systems should be controlled

rather than fixed. Otherwise, the OA quantity at full-loadoperation could be excessive. OA damper control should include

airflow measurement devices to ensure the required air quan-

tity is being provided under all load conditions. Specifications

should include provisions for initial test and balance of OA

quantities. Operations and maintenance manuals should include

prescribed schedules for periodic testing and adjustment of the

OA dampers and controls over the life of the systems.

Strategies to Improve Ventilation Efficiency

Standard 62.1-2004 has pointed out several areas that cause

reduced ventilation efficiency. The engineer has the opportunity

during the design phase to create the most favorable conditions

for effective ventilation air delivery.

Select diffusers and registers to deliver the primary air

to the breathing zone at recommended velocities, and at

worst-case conditions. This could include using diffusers

that automatically adjust discharge-free areas to maintain

distribution patterns as air volume is decreased.

Revise the method of building heating (such as hydronicbaseboard heating), so it is not necessary to provide ceiling

supply of heating air. Alternately, if using ceiling supply,

limit the heating air differential temperature by increasing

the heating airflows.

Use underfloor air systems and floor supply of cooled air

with ceiling return systems.

Revise VAV minimums based on optimization of ventilation-

critical zones. The engineer also should verify that ad-

equate heating is available for zones that are increased

during optimization to avoid overcooling of the spaces.

Add heat/cool energy recovery equipment, so even larger


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