A S H R A E J O U R N A L a sh r a e . o r g J U LY 2 0 1 65 2

COLUMN ENGINEER’S NOTEBOOK

Steven T. Taylor

It is common to see enclosed and underground parking garage parking garage parking exhaust systemsconsisting ofconsisting ofconsisting extensive of extensive of duct distribution systems with multiple exhaust inlets oftenducted to near the floor. The California Mechanical Code (CMC),1 for instance,includes this requirement:*

Exhaust Inlet Distribution. To ensure proper exhaust of con-

taminated air and fumes from parking garages, exhaust systems

utilizing multiple exhaust inlets shall be designed so that exhaust

inlets are distributed in such a manner that no portion of the park-

ing garage is more than 50 ft (15 240 mm) from an exhaust inlet.

Such exhaust inlets shall be installed so that the highest elevation

of the exhaust inlet is no greater than 12 in. (305 mm) below the

lowest ceiling level.

Exception: Garage exhaust systems designed without distributed

exhaust inlets may have their exhaust inlets designed based on

the principles of engineering and mechanics and shall provide the

minimum required exhaust rate in Table 4-4.

The goal of this requirement is clear, but the extensive

exhaust distribution system required is not necessary to

meet this goal. It misses two key ventilation fundamen-

tal concepts:

1. “You cannot suck out a match.” This is one of my

favorite expressions because it makes this fundamental

principle clear even to non-engineers. The idea is that

exhaust inlets cannot capture pollutants unless they

are generated right next to the exhaust inlet. Figure

1 (previously published in my February 2014 column

“Restroom Exhaust Systems”) shows a computational

fluid dynamics (CFD) simulation of a typical exhaust

grille. Note the velocity vectors are only high right near

the grille; more than 2 ft (0.6 m) or so away from the

grille face, the velocity vectors are close to zero. This

means that automobile exhaust emissions, which will

almost always be more than 2 ft (0.6 m) from exhaust

inlets, will not be captured by ducted exhaust inlets.

Hence, the location of the inlets has essentially no

impact on the source strength of the emissions into the

space.

2. Pollutants are diluted by supply air, not exhaust

air. It is the makeup air induced into the garage by the

exhaust air that is diluting auto emissions.† So it is the

makeup air distribution we need to pay attention to, not

the exhaust distribution. In fact, the distributed exhaust

inlets as mandated by the CMC can actually reduce

ventilation efficiency and increase average pollutant

concentrations depending on the location of the makeup

air inlets relative to the exhaust inlets.

BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE

“Sweep” Parking Garage Parking Garage ParkingExhaust SystemsExhaust SystemsExhaust

* This section was required in the 2010 CMC, forcing the use of the exception if alternative exhaust system layouts were to be used. The exception was interpreted by many code officials to mean that computational fluid dynamics had to be performed, such as that discussed in this article, to show alternative designs performed similarly. The 2013 version of the CMC includes this section as an alternative, so CFD is no longer required to justify alternative designs.† From the perspective of garage air quality, ventilation systems could directly supply outdoor air instead of inducing makeup air with ex-haust systems. However, the garage would then be positively pressurized, possibly pushing pollutants into adjacent occupiable spaces. Most codes, therefore, do not allow garages to be ventilated with supply air systems.

FIGURE 1 CFD analysis of exhaust grille; velocity vectors. (Courtesy of Price Industries.)

400

300

200

100

0

fpm

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COLUMN ENGINEER’S NOTEBOOK

Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a mem-ber of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.

For instance Figure 2 and Figure 3 show CFD predictions

of carbon monoxide (CO) concentration from a simple

garage with a center drive aisle with a continuous queue

of automobiles. The garage entry on the left side is the

source of makeup air. Figure 2 shows CO concentration

assuming multiple ducted exhaust inlets per the CMC,

while Figure 3 shows an unducted design with a single

exhaust inlet on the side opposite the entry. The exhaust

air draws makeup air from the entry across the garage

in a sweeping fashion; hence the name “sweep” exhaust

system.

The figures show that the sweep design has lower over-

all CO concentrations with maximum concentration

(~25 ppm) only at the very right side. The reason is that

the exhaust inlets on the left side of Figure 2 are extract-

ing air that has a low concentration of pollutants, wast-

ing this air and leaving less makeup air to dilute pol-

lutants generated on the right side of the garage. So the

sweep design in Figure 3 can provide better ventilation

than a fully ducted exhaust system, and it clearly will be

less expensive.

Example 1: One-Level GarageMany commercial and residential complexes have a

one-level underground garage below. Here is an exam-

ple of how we implemented a sweep garage exhaust

design on a 140,000 ft2 (13,000 m2) garage.

The garage entries will always be a source of makeup

outdoor air. So our first approach is to locate exhaust

points (tagged EA1, EA2, etc.) on the opposite side of

the garage so they can draw makeup air from the entry

down the drive aisles to the exhaust inlet, as shown in

Figure 4. This is the least expensive design. However, we

encountered some problems:

• The exhaust rate prescribed by the CMC (based on

ASHRAE Standard 62.1-20132) is 0.75 cfm/ft2 (3.7 L/s·m2)

for a total exhaust rate of 105,000 cfm (52,000 L/s).

The air speed through single garage entry would have

been ~2,000 fpm (10 m/s), which would be very notice-

able to people walking through and generate a higher

than desirable pressure drop. High velocity makeup

air also results in more stagnant areas caused by eddies

(discussed in the next example). Our experience has

been that the Standard 62.1-2013 garage exhaust rate

is extremely conservative; with fan speed controlled by

carbon monoxide (CO) concentration, as required by

ASHRAE Standard 90.1-20133 and California’s 2013 Title

24 Energy Standards4 for most garages, we find exhaust

rates seldom exceed half of the design rate and even

then only for short periods in the evening rush hour

(due to cold engine starts).

As hybrid, electric, and other low-emission vehicles

become more popular, the current Standard 62.1 garage

exhaust rate will become even more conservative. But

even half the 2,000 fpm (10 m/s) design air speed at

the entry seemed too high. So we decided we needed to

convert some of the exhaust points into additional sup-

ply air points. This increased first costs because the 0.75

cfm/ft2 (3.7 L/s·m2) exhaust rate still had to be main-

tained; the air that was previously exhausted at points

now used for supply had to shift to other exhaust points.

• The fact that we needed more supply air points

worked out well because two of our exhaust points, EA1

and EA2 in Figure 4, could not be made to work due to

architectural constraints. EA1 would have discharged air

into the ramp running down into the garage entry, caus-

ing exhaust air to recirculate. Converting it to a supply

air point solved that problem. EA2 was located near the

FIGURE 2 Multiple inlet garage exhaust system. FIGURE 3 Sweep garage exhaust system.

Exhaust Fan

Garage Entry

CO 0.00 4.55 9.09 13.64 18.18 22.73 27.27 31.82 36.36 40.91 45.45 50.00 ppm

Exhaust Fan

Garage Entry

CO 0.00 4.55 9.09 13.64 18.18 22.73 27.27 31.82 36.36 40.91 45.45 50.00 ppm

A S H R A E J O U R N A L a sh r a e . o r g J U LY 2 0 1 65 4

main entry to the campus, and we could not find an ex-

haust design that met code (e.g., 10 ft [3 m] above grade

and from building openings) and that was also archi-

tecturally acceptable. Converting this exhaust point to

a supply point solved that problem due to less stringent

code limitations on air intakes.

It also improved ventilation on the left drive aisle

between EA2 and EA3, circled in green in Figure 4, where

a stagnant‡ spot would occur with the original concept.

The final design concept is shown in Figure 5.

Example 2: Two-Level GarageMy second example is a 510,000 ft2 (47,000 m2) two-

level garage. The sweep design with two-level garages

can be made to work using transfer fans that can elimi-

nate stagnant spots on both levels without using any

ductwork. The garage is shown in Figure 6. The exhaust

fans are all located on one corner of the garage in a loca-

tion that is not architecturally sensitive. There are two

entries on the upper level through which all makeup air

is drawn.

The design consists of the following:

• Four exhaust fans totaling 110,000 cfm (55,000 L/s)

on upper level (shown as red squares in Figure 6);

• Ten exhaust fans totaling 275,000 cfm (140,000 L/s)

exhaust on lower level (shown as red squares in Figure 6);

and

• Five transfer fans, each 24,000 cfm (12,000 L/s)

drawing air from the upper level and discharging to the

lower level (shown as yellow squares in Figure 6).

The split in exhaust rate between the upper and lower

level was determined by experimenting with the CFD

model.

All fans are propeller fans with low-noise blades. Even

with the special blades, the fans are not very quiet at

full speed (35 sones), but they never get to full speed

when controlled off CO concentration (as noted earlier)

and garages are not acoustically sensitive spaces. Where

noise is a concern, e.g., if the area where the fans dis-

charge is acoustically sensitive, mixed flow fans can be

used; they are much quieter and somewhat more effi-

cient, but also much more expensive.

A CFD analysis was performed to justify the design

using the CMC code exception shown earlier. Both the

proposed sweep design shown in Figure 6 and a fully

ducted system compliant with the CMC were modeled.

The results, shown in Figure 7 for both upper and lower

levels, indicate that both designs work acceptably—they

both result in predicted CO concentrations well below

the 50 ppm CMC limit—but the sweep design results in

fewer stagnant areas caused by eddies with CO concen-

trations above 30 ppm.

Advantages of the sweep design over a fully ducted

CMC design include:

• Much lower mechanical costs, about $1.2 million

savings in this case, from $3.75/ft2 ($40.36/m2) down

FIGURE 4 One-level garage—initial concept. FIGURE 5 One-level garage—final concept.

‡ We used to call these “dead” spots but this did not go over well with clients concerned about CO poisoning.

COLUMN ENGINEER’S NOTEBOOK

J U LY 2 0 1 6 a sh r a e . o r g A S H R A E J O U R N A L 5 5

FIGURE 6 Two-level garage exhaust design. (Courtesy of CPP, Inc.) FIGURE 7 CFD CO concentration sweep vs. CMC design. (Courtesy of CPP, Inc.)

to $1.25/ft2 ($13.45 m2), one-third the cost. On other

projects, we have seen mechanical cost savings as high as

$4/ft2 ($43.06/m2).

• Lower floor-to-floor height due to the elimination

of ductwork. The cost savings of reduced floor-to-floor

height can exceed the mechanical savings.

• Generally, lower fan energy costs. The connected

power for the sweep design with propeller fans in this

example was ~100 kW vs. 125 kW for a CMC ducted ex-

haust system with mixed flow fans, 25% lower despite

the less efficient propeller fans and the added fan

energy of the transfer fans. This is due to the much

lower pressure drop of unducted systems. The energy

savings, however, can degrade if there is a large vari-

ance in emissions throughout the garage; the speed

of all fans must be controlled based on the highest

CO reading, which can cause overventilation in other

areas. A ducted system, if provided with multiple fans

serving discrete areas, may be able to be controlled at

different speeds depending on local CO readings. But

our experience has been that, for office buildings at

least, fans usually run at minimum speed (20% or 0.15

cfm/ft2 [0.74 L/s·m2] per California’s Title 24 Energy

Standards) almost all the time with fairly uniform

increases in CO concentrations during evening rush

hour.

• Improved architectural appearance due to the

elimination of ductwork.

• Fewer exhaust discharge locations. For most sweep

designs, garage exhaust fans can be located in one or two

locations. Ducted systems usually require many more

discharge locations to limit duct sizes to minimize floor-

to-floor height. Exhaust discharge locations are difficult

to coordinate architecturally due to code minimum

separation distances (e.g., 10 ft [3 m] above grade and

from openings into the building) and architectural resis-

tance to exhaust stacks, louvers, etc., which can be very

large for large underground garages.

ConclusionsCode requirements and standard practice in many

areas include fully ducted garage exhaust systems with

multiple intakes distributed around the garage. These

designs are unnecessary for minimum garage air qual-

ity and can even result in reduced air quality while

increasing first costs 300% or more and increasing

energy costs 25% or more compared to sweep garage

exhaust system designs. Sweep designs can be made

to work for almost any garage architectural layout,

including multi-level garages with transfer fans used

to prevent stagnant areas by moving air from one level

to the next. In most cases, the sweep system can be

designed without modeling the system using computa-

tional fluid dynamics, as in Example 1, but CFD can be

a valuable design tool for more complex garage layouts,

such as Example 2.

References1. California Code of Regulations, Title 24, Part 4 California Me-

chanical Code, California Building Standards Commission.2. ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air

Quality.3. ASHRAE Standard 90.1-2013, Energy Standard for Buildings Except

Low-Rise Residential Buildings.4. 2013 Building Energy Efficiency Standards for Residential and

Nonresidential Buildings, Title 24, Part 6 CEC-400-2012-004-CMF.

Transfer FanTransfer

Fan

Transfer Fan

Transfer Fan

Transfer Fan

EntryEntry

Exhaust Fans

50.0046.4342.8639.2935.7132.1428.5725.0021.4317.8614.2910.717.1433.5710.000

UL

LL

Sweep Design CMC Design ppm

COLUMN ENGINEER’S NOTEBOOK