Greenheck VENTILATION GUIDE

September

2005

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Table of Contents

SECTION PAGE NUMBER

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1: Types of Commercial Kitchen Ventilation Hoods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2: Determining Exhaust Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3: Supply and Make-Up Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4: Room Balance and Airflow Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5: Grease Extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6: Fire Suppression Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

7: Energy Management Systems (Variable Volume Driven) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

8: Unit Selection

Exhaust Fan Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Make-Up Air Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

9: Ductwork and Pressure Losses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

10: Utility Distribution Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

11: Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

12: Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

13: Quick Reference Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49-52

Greenheck Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Free Foot Area Consideration and Hood Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

References, Codes, and Informational Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Commercial Kitchen Ventilation Web sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Our Warranty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

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INTRODUCTION

A kitchen hood is not just a box. Every commercial kitchen requires ventilation, and in the past, the importance
of a proper ventilating system has been overlooked. Today, designers, installers, and operators are recognizing
the value in well-designed commercial kitchen ventilation (CKV) systems. Emphasizing “system” because it is
not just a box, it is an engineered system of exhaust hoods, ventilators, make-up air ventilators, grease removal
apparatuses and more. Taking time to properly design a CKV system will increase the health and safety of the
kitchen operators and increase the efficiency and energy savings for the owner.

This guide discusses many of the factors that must be analyzed when designing an efficient kitchen ventilation
system. It offers a background in the basic theories of CKV design, product types with their application,
necessary calculations with examples, troubleshooting, and more. This guide will assist in the development of a
well-balanced and functional system.

Although this guide will aid in a successful design, it is important to keep in mind the variation in standards
and codes which have been adopted. Each county may have slightly different requirements for the designer
to meet. The local authority having jurisdiction (AHJ) should be consulted to ensure the final design meets the
requirements set forth. See the design and code reference section on pages 52 and 53 of the guide for a listing
of common codes. If you would like to discuss any of the topics to further detail please contact Greenheck.

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Make-Up Air Unit

Upblast Exhaust Fan

Vented Curb Extension

Variable Volume Controls

Fire Suppression Controls

Commercial Kitchen Hood

Filter Bank

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TYPES OF COMMERCIAL KITCHEN VENTILATION HOODS

Two Types of Hoods

Two different types of kitchen hoods are used in the commercial kitchen. These hoods are classified as a
TYPE I or TYPE II ventilation hood. TYPE I hoods are used over cooking equipment producing heat and grease
laden effluent. These hoods require a fully-welded ducting system. TYPE II hoods are used over non-grease
producing cooking equipment exhausting heat and condensation. Various categories of TYPE I and TYPE II
ventilation hoods exist for different applications and personal preferences.

TYPE I Canopy Hood

The canopy hood uses the updraft concept to capture and contain the contaminated
air generated by the cooking process. Heated air is less dense than
the surrounding air causing it to become buoyant. If no
cross drafts are present, the contaminated air will
rise up into the hood where it is captured and
contained until it can be exhausted through
the grease filters to the outside. Wall, single
island, and double island represent the three
configurations of canopy hoods. Although
each configuration is mounted from the ceiling
directly above the cooking equipment, each is
used for a different application.

Wall Canopy Hoods

The wall canopy hood is used when the cooking equipment is placed against a wall. Hoods that are used
against a wall have a tendency to capture and contain the effluent using less airflow than in an island type
application. Make-up air from the kitchen enters the area below the hood replacing the air being exhausted.
The wall located on the backside of the hood will cause the make-up air to enter at the front and sides of the
hood creating a front-to-rear airflow pattern. The plume will rise from the appliance and will be attracted to
any surface parallel and near the cooking equipment, in this case, the wall. This phenomena is known as the
Coanda Effect. The plume is then directed into the hood, enhancing capture and containment. Cross drafts still
threaten spillage, but to a lesser degree than island style hoods.
National Fire Protection Association (NFPA 96) and International
Mechanical Code (IMC) should always be consulted when using a
wall canopy hood. Wall canopy hoods may or may not be mounted
directly against the wall depending on the type of wall. NFPA 96
defines three types of walls; non-combustible, limited combustible,
and combustible. Most commercial kitchen applications have
limited combustible walls which require a 3-inch air space between
the back of the hood and the wall. Most hood manufacturers
provide a 3-inch space with the hood enabling it to be placed
against the wall.

IMC requires a minimum hood overhang of 6 inches from
the cooking equipment on each end of the hood, and it is
recommended that there is a minimum overhang of 6-12 inches
beyond the widest cooking appliance for the front. Greater
overhangs will increase capture and containment.

Wall Canopy

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Single Island Hoods

A single island hood is used over one row of cooking
equipment placed where no walls exist. Single island
hoods can be seen from all directions, therefore, have
four finished sides. With four exposed sides, this type of
hood is more susceptible to cross drafts, spillage, and is
dependent only on the thermal updraft of heat from the
cooking equipment and how quickly the exhaust fan can
rid the hood of contaminated air. These hoods should be
sized larger and use more airflow than a wall canopy hood
with the same cooking battery. The single island hood must
overhang the cooking equipment by a minimum of 6 inches
on all four sides of the hood. However, it is recommended
that the overhang be extended to 12 inches on all sides
of the hood. Extending hood overhangs increases capture
volume which aids capture and containment. To eliminate
the front to back airflow on a single island, a V-bank of
filters improves capture and containment by directing the
contaminated air to the center of the hood.

A wall canopy can be installed as an island hood with
a finished back enhancing its aesthetic appearance.
However, it is not recommended because the front to
back airflow pattern of a wall canopy will cause capture
problems when being utilized as a single island canopy.

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Single Island Canopy

Double Island Hoods

A double island hood is placed over two rows of cooking
equipment placed back to back. This configuration
is made up of two wall canopy hoods placed back to
back, thus creating four finished sides. This category of
hood performs similar to the wall canopy hood due to
two thermal plumes rising against each other, but is still
susceptible to cross drafts. A double island hood must
overhang the equipment by a minimum of 6 inches on all
four sides of the hood but would benefit from additional
overhang.

Water Wash Hoods

Available in a wall canopy or double island configuration,
water wash hoods are a cartridge type canopy hood,
meaning the grease filtration device is built into the
exhaust plenum. Water wash hoods utilize water spray
nozzles in the exhaust plenum to clean the grease
collected by the filtration system (see Figure 35 on page

24) after a certain period of operation. These wash cycles
can be programmed to run for a specified length of time
and can be set to run automatically at the end of the day.
Continuous water mist can be used to extinguish embers
on a solid fuel cooking operation. The hood is controlled
through a remote mounted control box including an
adjustable flow detergent pump, a wash cycle timer in
a solid state programmable controller, and a detergent
reservoir. These hoods have a high up-front cost and have
a higher operating cost than other types of hoods.

Double Island Canopy

Water Wash Hood - End View

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A

C

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Short Circuit Hoods *Warning: NOT RECOMMENDED*

Short circuit canopy hoods were once thought of as an energy saving device. The theory, by introducing
untempered make-up air inside the hood reservoir it would reduce the amount of tempered air being exhausted
from the kitchen, minimizing heating and cooling loads. This was done to get around old codes which set a
minimum exhaust rate that was much higher than needed to achieve capture and containment. Make-up air
was short-circuited by as much as 80-90% of the exhaust rate resulting in spillage of the contaminated air.
*Schlieren Imaging (see Schlieren Imaging on page 8) confirms that
only 15% of the minimum capture and containment airflow can be
brought through the hood without causing spillage.

Short circuit hoods are ineffective because they do not discharge the
make-up air in the correct location. Think of the cooking equipment
as a generator of contaminated air. The purpose of make-up air is to
replace the air that is being generated at the cooking surface. Both
the hood and exhaust system are designed to capture and contain the
airflow generated by the cooking equipment. Short circuit hoods dump
make-up air into the capture and containment area, thus overfilling the
hood and releasing a mix of make-up and contaminated air into the

Short-Circuit Hood - End View

room causing greasy surfaces and increased heat loads. For these
reasons, short circuit hoods are not recommended.

*Data provided by Architectural Energy Corporation,
and Fisher-Nickel, Inc.

UL Vertical Distance

Above Cooking Surface

Proximity Hood

Hanging Height

Proximity Hoods (Backshelf)

Proximity hoods are TYPE I hoods that are shorter in

1.5 INCH OPTIONAL SHELF

height and depth than a typical canopy hood. The name
“Proximity” or “Backshelf” refers to the close location of
the hood with respect to the cooking equipment. Actual

6 OR 12 INCHES

distance from the cooking equipment varies between
manufacturers due to their UL listing; typically mounted
at 10-36 inches above the appliance. Cooking equipment
may extend past the face of the hood creating underhang, therefore cooking

17 TO 36 INCHES

equipment such as large skillets and ovens may not be used. See the
manufacturer’s UL listing. Even with underhang, these hoods are still able
to capture the contaminated air due to their close proximity. Large surges

3 INCH TO EQUAL TO

HOOD DEPTH

of contaminated air may escape from the hood, therefore proximity hoods
are best suited for light and medium duty cooking applications such as
griddles. The major benefit is reduced airflow required to obtain capture and

COOK SURFACE

containment compared to a canopy hood with the same cooking lineup. The
savings are realized through reduced heating and cooling loads.

Flue Bypass Proximity Hoods

Proximity hoods are mounted closer to the cooking equipment subjecting the grease filters to
abnormal heating loads from appliance flues. When gas-fired cooking equipment with flues
are used, flue bypass proximity hoods offer another advantage. Instead of allowing the heat
from the flues to pass-thru the face of the filters, the flues are ducted to discharge the hot air
directly to the back of the exhaust plenum, bypassing the grease filters. Normally, this heat
would hit the filters, causing them to radiate heat onto the cooking personnel. Radiant heat
loads are greatly reduced with the absence of the hot flue gases and grease is less likely to
bake to the filter face which enables the filters to be cleaned more easily. Airflow requirements
are lowered because the hood does not have to capture the excess heat, only the heat and
grease from the cooking surface must be contained.

The flues on the equipment must be sized correctly to the bypass chamber to ensure grease
is not pulled into the flue. Dampers control the amount of hot flue gases that exit through
the exhaust plenum. This airflow is critical to cooking equipment performance, therefore,
the dampers must be set according to the cooking equipment beneath the particular hood.
Greenheck should be consulted prior to ordering flue bypass proximity hoods to ensure a
proper hood-to-equipment fit. Flue bypass is recommended for fryers and griddles.

23 TO 36 INCHES

Flue Bypass

3 INCH

OPTION

CLEARAN

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TYPE II Hoods

TYPE II hoods are commonly referred to as oven or condensate hoods. In essence, these are stripped down
exhaust only canopy hoods. The purpose of the TYPE II hood is to remove heat, moisture, and odor-ridden air
from non-grease producing appliances. The hoods do not contain grease filter banks but rather a duct collar
to exhaust the contaminated air. A TYPE II hood duct does not
need to be fully-welded, instead it can be a standard galvanized
duct because there is no grease loading. Flex-duct is not
allowed for TYPE II hoods.

Oven Hood

The oven hood is an exhaust only canopy hood with an exhaust
duct collar for the removal of heat and vapor. These hoods are
the simplest of all hoods and are usually placed over ovens or
small appliances only producing heat and odor. For complete
capture and containment, overhangs should be measured with
the oven door open.

Condensate Hood

The condensate hood is an exhaust only canopy hood with
U-shaped gutters to capture and direct condensate to a drain.
It also has an exhaust duct collar for heat, moisture, and
odor-ridden air to exit. Many manufacturers have options for
condensate baffles in the hood to help condense the moisture
laden air, one or two baffle configurations are typical, depending
on the moisture content of the contaminated airstream.
Condensate hoods are usually found mounted over dishwashers.
For complete capture and containment of large plumes of heat
and steam, 18-36 inches of overhang are recommended.

Oven Hood - End View

Condensate Hood - End View

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HOOD CERTIFICATION

Most jurisdictions require TYPE I exhaust hoods to bear the Underwriters Laboratory (UL) label. UL 710 is
the test criteria in which UL listed exhaust hoods are tested. Tests include temperature, cooking, flare-up,
fan failure, fire, and burnout testing. In order to complete the analysis, these hoods have to be operating at a
minimum exhaust airflow rate to obtain capture and containment determined under laboratory conditions.

This is where the misconception of the UL listing becomes apparent. The minimum airflow that UL uses to
test hoods is obtained by first adjusting airflow to the manufacturer’s recommendation, then fine tuning it to
ensure complete capture and containment of the effluent generated by cooking hamburgers. This airflow is
then assumed to be the minimum capture and containment value for the UL testing of a particular hood in
a controlled laboratory environment. This airflow is then considered safe for the flare-up and burn tests that
follow. The temperature of the hood shall remain in a range that does not compromise the structural integrity
of the hood at the listed airflow. The airflow does not guarantee capture and containment.

UL minimum airflow ratings in the hoods exist only as a safety rating. It does not guarantee capture and
containment and therefore, should rarely be used as design criteria. It is important to realize that UL
established airflow rates are determined and utilized under laboratory conditions. More exhaust and/or
lesser supply rates may be required in real environments. There are situations where extremely light cooking
applications exist where the exhaust rate may be at or near the UL listings, but only in these light, low
volume, and light cooking battery situations.

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DETERMINING EXHAUST RATE

Having the proper exhaust rate is one of the most crucial calculations in a kitchen ventilation system. Not only
will it allow the system to capture as it is designed, but can save money each year through energy savings as
well as initial start-up costs. Throughout the industry there are two common methods to determine exhaust

2

airflow, which will be discussed in this section. It is important to establish how contaminated air is generated
and how it behaves.

Cold vs. Hot

Concepts used to determine airflow rates in the past were developed through
testing on non-operating (cold) equipment. As shown in Figure 1, smoke
bombs were commonly placed at the front edge of the cooking equipment.
The exhaust rate would then be increased until all the smoke was captured,
thus determining a minimum capture velocity. Additionally, it increases the
amount of make-up air needed to balance the room which decreased overall
system efficiency.

Fortunately, the majority of tests are required to be completed over operating
(hot) equipment. Greenheck’s approach utilizes the thermal updraft produced
through the transfer of heat from the surface to air. The updrafts help to
transfer the contaminated air (heat, grease vapor, smoke, steam, and gas
combustion) to the kitchen hood as seen in Figure 2. Implementing this
approach can significantly reduce the airflow required to obtain capture and
containment. By realizing that hot air rises, a more efficient and less costly
system can be achieved.

Smoke

Cold

Cooking

Equipment

The Cooking Equipment

When using the “Greenheck Method” the cooking equipment can
be thought of as a generator of contaminated air. The quantity
of such air generated by each appliance is ultimately dependent
upon the temperature and size of the physical cooking surface.
Gas flues on gas equipment are also considered generators.
Heat from appliance surfaces cause a change in the density of
surrounding air creating a thermal updraft. While hot air rises, it is
replaced by air in the immediate vicinity as shown in Figure 2. The
replacement air assists in establishing minimal capture velocity at
the cooking surface and contains the contaminated air generated
by the cooking equipment. There are many factors that can alter
the direction and velocity of this air which are discussed in the
design consideration (pages 42-45) section of this guide.

Hood

Updraft

Heated Surface

Figure 1

Figure 2

Schlieren Imaging

Schlieren Imaging is a powerful tool now being used for research in the commercial kitchen ventilation industry.
Schlieren Imaging enables people to see the heat given off by appliances and watch its flow pattern in the
hood. It is an excellent way to test for capture and containment due to the ability to zoom in closely along
edges of the hood and observe any escaping effluent. Schlieren images are made visible by taking advantage
of the different air densities
yielding a high contrast
optical image.

Figure 3A & 3B are an example of
the heat load seen in a common
kitchen application. Both hoods
use the same exhaust rate,
however, the hood in Figure 3A is
spilling heat into the space.

Figure 3BFigure 3A

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Previously undetectable to the eye, it is apparent that the heat gain to the space can be quite significant. Figure
3B demonstrates Greenheck’s PEL lip technology, which is a 1.5 inch return lip along the bottom edge of the

hood. This lip directs airflow back into the hood allowing cfm requirements to be reduced without spillage. A
properly designed system should look like the image on the right.

Utilizing Exhaust Flow Definitions

Using any of the concepts, every piece of cooking equipment can be placed into a category which assigns
a value to the actual updraft velocity or airflow volume per foot. These values can be used for CFM hood
calculations. Figure 4 categorizes common kitchen appliances and provides the updraft velocity factors and
International Mechanical Codes airflow volume per linear foot necessary to complete the calculations.

Recognize that the extra-heavy category contains nearly all solid fuel cooking appliances. Solid fuel is the most
volatile and uncontrollable fuel source in a commercial cooking operation. There is no on/off switch like most
appliances, but rather one can add fuel or let the fuel burn out. Thus, the load is extremely variable and may
exceed projected exhaust requirements. In these situations it is important to have additional airflow up front and
size exhaust and supply fans so their airflow can be increased if needed. Lastly, look into standards and code
requirements such as: Local Codes, State Codes, NFPA 96, IMC, or any other required agencies in the area to
ensure proper installation.

LIGHT MEDIUM HEAVY EXTRA-HEAVY

2

Equipment

(Greenheck’s Appliance

Classification)

Greenheck Method

(updraft velocity in feet

per minute)

International Mechanical

Code

2003 Edition

(cfm per linear foot)

Gas & Electric Ovens

Gas & Electric Steamers

Gas & Electric Ranges

Food Warmers
Pasta Cookers

Pizza Ovens

Non-Cooking Appliance

Smoker

Rotisserie

50 85 150 185

200 300 400 550

Combi-Ovens

Gas & Electric Fryers

Griddles

Tilting Skillets

Tilting Braising Pans

Grill

Hibachi Grill
Salamander

Upright Broiler

Electric Char-Broiler

Gas Char-Broiler

Mesquite

Infrared Broiler

Lava Rock Char-Broiler

Wok

Chain Broiler

Figure 4

A typical cooking battery has been created and will be used to illustrate the IMC code method, Greenheck
Method, Free Foot Consideration, and Greenheck’s Airflow Volume per linear foot method. Figure 5 illustrates
the typical cooking battery.

ASSUMPTIONS

Fryers

36 in. x 30 in.

Griddle

24 in. x 30 in.

Oven

36 in. x 36 in.

• 3 inch spacing behind hood

• Typical 6 inch overhang required on sides and front

• Hood Dimensions: 9 ft. 0 in. long x 4 ft. 0 in. wide

• Greenheck Method uses actual containment area of
45 in. because of 3 in. integral air space

IMC EXAMPLE

Figure 5

IMC requires that when calculating the exhaust rate for a combination of appliances, the highest exhaust rate
be applied over the entire length of the hood. Therefore, in the case of the hood above, IMC requires 300 cfm/
ft. for the 9 feet of hood yielding a total of 2700 cfm. Furthermore, it is recommended by IMC and NFPA that any
hood over a solid fuel cooking battery shall have a separate hood, duct, and exhaust system.

9

Greenheck Method

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Fryers

36 in. x 30 in.

Griddle

24 in. x 30 in.

Oven

36 in. x 36 in.

QC - Quantity of contaminated air
generated by the cooking equipment.
Using Figure 4, identify the appropriate
updraft velocity and multiply it by the
area of the appliance.

10

QF - Quantity of air required to contain
surges and drafts. Use the minimum
updraft velocity of 50 fpm and multiply
it by the difference in area between
the hood containment area and the
appliance area.

Q

E

Q

F

Q

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Greenheck airflow volume per linear foot (cfm/linear foot) method

The commercial kitchen ventilation industry uses several different ways to calculate the exhaust airflow required
to obtain capture and containment. A vast majority of companies, standards, and codes use airflow in
cfm/lineal foot. To help do a comparison, Greenheck suggests the following cfm/lineal foot values shown in
Figure 6 below. The example calculation below uses the typical cooking battery from the Greenheck Method
example on the previous page. Like the Greenheck Method, the linear foot method is appliance specific. Use
the appropriate airflow volume rate across the length of each appliance. Use the light duty airflow for end
overhangs and the sum of the individual rates is the total airflow. Lastly, note that hood factors should be
applied after obtaining an exhaust value using cfm/lineal foot.

HOOD TYPE LIGHT MEDIUM HEAVY EXTRA - HEAVY

Wall Canopy 200 250 350 450

Backshelf 150 200 300 N/A

Note: 1. Double Island hoods are considered two wall canopy hoods

2. Single Island hoods need to be multiplied by the hood factor after using the wall mounted canopy value

Figure 6

EXAMPLE CALCULATION

2

Limits and Assumptions

Airflow (cfm) = Length (feet) ∗ Airflow Volume

cfm

linear foot

(for cfm per lineal foot calculations)

1. Used for hoods 54 in. or less in width

Left Overhang Airflow = 0.5 feet ∗ 200 = 100 cfm

cfm

linear foot

2. Cannot be used for pizza ovens

3. Cannot be used for cook chill

4. 6 ft. 6 in. hanging height

5. Vertical updrafts

6. Proper room ventilation

7. Proper overhangs

Fryer Airflow = 3 feet ∗ 250 = 750 cfm

Griddle Airflow = 2 feet ∗ 250 = 500 cfm

Oven Airflow = 3 feet ∗ 200 = 600 cfm

Right Overhang Airflow = 0.5 feet ∗ 200 = 100 cfm

cfm

linear foot

cfm

linear foot

cfm

linear foot

cfm

linear foot

Total Airflow = 2050 cfm

FREE FOOT AREA CONSIDERATION

The Greenheck Method assumes in most cases a 6 inch overhang on the front and ends of the hood under the
typical cooking battery and open end conditions. Unfortunately, the method penalizes for additional square feet
of empty hood. Additional overhang can help increase capture by utilizing a larger “holding tank” for the effluent
before it’s exhausted, helping with large momentary surge. However, there are limits to how much overhang is
effective. Therefore, Greenheck will allow a 1 foot extension beyond the built-in 6 inches of overhang creating
a free foot area without increasing the required airflow. This can be used for both Greenheck Method and CFM
per lineal foot. See Figure 7 below.

CORRECTING FOR FREE FOOT AREA

11

12 in.

6 in.

12 in.

Steam Kettle

30 in. x 30 in.

(50 cfm/ft

“Greenheck Method” Area

(50 cfm/ft2)

2

)

Free Area Overhang (0 cfm/ft2)

Extended Area Outside Updrafts (50 cfm/ft

Oven

30 in. x 40 in.

2

(50 cfm/ft

6 in.

6 in.

)

12 in.

2

)

12 in.

Figure 7

The table to the right illustrates the difference in exhaust
CFM between the Greenheck Method and the IMC
method. At a cost of approximately $2.00/cfm each year,
the Greenheck method offers significant savings. The

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Greenheck Method considers all the cooking equipment
as a separate item to determine the best exhaust rate.
It not only is the most accurate, but is the lowest airflow
volume of the three methods. The Greenheck Method
is appliance specific to determine the proper amount
of exhaust required so that the system is not under­exhausting or over-exhausting, costing more money in
either operation, cleaning, or upgrading costs.

Method Exhaust CFM

International

Mechanical Code

Greenheck

Method

Greenheck

cfm/lineal ft.

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2700

2013

2050

HOOD FACTORS

There are several design factors that can either hinder
or enhance the performance of the kitchen ventilation
system. For that reason, Figure 8 has customized
the exhaust airflow to the application. Notice there
are some scenarios that will increase the airflow
significantly and others that do not change or will
decrease the airflow. Use this chart after you have
already determined the airflow using the Greenheck
Method, or Greenheck airflow volume per lineal foot.
Simply multiply the cfm by the multiplication factors
for each item that applies.

Wall Canopy 1.0
Wall Canopy - Finished Back 1.3
Single Island - V-Bank 1.2
Double Island 1.0
Mini Skirts - 2x2 0.92
End Skirts - Full / Wall 0.90
Exhaust Only 1.0
Supply Plenums 1.1
Char-Broiler at end of Canopy or

under a Single Island hood
Hanging Height 6 ft. 6 in. 1.0
Hanging Height 7 ft. 0 in. 1.1

Condition Multiplying Factor

EXAMPLE HOOD FACTOR CALCULATION

Given a determined exhaust rate of 3000 cfm, it has now been determined that the application will
have the following conditions:

1. Char-Broiler at the end of the canopy (1.2)

2. Full End Skirts (0.9)

1.2

Figure 8

Therefore the new airflow will be: Airflow (cfm) = 1.2 * 0.9 * 3000 cfm = 3240 cfm

12

®

Putting it all together - complete example

To put all of the information
from this section together, the
following is a full example from
start to finish. Examine first the
cooking battery and hood to
the right and follow through the
calculations illustrated below.

Fryer

30 in. x 30 in.

144 in.

Griddle

36 in. x 30 in.

Free Foot Area

Char-broiler

36 in. x 30 in.

2

Range

30 in. x 30 in.

39 in.

48 in.

13

QC - Quantity of contaminated air
generated by the cooking equipment.
Using Figure 4 on page 9, identify the
appropriate updraft velocity and multiply
it by the area of the appliance.

2

®

QF - Quantity of air required to
contain surges and drafts. Use
the minimum updraft velocity
of 50 fpm and multiply it by the
difference in area between the
hood containment area and the
appliance area.

All calculations shown in this example have been built into Greenheck’s
Computer Aided Product Selection (CAPS) program. These examples
illustrate the logic used to properly determine exhaust rates. To obtain
CAPS, request online at www.greenheck.com or consult your local
Greenheck representative.

14

®

SUPPLY AND MAKE-UP AIR

The design of the make-up air system will have the single largest affect on hood
performance. Supply air is defined as air that is brought into the space, but make-

Transfer

Fan

up air is dedicated to “making-up” the air being exhausted. Make-up air is brought
into the kitchen at approximately an equal rate to the air being exhausted by the
kitchen hood. This means that 100% of the air being exhausted must be made
up. This can be accomplished through one supply type, transfer air, or multiple
sources. A slight negative pressure is desirable in the kitchen with respect to the
dining room to keep odors out of the dining area, but pressure levels should not
exceed negative 0.02 in. wg. The key to designing a system is to introduce make-up air in the most

Figure 9

economical way without affecting the capture and containment of the hood. Maximum hood performance can
be obtained by distributing air at low velocities evenly throughout the room as seen in Figure9. This section will
layout which type of make-up air system is desirable for a restaurant’s particular needs.

Tempered or Untempered?

Air that is heated or conditioned before it is brought in from

Goal is Comfort Goal is Low Cost

Tempered MUA No Air Conditioning

the outdoors is called tempered air. If the goal is to make
the kitchen comfortable, then utilize tempered air. If the
goal is low cost, then use untempered air. Both tempered
and untempered can be introduced, however, selecting
the proper supply types will affect comfort and economic
efficiency. Once this decision has been made a type of
make-up air system can be selected, but always keep two
things in mind. When tempering the air, use a source that
will distribute the air throughout the kitchen to increase

Perforated Ceiling GOOD

Perforated Face Air Supply Plenum

Back Supply

Plenum

Air Supply Plenum Perforated Ceiling

Back Supply

Plenum

Perforated Face

employee comfort. When using untempered air, use a source
that will keep the air near the hood so it can be exhausted
quickly without mixing in the space causing discomfort and

Variable Supply

Plenum

Variable Supply

Plenum
increased heating/cooling loads. Make-up air temperature
should not vary more then 10º F from the air in the space.
However, this limit can be exceeded if the make-up air does
not decrease the comfort of the occupied space or is part of
an air conditioning system.

Northern climates with cold winters and short mild
summers will almost always require heated make-up air
and no cooling. Southern climates with long hot and humid

Register Face Air Curtain

4-Way Diffuser Short Circuit

Short Circuit Register Face

Air Curtain BAD 4-Way Diffuser

Figure 10

summers and short mild winters will want to minimize the amount of hot, humid air that has to be
conditioned by keeping make-up air near the hood. In a hot and dry environment the air can be
cooled in the make-up air unit using an evaporative cooler, which will greatly reduce air conditioning loads.

3

Figure 10 has two columns. The left column ranks various ways of bringing tempered air into the kitchen. The
best options all distribute the tempered air throughout the room. The right column ranks various ways of bringing
untempered air into the kitchen. The best options keep make-up air near the hood to decrease heating/cooling
loads seen in the rest of the building. The supply options listed at the top of each column are proven through
testing and research to be the best ways of introducing make-up air. Choosing the supply options listed near the
bottom will not work as well as options listed near the top.

Supply Options

Make-up air can be introduced through the hood with an
integrated supply plenum or an external supply plenum.
The advantages of using an external supply plenum verses

Integrated Supply

an integrated supply plenum can be seen in Figure 11.
The shaded region represents the volume of the hood. Increasing the volume allows more smoke and
heat to be held in the hood until it can be exhausted. This is important over cooking equipment that
produces a great deal of heat and smoke, such as a char-broiler. External supply plenums are usually less
expensive and can be retrofitted to most exhaust only hoods.

15

Exhaust Only

Figure 11

®

Exhaust Only Hood with
Non-Directional Ceiling
Diffusers

This system will work best when
bringing tempered air into the
kitchen or can be used in climates
where outside air closely matches
desired indoor conditions. An
exhaust only hood has no make-up
air entering the room through the

3

hood. This system is the least complex and in most cases works the best, however, may not be the
most economical. The amount of exhausted air must be made up, therefore non-directional perforated ceiling
diffusers and/or transfer air would be used to make-up 100% of the air. The most important thing to remember
is to place many non-directional perforated diffusers throughout the room to keep air velocities low and uniform.
Uneven air distribution will cause drafts in the kitchen causing capture and containment to suffer (Figure 12).
Make sure the transfer air from another room, especially if supplied through a pass-thru window, is kept at
a low velocity. This can be accomplished by increasing the
amount of air through ceiling diffusers in the kitchen.

Figure 12

Face Supply

150 cfm/ft.

Located on the front of the hood (Figure 13), face discharge
is designed to throw make-up air across the room. Use
face supply when tempered air is brought in through MUA
into a tempered kitchen or when the MUA and kitchen

Integrated Face Supply Plenum

Figure 13

are untempered because mixing will occur with the air
in the space. Registers can be used for larger kitchens with longer throws, but perforated face panels
are recommended for lower air velocities, which will minimize drafts in the kitchen. The maximum supply

Hot

Air

Ceiling

rate is 250 cfm/ft. through perforated panels under
ideal conditions. For optimum performance design to
recommended values of 150 cfm/ft. Face supply should
not be used when a wall, another hood, menu board,
or other object is less than 6feet from the face.

Figure 14

The problem with bringing hot untempered air into an air-conditioned room can be seen in Figure 14. Hot air will
not fall into the room and cycle back out through the hood, rather the hot air will hug the ceiling because it is
more buoyant. If humidity is present in the hot make-up air, it will condense on the metal ceiling diffuser when
it mixes with the air-conditioned air brought through it. Most of the hot air along the ceiling will be taken in at
a return grill by the roof top unit (RTU) and conditioned before it is introduced back into the room, thus totally
defeating the purpose of bringing in untempered make-up air.

Integrated Air Curtain

The hood integrated air curtain (Figure 15) discharges air at the bottom­front edge of the hood and directs air downward. If spot cooling for the
cooking personnel is desired, use tempered air. This type of hood can
also be used to keep untempered air near the hood, although employee
comfort will suffer. Buoyant, hot, humid air will have a tendency to travel
out into the room with this type of hood as Figure 14 illustrates rather
than back into the hood. The maximum supply rate is 125 cfm/ft. through
perforated panels under ideal conditions. For optimum performance
design to recommended values of 65 cfm/ft. Caution must be used with
the design of air curtain hoods (See Figure 17).

16

Figure 15

Integrated Air Curtain

65 cfm/ft

®

External Air Supply Plenum

The external air supply plenums (Figure 16) provide spot cooling when
using tempered air, but can also keep untempered air near the hood,
which will save on heating/cooling loads. There are advantages over
the integrated air curtain. Mounted 14-20 inches above the bottom
edge of the hood or flush with drop ceiling, external air supply plenums
can supply airflow at a maximum rate of 180 cfm/ft. For optimum
performance, design to the recommended rate of 110 cfm/ft. In addition,
external plenums can be attached to the face or ends of an exhaust only
hood to create a curtain of air on all exposed sides of the hood, thus

Figure 16

increasing the volume of air brought in at the hood.

In Figure 17 notice the pocket of low pressure caused by the air flowing from
the external air supply plenum. When velocities are too great, there is enough
pressure differential to cause the hood to spill heat and contaminate. This effect
can be observed on external and integrated air curtains, however, integrated
air curtains are more susceptible to it due to the location of discharge.

Combination Hood

Combination hoods (Figure 18) are a combination of face supply and air
curtain supply and are better suited for cooler climates where outside air
can be used to cool the kitchen. See Face Supply (pg. 16) and Integrated
Air Curtain (pg. 16) for design considerations for the different parts of
the combination hood. More make-up air can be brought through a
combination hood than a face or air curtain alone, but the same limits
exist for each part of the plenum, maximum 250 cfm/ft. from the face
and maximum 125 cfm/ft. from the air curtain. Perforated panels should
always be used to reduce air velocities and eliminate spillage from the
hood. Supply rates should be designed to recommended values of 150
cfm/ft. through the face and 65 cfm/ft. through the curtain for optimum
performance. An exhaust
only hood with a variable
supply plenum (Figure 19)
can be used instead of a
combination hood which
will increase maximum
supply rates (see external
air curtain, face supply) and
not take up valuable capture
and containment volume.

0-80 cfm/ft.

80-160

cfm/ft.

adjustable

damper

External Air Supply Plenum

>180 cfm/ft.

Low
Pressure
Pocket

Figure 17

65 cfm/ft.

Combination Hood

110 cfm/ft.

3

150

cfm/ft.

Figure 18

External Variable Supply Plenum

Figure 19

Back Supply Plenum

An effective way to introduce untempered make-up air into the
kitchen is from the rear of the hood through a back supply plenum
(Figure 20). These plenums are also ideal for heating air during the
colder months since hot air will rise from its low discharge position.
This plenum is mounted 31.25 inches above the finished floor and
directs airflow through perforated panels behind and below the
cooking equipment without affecting capture and containment,
cooking surface temperature, or pilot lights. When using untempered
air, utilizing low air velocities will keep the air near the hood. These
plenums are 6 inches deep and stretch across the entire length
of the hood, therefore they function as a backsplash panel and
provide the 3-inch clearance to limited combustibles needed in most
circumstances. Back supply plenums are able to supply a maximum
of 250 cfm/ft. For optimum performance design to the recommended
rate of 145 cfm/ft.

17

145 cfm/ft.

Back Supply Plenum

Figure 20