Frequently Asked Questions About Kitchen Ventilation Systems
Q: What causes smoke and grease laden air to move horizontally and spill out from under a kitchen hood?
A: There can be many different reasons for this condition to occur. Consider the following:
- Not enough exhaust air for the cooking load.
- The kitchen hood is undersized for the application.
- Cross drafts in the kitchen.
- Excessive velocities from the face and/or air curtain registers.
- Not enough supply air brought back into the kitchen.
Q: When is it necessary to provide make-up air to a system?
A: Always. Codes like NFPA 96 require the following:
"Replacement air quantity shall be adequate to prevent negative pressures in the commercial cooking area(s) from exceeding 0.02 in. water column (4.98 kPa)."
Q: What are typical applications for a Type I and Type II kitchen hood(s)?
A: According to the International Mechanical Code:
"A Type I hood shall be installed at or above all commercial food heat-processing equipment that produces grease vapors or smoke. A Type I or Type II hood shall be installed at or above all commercial food heat-processing equipment that produces fumes, steam, odor or heat."
Q: What are the minimum and maximum allowable airflow velocities at the exhaust duct collar of a Greenheck hood?
A: Minimum = 1500 feet per minute. Maximum = 2100 feet per minute.
Q: According to the National Codes what are the minimum allowable clearances for a kitchen hood?
A: Hoods, grease removal devices, exhaust fans and ductwork must have a clearance of at least 18 inches to combustible surfaces, 3 inches to limited-combustible surfaces and 0 inches to non-combustible surfaces.
Q: Are round duct collars available on U. L. Listed kitchen hoods?
A: Duct collars are available as square, rectangular and round configurations on U. L. Listed without fire damper type kitchen hoods. The maximum allowable size is dependent on the style of hood.
Baffle Filter Canopy Hoods 16" Dia.
Baffle Filter Proximity Hoods 10" Dia.
Dry Cartridge Canopy Hoods 10" Dia.
Water Wash Canopy Hoods 10" Dia.
Round duct collars are not available on U. L. Listed with fire Damper type kitchen hoods.
Q: Can Greenheck provide an exhaust duct collar mounted on the rear or back of the hood?
A: Exhaust duct collars can be mounted on the rear of baffle filter style hoods at no additional charge. However, there is a limitation to this alternate location. The rear of the hood limits the maximum height of the exhaust collar to be 12" for canopy style and 10" for proximity style. Rear mounted exhaust collars are not available on Dry Cartridge or Water Wash Style kitchen hoods.
Q: Can the exhaust duct collar be moved to the right or left of the standard centered position?
A: The U. L. Listing allows the exhaust duct to be located within 48" from the center of the hood to the center of the collar. However, construction limitations prevent the exhaust collars being placed within eight inches of the hood end panels.
Q: Can the exhaust duct collar be moved forward on the hood to give a greater clearance behind the duct?
A: The UL Listing allows the exhaust duct to be moved forward from the standard location. The maximum distance from the back of the hood the collar can be moved is based on the style of hood and the width of the collar. The farther forward the collar is placed the narrower the collar must be. The following is the listing of maximum dimensions that the front of the duct collar can be moved from the rear of the hood.
| w/o fire damper | w/fire damper |
|---|
| Baffle Filter Canopy Hoods | 18" Max. | 16" Max. |
| Baffle Filter Proximity Hoods | 4" Max. | 14" Max. |
| Dry Cartridge Canopy Hoods | 12" Max. | 12" Max. |
| Water Wash Canopy Hoods | 12" Max. | 12" Max. |
Q: Can a flexible duct connector be used to attach an exhaust duct to a utility set exhaust fan?
A: NFPA 96 states the following:
5-1.3 ...The fan shall be connected to the exhaust duct by flanges securely bolted as shown in Figures 5-1.2(b), (c), (d), or (e) or by a system specifically listed for such use. Flexible connectors shall not be used.
Q: Is a fire damper required to be installed in the supply duct collar of commercial kitchen hoods?
A: The requirement of a fire actuated damper installed in the supply duct collar is based on how the hood is constructed. NFPA 96 states the following:
2-3.1 A fire-actuated damper shall be installed in the supply air plenum at each point where a supply air duct inlet or a supply air outlet penetrates the continuously welded shell of the assembly. The damper shall be listed for such use or be part of a listed exhaust hood with or without exhaust damper. The actuation device shall have a maximum temperature rating of 286*F (146*C). Supply air plenums that discharge air from the face rather than from the bottom or into the exhaust hood and that are isolated from the exhaust hood by the continuously welded shell extending to the lower outermost perimeter of the entire hood assembly do not require a fire-rated damper.
In accordance with NFPA 96, Greenheck provides supply duct fire dampers as standard on the following Make-Up Air hood models: Baffle Filter, Dry Cartridge and Water Wash Hoods in both wall style and double island configurations.
Due to the construction techniques utilized on the face discharge style hoods, Greenheck does not furnish fire dampers as standard.
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