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The question of why the hood is not performing properly in both new and existing operations has plagued many sanitarians, fire safety inspectors and owners for as long as there have been kitchen exhaust hoods.
The following is a simple suggested checklist that an AHJ may wish to save and apply when a smoky or greasy kitchen is found on a normal inspection. It is typical for all of us to look up at the hood and state that “the hood isn’t working”, but it goes far beyond that simple statement.
Hoods, canopies or ventilators in and of themselves constitute only one small part of a complex system. A system when well engineered and designed will work for years and years. Only routine exhaust cleaning by a trained, qualified and certified exhaust system cleaner, fan belt replacement and semi-annual fire suppressions system check are required.
Numerous other factors may be at work that may cause smoke to escape the hood. We will break this into two sections, the first for new installations, and the second for existing operations. This guide is not intended to cover all situations. In many instances, it will take trained eyes and further investigation to understand “Why the hood isn’t working”?
NEW INSTALLATION CHECK LIST |
COMMENTS |
Sufficient front overhang covering all appliances? |
While 6” is minimum, ASHRAE research indicates 9”-12” to as much as 18” is better. |
Oven overhang? |
When the oven doors are opened, be sure that the blast of steam is covered by sufficient overhang. |
Any 4-way ceiling diffusers installed within 10’/15-0” of hood face? |
Any air directed at hood over 50 CFM will disrupt the hoods internal draw pattern. |
Are side panels installed? |
ASHRAE testing indicates side panels significantly reduce air loss and side drafts at hood ends. (A wall qualifies). Cheap and easy to install. |
Open space behind equipment? |
New findings indicate that the elimination of open space behind appliances can improve hood performance, sometimes dramatically. |
Is the hood listed? |
Unlisted hoods normally require higher CFM. |
Was specified hood installed? |
Contractors may substitute and switch manufacturers including so called back alley shops. |
Check CFM exhaust rating. |
Hood should be designed for highest rated appliances (This rule has room for discussion and can be adjusted). |
Test and balance procedure. |
T&B must be performed with all appliances installed, fired up and hot to duplicate the thermal plume process during cooking. |
| HVAC System Commissioning. |
Was the system commissioned by a third party or self-commissioned by the installing HVAC contractor? See Building Commissioning Association website at www.bcxa.org |
| Exhaust ductwork. |
Installed according to design drawings? |
| Additional bends in duct run? |
Any additional bends adding to static pressure? (Fan must work harder). Each extra bend adds about 10’-0” to the static pressure calculation and can easily cause the fan to work harder. |
| 90 degree duct turns? |
Curved duct turns are far more effective than a hard 90 degree turn that significantly increases turbulence within the duct system. |
| Exhaust duct welding. |
Leaks in welded ducts are very common. Has the duct run been pressure tested or at minimum had the ICC light bulb test? |
| Rooftop exhaust discharge location? |
Is at least 10’-0” away from any fresh air or make-up air intake grilles? Do prevailing winds direct discharge towards a fresh air or make-up air grille? |
| Multiple Hoods. |
While common to have multiple hoods exhausted by one fan, multiple hoods make it more difficult it to balance each hood properly. |
| Side drafts? |
Beware of side drafts from doorways, pass-through openings, hallways or other supply registers. Both constant and intermittent. Test with side panels. |
EXISTING KITCHEN HOODS |
COMMENTS |
All of the above apply. |
As above. |
Fan switched on? |
Don’t laugh. It happens. |
Broken fan belt? |
While common, sometimes overlooked. |
| Re-commissioning! |
Things change during a systems lifetime. Maintenance personnel make adjustments to make-up air, systems may be modified, equipment added, fan drive belts broken. Owners modify hoods, see actual example below. Filters need replacing. Re-commissioning is suggested after the initial 6 months after opening, then periodically afterwards. Contact the Building Commissioning Association at www.bcxa.org |
These are but a few of the many possibilities that can seemingly cause a hood or canopy to malfunction or perform poorly. All possible reasons “Why the hood isn’t working”. However, there are often multiple reasons that lead to a malfunction.
We recently observed modifications made to the inside of listed hood where the owner blanked off two make-up air holes on the face of his hood and cut out the inside portion of the make-up air chamber to redirect the air. It looked at is if he had used a can opener and just peeled the metal down. The edges were all raw. And yet when we suggested that he had voided the UL listing and would probably be shut down, he answered that the fire inspector had just been there and did not notice anything. This was a real good case for inspectors needing to look up.
Some solutions may be simple such as experimenting with cardboard side panels to determine if they can help resolve any smoke problem. If so, stainless steel side panels are simple and inexpensive to permanently install. This solution could save thousands of dollars.
Simply pushing appliances back towards the wall can significantly improve exhaust performance. While many cooks like to line up all the fronts, recent research by ASHRAE indicates that when equipment is pushed back and a cap added to close off any remaining space, hood effectiveness immediately increases.
Consider experimenting with the two simple ideas above before looking towards expensive remedies. They can make the difference between smoke, vapor, heat and VOC’s escaping or being contained. The secret is keeping the thermal plume drawn to the back wall.
Recent testing regarding equipment positioning under the hood is very enlightening. Tests clearly proved that when high heat and emission devices are placed at or near the ends, it is necessary to use much more CFM to contain the exhaust effluent stream than if the device is near the center of the hood.
Once a hood is installed, most other research, experimentation and testing is far more difficult, time consuming and expensive. It is recommended that owners hire engineers and system analysts with proven track records to help them determine first of all, what is causing a problem, then determine solutions to fix the problem.
Author:
George Zawacki, DFS, CFSP
Publisher and Editor
UP YOUR STACK®
CKV SOLUTIONS
Member - ASHRAE TC-5.10 Kitchen Ventilation
NSF Joint Committee for Foodservice Equipment
Click on the following LINKS to previous UP YOUR STACK® articles:
“Why you should consider adding hood side panels!”
“Rich Swierczyna Presents Tools for the Tool Box?”
“Is it a Canopy, Ventilator or a Hood?”
“Testing and Balancing a New Restaurant Exhaust System!”
“Require Exhaust System T&B to be performed by an independent firm!” |
