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Commercial
kitchen exhaust fans are a vital part of your ventilation
system. They remove odors and improve indoor air
quality. Commercial kitchen fans also remove moisture, which can increase the level of
humidity. High humidity can cause mold, mildew and bacteria growth
which can ultimately result in major health code violations.
Common Exhaust Fan Systems
The two most common types of fans are impeller fans and blower fans.
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Impeller fans move air with blades similar to airplane propellers.
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Blower fans look like hamster wheels - they are often called
squirrel cage fans - and generally are more efficient than impeller
fans.
Most commercial kitchen exhaust systems
have an exhaust fan, ducting and a hood. Some buildings have a central exhaust system, which
is one or more fans, drawing air from the entire building (or parts of it)
using a network of ducts.
A Heat Recovery Ventilator (HRV) also exhausts moisture and
odors.
An HRV is a self-contained ventilation system that provides balanced
air intake and exhaust. Like a central exhaust fan, it can be
connected to several rooms by ducting.
How good is
your commercial kitchen exhaust fan: Is your fan...
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too noisy?
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not powerful
enough?
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not energy efficient?
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a
fire hazard?
How Do I Choose the Best
Restaurant Kitchen Exhaust Fan System?
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Choose the quietest, most energy-efficient
exhaust fan or blower in the size
range required. Most exhaust fan and blower labels have ratings so you can compare noise and energy efficiency. Look for a
fan with replaceable parts and permanent lubrication. A fan suitable
for continuous use is preferable. Be prepared to pay more for a
quality fan.
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Select low-resistance (smooth) exhaust ducting. Seal the
joints and insulate sections that run through unheated spaces.
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Place the exhaust hood where it will not cause moisture
damage on exterior surfaces.
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If you have heating appliances with chimneys, make sure that
fans won't cause the appliances to backdraft.
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Install the proper controls.
Things to
Consider when Choosing a Commercial Kitchen Exhaust Fan or Blower:
Noise
Look for
fans labeled 'low noise' or 'quiet,' and check for the HVI or db
(decibel) rating.
If it is not rated, there is a good chance that it will be noisy.
Noisy fans are usually of lower quality, use more electricity and
deteriorate until they cease to function. Older
noisy fans can also be a fire hazard. Anti-vibration pads or foam tape can insulate the fan housing from
wood joists and drywall.
Fan Power Requirements and Airflows
There is more to energy efficiency than selecting an
energy-efficient fan. Ducting can affect fan performance.
Un-insulated, undersized, or droopy flex ducting, ineffective or
dirty backdraft dampers and exhaust louvers can cut rated airflow by
more than 50 percent!
Cleaning
Fans create static electricity which attracts dirt like a magnet to
the fan and housing. The dirt can encourage mold growth and restrict
air movement. Clean fans, housings, back draft dampers and exterior
flaps seasonally.
Weather Hoods, Grills And Backdraft Dampers
Even when fans are off, stack effects and wind loads may cause
outside air to enter or inside air to exhaust through fan ducting. Check the flaps from time to time to make sure they
are clean and working. The exterior exhaust flap or louvers should
be clean and in good repair to maintain unobstructed airflow and
reduce air infiltration. Most exhaust ducts are fitted with a single
flap exhaust hood or triple louver aluminum or plastic exhaust
grill. Use weather hoods that lie flat on the wall in driveways and
other places where hood-type units could be damaged. Clean
exhaust hoods of lint and nesting materials seasonally to ensure
that the flap or louvers are not blocked or stuck open.
High Capacity Systems
High capacity, industrial or oversized exhaust fans, and range-top
barbecue fans can cause chimney backdraft. Backdrafting occurs
when air is drawn down chimneys, bringing dangerous combustion
exhaust gases into the house. Avoid this by selecting sealed
combustion heating appliances. If you have appliances with chimneys
in your house, and you wish to install high capacity exhaust fans,
you will need a matching supply air fan to balance house pressures.
Many ventilation contractors or salespeople are unaware of the
effects of large exhaust fans on other house appliances. Make sure
that your system is properly installed with supply air and that you have smoke alarms and carbon monoxide
detectors to warn you if you have severe chimney back drafting.
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Ventilation Versus Evacuation
Vented kitchen fans help to maintain good indoor air quality. They
remove combustion pollutants, moisture, cooking odors, and grease
from the restaurant or house. Commercial kitchen range hoods must deal effectively
with grease, in particular. One reason that commercial kitchen fans are so
powerful is that they must maintain a high enough velocity to draw
all contaminants into the hood. If this is not accomplished, grease
gets deposited throughout the kitchen, creating a risk of fire. There are various rules of thumb for matching
the size of the cooktop to the size of the fan. A common one is 300
CFM per linear ft of cooktop. Thus a 48-inch cooktop needs a 1,200
CFM fan. This standard comes from the commercial sector where
codes are very specific about the installation of kitchen fans.
This is in sharp contrast to how kitchen fans are installed in a
residence. There is no language in the residential codes that ensure
the safe installation of large commercial fans. The residential
kitchen ventilation requirement in all three Model Codes and ASHRAE
Standard 62 is for a 100 CFM fan or range hood. The industry
recommendation from the HVI is for a minimum of 40 CFM per linear ft
of cooktop. According to this rule, a 48-inch cooktop needs only a
160 CFM fan.
Please see an online article from Home Energy for some insight into
using
oversized commercial kitchen fans in a residence.
Makeup Air
To operate any fan safely, the air that is exhausted from the inside
to the outside must be replaced. This is why the Uniform Mechanical
Code (UMC) requires makeup air for commercial kitchens. Makeup air
is air that is intentionally pushed or pulled into the building in
an amount more or less equal to the amount of air that is exhausted
by the kitchen fan. Makeup air prevents fuel spillage and
backdrafting from vented combustion appliances, such as gas water
heaters or fireplaces, within the restaurant. Another purpose of
requiring makeup air is to ensure that the fan can actually exhaust
at its rated capacity. If the fan isn't running at its rated
capacity, it does not maintain sufficient air velocity to trap the
various emissions from the cooking surface. A building's tightness
and exhaust duct configuration will affect the amount of air that
fans inside it can exhaust. The tighter the building, the greater
the pressure the fan has to operate against and the less air the fan
can move. The amount of air that a fan can move against any given
pressure is called its fan curve. Get too low on this
fan curve, and a 1,200 CFM fan might exhaust only 1,000 CFM.
Makeup air is usually tempered (heated or cooled to offset the
outside temperature) to ensure the comfort of kitchen staff.
Installing makeup air devices also ensures that large amounts of air
from the eating area of the restaurant are not drawn into the
kitchen. This can create drafts that affect the comfort of the
diners.
In many jurisdictions, the owner of a commercial building must pass
a performance-based test that certifies that the makeup air is
sufficient for the fan and grease filter to work safely. Testing and
balancing companies typically administer these tests. They measure
the flow of both the exhaust and supply fans and check to make sure
the restaurant is not depressurized.
Performance
The right path
is performance. This means that a test is used to determine whether
a given installed system works as specified. Plumbers must
pressurize the plumbing system, and it must hold that pressure for a
given amount of time before an inspector approves it.
Determining the leakiness of a plumbing system using a prescriptive
approach would result in leaky piping and high utility bills.
An example of a code that is taking the path of performance is the
Canadian General Standards Board 51.71-95. This code sets limits for
depressurization caused by any source for a variety of combustion
appliances. The limits are 5-20 Pa for various types of combustion
appliance, such as fireplaces and condensing furnaces. This code
requires testing. Nothing is assumed about the leakiness of the
house; nothing is assumed about how much air a fan is exhausting.
The worst-case depressurization test tells the inspector whether the
system passes or fails. The 1999 Minnesota Energy Code uses a
similar performance table with limits of 2-25 Pa for different
appliances. It also allows the use of a prescriptive table or
performance testing. The code requires a supply fan for kitchen
exhaust flows over 250 CFM where direct vent appliances are used and a
supply fan for all kitchen exhaust for other appliances. Flows must
be matched within 10%.
Do It Right
Kitchen fans are important to maintaining good indoor air quality.
At least one manufacturer says that its larger fan promotes a
healthy house by "helping the house breathe." Unfortunately, if what
the residents are breathing is combustion by-products drawn into the
house by a kitchen fan, then these powerful fans only serve to
undermine the indoor environment and endanger their health.
If large kitchen fans are installed at all, they must be installed
as a part of a kitchen ventilation system that includes makeup air,
and these systems must be performance-tested in the field.
Steps to take
into consideration when designing a
makeup air system:
Step 1:
Many companies install multiple fans and multi-speed fans in the
same hood so that they can be switched on incrementally.
While more power is important, what is
probably driving the market is the aesthetic appeal of
architecturally designed range hoods. Hoods are often the focal
point of the modern open commercial kitchen.
Step 2: Decide where to introduce the makeup air. The
logical choice is to bring air into the restaurant kitchen
somewhere.
Step 3: Determine how much makeup air is needed. If
sufficient makeup air is provided, no depressurization will occur.
Obviously, some makeup air will be provided through leakage.
However, it is best not to count on that. Size the makeup air to
match the fan.
Step 4: Decide whether or not to temper. Except in very
mild climates, it will probably be necessary to temper the incoming
air during cold weather. This is most easily done by using inline
duct heaters, also known as resistance heat, that kick in when the
incoming air is below a set temperature.
Step 5: Develop a control strategy. The makeup air
system must work whenever the kitchen exhaust is running. When the
fan is turned on, a motorized damper opens and the supply fan turns
on.
The supply fan moves more air when the kitchen fan is turned to a
higher setting. If the temperature of the air is below preset limits
(50° of supply air), the inline heater comes on.
Step 6: Test the system. Worst-case depressurization
testing is critical. It is impossible to know the tightness of the
building and the interactions between the makeup air system and other
mechanical systems without testing. The worst case depressurization
test tells you if the makeup air matches the exhaust air, and if
other combustion appliances will backdraft.
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