By Nalliah Thayabharan
Expert Building inspections Ltd
905 940 0811

Kitchen fans are very important to maintain good indoor air quality, since they remove  odours,  
excess moisture,  grease, combustion products  and pollutants from cooking.  

A typical  builder model kitchen fans  are rated about  200 cubic feet per minute (CFM) of
exhaust  and is generally adequate  to eliminate  excess moisture and odours. Manufacturers
provide air delivery ratings, but actual flows vary depending on  how much  restriction is created
by filters, ducts, grills, screens, and louvers.

Width of the kitchen fans should be same as width of the stove, and should be installed  less than
30in  above the stove top.    20in deep kitchen fans pickup from the front area of the stove much
better than 17in deep kitchen fans.

Low profile designer kitchen fans  and  kitchen fans combined with  micro wave oven  are less  
effective  because  they  cover  less stove area.   The performance of these  kitchen fans  does
not improve  even when fans are installed only 15 inches from the stove top.

Manufacturers recommend correct size smooth galvanized ducts and short duct runs to maintain
the rated flow.  Ducts  should be secured with screws and hangers.  Duct joints should be taped
with long life foil tape.

Though  urban Home owners  like low wattage fluorescent lights  and  low flush toilets,   when it
comes to  kitchen,  they like bigger refrigerators and bigger kitchen fans which typically exhaust
more than 500CFM.

Most Home owners  are not aware  that  these  bigger  kitchen fans  create depressurization  
which causes  back drafting (pulling fumes back into the house), flame roll out and fuel spillage
with vented combustion appliances such as fireplaces, furnaces and water heaters. Backdrafting
is a health, safety  and comfort concern.   Houses need to  inhale as  easily as exhale.   If there is
a law  to describe  proper ventilation,  that  might be    "Air  out equals air in".

In addition to serious back drafting hazard, the bigger fans are causing high utility bills by  
exhausting very large amounts of  conditioned air  from the house.  A typical 2 storey 1600sqft
house (800sqft of area in 3 levels) with 8ft high ceiling has 19200cubic feet (800x8x3) of indoor
air which can be completely removed by a  700CFM kitchen fan  (19200 / 700 =27)  in less than

Depressurization can speed the entrance of radon and other soil gases into the house. It can also
create air infiltration through the building envelope.

After the  bigger kitchen fan is installed,  the installer must  verify  that the kitchen fan is not
causing any vented combustion appliances to backdraft.

While  holding  a smoke stick of neutral density  up to the draft diverter  or dilution port of  each
vented combustion appliance,   the  installer should turns on all exhausting devices in the house
including kitchen exhaust fan, bathroom fans, clothed dryer and central vacuum.  Test must be
continued with the furnace blower fan both on and off because unbalanced air flows in forced air
duct work can contribute to depressurization problems.

If the test reveals back drafting, the installer should  open a window within the depressurized zone
until back drafting stops.   If it required only a little bit of opening (10 to 20sqin),  a passive duct  
with  automatic damper will supply adequate makeup air to prevent depressurization.

According to "R2000 Program - Makeup Air Guidelines" a 200CFM kitchen fan requires a 10 in
diameter passive duct and 300CFM kitchen fan requires 12in diameter passive duct.  Bigger
kitchen fans (greater than 600CFM) are not even listed in the makeup air guidelines.

If it  takes  more than  20 sq in  of  window  opening  to  alleviate  the back drafting, a fan forced
makeup air system is required.  Partially tempered fan forced makeup air can be introduced into
any adjacent space not blocked by a closable door or return side of the forced air duct work.

As a  result of  extensive research  on  depressurization  in the 1980's,  the Canadian General
Standards Board has a useful standard to refer to- "The Spill  Test:  a method  to  determine  the  
potential  for  pressure  induced spillage from vented fuel fired space heating appliances, water
heaters and fireplaces".  The standard was adopted in 1995 and resulted from extensive
research on venting by Canada Mortgage and Housing Corporation -CMHC.  Most of the
procedures described in the standard are familiar to any blower door  operator  who  has  done  
a  worst  case  depressurization test. The standard sets different limits for continuous and
intermittent depressurization.

The National Building  code has  two relevant prescriptive sections about depressurization.   
"Protection  against  depressurization"  requires  that in houses  with  fuel  fired  appliances  
vented  through  a  chimney,  " any mechanical air exhausting device or group of devices,
operated by a single control, with net exhaust capacity greater than 160CFM shall be provided
with makeup a supply fan rated to deliver not less than amount by which the net exhaust
rate of the device exceeds 160CFM....wired so that it is activated whenever the device is

The National Building Code also requires that "The return air system shall be designed   so  that  
negative pressure  from the circulating fan can not (a) affect the furnace combustion air supply nor
(b) draw combustion products from joints or openings in the furnace or flue pipe".

The National Building Code requires only that the makeup air reduces the air flow difference to
160CFM and does not require that all major exhaust devices to be interlocked to the makeup air
fan.  The kitchen fan installer can install either a sail switch in the duct or relay to control both the
exhaust and makeup air fan.

I have inspected many houses where chimney backdraft virtually every time these bigger kitchen
fans are turned on.  

The biggest backdrafting  concern with  natural gas furnace is  poisonous carbon monoxide
-"CO".  A poorly serviced, broken or dirty gas furnace can produce lots of CO.   Some fireplace  
backdraft  contain  CO  and cancer causing benzene.  

Losing about 20% of the air in the home due to powerful kitchen exhaust fan is equivalent to
being lifted to place which is 13,000 feet (4,000m) higher in elevation. In new energy efficient air
tight houses, it is impossible to infiltrate an adequate amount of air at a reasonable pressure to
satisfy the exhaust demand to prevent serious depressurization.

Chronic depressurization may cause both mental and physical effects including a loss of
judgement,  muscular incoordination and an impairment of colour, night and peripheral vision.
Hearing also deteriorates with chronic depressurization . Occupants also experience hot flushes
and turn bluish at the extremities (cyanosis). Simple tasks become extraordinarily difficult and
performance fails. Occupants of a severely depressurized home are more susceptible if they are
sick, stressed, fatigued, under the influence of drugs. Oxygen shortage due to depressurization
can cause several major illnesses including heart problems, cancer, digestion and elimination
problems, respiratory disease, inflamed, swollen and aching joints, sinus problems, yeast
infections and even sexual dysfunction.

From a rational point of view,  a working conventional fireplace  does not belong in a modern
house with bigger kitchen exhaust fan.  A Home owner who plans to use a fireplace frequently
should not only make sure the house has balanced ventilation, but should also install a reliable
CO alarm in the same room  as  fireplace.  If  the alarm sounds  frequently,  occupants will quickly
learn to change the habits,  or install a reliable ventilation system.

I have also noted at some houses, the makeup air vent is covered up by the  chilly incredulous

Although recirculating kitchen fans  which are vented inside do not cause depressurization, they
remove only a little grease and odour at best.

To avoid depressurization problems with vented combustion appliances, the kitchen fan  must
be  correctly sized  and installed, and if needed provide dedicated makeup air.