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 10in
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 20sqin 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 air.....by 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 combusting 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.
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.