My
offer to range
hood manufacturers, dealers and sales and marketing people.
"...sticking a 1200 cfm
exhaust fan into a typical residential kitchen is not like
playing with a hairdryer it’s more like having a home
appendectomy with the suction side of an industrial grade
leaf blower."
"...what these
hood S & M folks (sales and marketing) need is a simple
lesson from the Whitney Houston School of Building Science;
you know the “Crack is Whack” rule…and when suck trumps
blow, holes and cracks in the enclosure become the conduit
to contamination."
"...commercial
range hoods in residential applications without proper
make-up air fly’s in the face of everything Health Canada
wants Canadians to avoid."
Outdoor air does not equal "fresh air"
Only when the outdoor air is free of
contaminates can it be considered fresh. It should never be
assumed to be "fresh" when common outdoor pollutants include
particulate matter (organics and non organics), ozone,
moisture, polycyclic aromatic hydrocarbon (PAH's) etc...
The following industry design guides do not
replace the need for a design professional - in all cases
the authority having jurisdiction must be consulted and
where guidance is absent
consult with a design professional.
Gas ranges should have at least 100 cfm
exhaust air per 10,000 Btu/hr input and a minimum of 500
cfm.
Add 200 cfm for cook tops with grills.
Never have less than 100 cfm per 12" width of
cook top surface area. See Schlieren images
at different exhaust rates
per linear foot.
The cfm required is the net flow after allowing
for duct and fitting pressure losses, i.e. its not
sufficient to buy a fan for a required cfm only to loose its
capacity to duct resistance.
Mount hoods between 18" to 24" for standard
units and no more than 30" for commercial models.
Install a make up air system equal in
capacity to the exhaust air system.
For gas ranges provide combustion air as per
the authority having jurisdiction.
Do not use recirculating vent hoods, vent all
products to the outdoors.
For quieter operation use a remote fan
instead of a fan located above the stove top.
For residential units select the fan using standard selection
procedure accounting for duct losses at 600 fpm to 900 fpm.
Use 16 guage or 18 gauge insulated stainless
steel ducting, i.e. do not use flex duct.
Pitch ducting downwards back towards the
stove top.
Install an appropriate back draft damper at
the exterior surface of the enclosure.
Use a hood that covers the entire stove top
plus a minimum additional 6" on each side.
Reduce drafts around the range hood from
ceiling fans, windows, doors or supply air outlets.
Do not blow make up air directly at the hood or stove
top. Let the range hood do its job of capturing fumes and let the range hood fan do the exhausting.
Air speeds external to the range hood should not
exceed 100 fpm.
Preference is given to 100% dedicated make up
systems. Alternatively make up air could be delivered from
supply air grills within the vicinity of the hood plus air
supplied from floor registers used to condition the space.
Provide a dampered make up air duct to the
return air stream of the air handler. Interlock the damper
with the range hood, i.e. end switch on damper closes
contacts on range hood fan.
Conditioning of make up air must account for
energy and mass transfer, i.e. heat, moisture, particulate
etc.
For additional
support on this topic visit our
visitor services
page.
"Measured airflows were substantially lower
than the rates claimed in product literature for most of the
fans evaluated. Only two of the 13 independent models (three
installations used
same model) had actual airflows that were 90 percent or
greater of the advertised values."
LBNL, 2011
Offer:
Are you a range hood manufacturer, dealer,
sales and marketing person?
If you have professional design guidance for
residential range hoods that can pass my sniff test I'll
give you the opportunity to submit it to me for
consideration.
If I like what you have to say I'll include a
link from this page to your page at no cost to you.
Introduction
Am I the only one noticing an alarming rise in the misguided use
of commercial type range hoods inside residential buildings? I
mean it’s always been a problem but twice in the last week I’ve
had people call up asking if their already installed monster
sized range hood is going to be ok?
“What do you mean by ‘Ok’ I
ask?” “Do you mean is it going to need therapy?” So I follow up
in my most empathetic voice, “Do you know what happens when you
try to Hoover up the inside of your home without considering the
ramifications of pressure differentials?” “Hmmm mubble mubble…pause…ah
no” is all I can barely hear on the other end.
So once again in my famously soft
spoken demeanour (choke and cough) I ask, “So now you want my
advice, after you bought and had it installed, am I right? Look
my new friend – that’s a question you should
have asked a building science professional before you became
enamoured with the
manufacturers glossy range hood brochure.”
Figure 1. The suction power of a commercial range hood is
sufficient to put a home into deep negative pressure relative to
the outdoors. Such differentials can cause back drafting from
combustion based appliances and promotes entry of other IAQ
pollutants and contaminates.
For those looking for a way to explain it to appliance addicts
in a consumer friendly way, tell them sticking (for example) a 1200 cfm
exhaust fan into a typical residential kitchen is not like
playing with a hairdryer it’s more like having a home
appendectomy with the suction side of an industrial grade leaf
blower.
Figure 2. "Trends in Kitchen Design" make note of the range hoods...it's exactly this type of
trend that creates building and health science issues for home
owners. If the video doesn't play in IE try Firefox.
What we have
occurring in the marketplace is ignorant range hood sales people supported by
their just as naïve range hood marketing departments selling
range hoods to “doe-eyed” consumers (as per video
above) and when it comes to
building science - all of them put together don't know their
water column from their water dispenser. In my 40 grit opinion the
potential health and building
problems created by hood induced negative building pressures should rest
squarely on the appliance manufacturers, their dealer’s
and the kitchen designers shoulders. The HVAC industry needs
to step up and tell these service providers that when you
continuously suck
way more than you blow you’re going to create problems for
the occupants and the building - full stop.
Really, what these hood S & M folks (sales and marketing) need
is a simple lesson from the Whitney Houston School of Building
Science; you know the “Crack is Whack” rule…and when suck trumps
blow, holes and cracks in the enclosure become the conduit to contamination. Why?
Because when it comes to buildings you can always count on the
inside wanting out, and the outside wanting in; and just what in
pray tell do these hood salespeople think is going to come
through the cracks when you suck the insides out of a home?
According to my conversation with one dealer of hoods, "those
are
fresh air cracks"…crack head comments like that give me
a headache.
Figure 3. When the outdoor pressure exceeds the indoor pressure,
mass and energy will flow from the outside in. The mass could be
organic and inorganic contaminates (water
vapour, ozone,
mould spores, odours,
particulate matter etc.) and the energy will be heat and
sound.
At a time when the Federal Government is spending our tax
dollars on a coast to coast
“got bad gas” campaign alerting our
fellow citizens to the dangers of having radon - an entire
appliance industry is flogging kitchen bling which enables
environmental contaminates like gases and particulate matter to
enter at free will. It's like having your own illegal immigrant
border crossing problem where the home owner and fan are the
unsuspecting smugglers; victims of their own unintended
consequences.
So let’s put this problem into perspective by taking a nice
sized hypothetical 20 ft. x 30 ft. x 9 ft. (6m x 9.14m x 2.74m)
kitchen and let’s play my favourite game of, “what if?”
What if
that 5400 ft3 (153m3) kitchen was sealed up and the 1200 cfm
(0.57m3/s) kitchen exhaust fan activated. Well it would only
take 4.3 minutes (5400ft3/1200cfm) to remove all the air such
that the pressure would approach 0 psia (assuming the home was at
sea level and for giggles assuming the fan was capable of
drawing down that low).
Now you know and I know that it takes
longer than 4.3 minutes to barbeque a pig inside your home and
we can all agree homes are not hermetically sealed and Mother
Nature abhors a vacuum; so what must happen is over the time the
fan is on, replacement air of some quantity and quality must find a way
through the enclosure into the space.
If the replacement air
were supplied at the same rate as the exhaust air it would be
equivalent to 14ACH or almost 70 times more than its natural
rate for a well-built home. Now I know this has been greatly
simplified (ignoring the total volume of the home, run time,
size of pig, etc.) but the
fact remains if the supply air volume were less than the exhaust
air volume, then interior space pressures must drop below
atmospheric pressure which motivates the outside to move
inwards (see Table 1 below).
But what if the above grade enclosure and all its
corresponding penetrations are for arguments sake “sealed” or
“sealed” to such an extent that any cracks in, at and around the
below grade enclosure became the prime source of relief and what
if that air is contaminated with your favourite cancer causing
gas – like radon? Or what if the combustion venting for
heaters, stoves and fireplaces became a back drafting relief connection
between the outdoors and indoors? No one survives an extended
exposure to carbon monoxide. This is just one of many examples of where
commercial range hoods in residential applications without
proper make-up air fly’s in the face of everything Health Canada
wants Canadians to avoid…time again to rewrite Oh Canada into
Woe Canada.
Table 1. Approximate total
exhaust airflow (cfm) needed to induce depressurization
Notes: assuming
cfm is delivered flow at design static pressure, and ignores
cycle time:
Example 1. A 215
cfm exhaust fan is capable of inducing -10Pa
depressurization in a transitional (very tight) home
constructed to 2.0 ACH50.
Example 2. A 185 cfm
exhaust fan is
capable of inducing -2Pa depressurization in a traditional
(typical) home constructed to 5.0 ACH50.
At 5 ACH50, 150
cfm range hood + 200 cfm dryer fails combustion safety test
Adapted from:
Singer, B., Kitchen Ventilation Solutions to Indoor Air
Quality Hazards from Cooking, Lawrence Berkeley National
Laboratory, October 10, 2013
Table 1. Provides an estimation of how much airflow and
at what ACH50 to induce depressurization. At -5Pa its enough to
cause a health and safety risk due to back drafting of naturally
vented appliances. If you don't want to induce negative
pressures you will have to provide relief air - and in many
climates this air will need conditioning.
All right what if we could convince the crack head hood dealers and
their innocent users that sucking bad stuff into the home isn’t the smartest of
moves and have them put in a proper make up air unit, then what?
Well let’s calculate the winter sensible heating load
using;
q = Q * 60 min/hr * p * cp * (ti –
t0)
Where
q = load, Btu/hr (kW)
Q = flow, cfm (l/s)
p = density, lbm/ft3 (kg/m3)
cp = specific heat, Btu/lbm F (kW/kg C)
ti = air temperature inside, °F (°C)
to = air temperature outside, °F (°C)
Assuming an outdoor air temperature (to) of -30°F(-34°C) and an
inside (ti) of 70°F(21°C), and standard air conditions, the
winter sensible load (q) at 1200 cfm (0.57m3/s) becomes in IP
units;
q = 1200 * 60 * 0.075 * 0.24 * (70 - -30)
q = 1200 * 1.08 * 100
q = 129,600 Btu/hr (38kW)
Figure 4. For the
non-technical person - imagine 129,600 wooden matches burning
per hour because that's a visual heat approximation for one hour
of pig roasting time.
Putting this into perspective - with that amount of output you
could heat a floor space over 10 times that of the kitchen it is
serving. If you did the same exercise but for summer time
sensible and latent cooling you would likely find a similar load
for dehumidification of incoming outdoor air. How many people
would put in for giggles a 10 ton cooling plant just to wring
out the moisture from the make-up air in a residence? Not
likely, but if they don’t dehumidify the incoming air, it’s very
probable in a conservative sized home that the indoor conditions
would soon represent those of the outdoor conditions, and having
a space humidity exceeding 70% RH is just begging for mould (mold
for my U.S. readers) to
thrive and multiply.
Furthermore if adequate relief is not
provided, outdoor moisture will be pulled into the enclosure
cavities where it will also likely encounter a vapour barrier
which serves as a capture plane. If the home is air conditioned
and the outer most surface of the drywall is below dew point
then the problem is exasperated further.
Ok I get once again that I’m taking this to a worst case (but
not impossible) scenario but it needs to be stated so that
appliance dealers and their unsuspecting users don’t find
themselves as an “in-duct-ee” of Canada’s overburdened
healthcare systems from the unintended consequences of monster sized range hoods installed without
proper make up air.
Figure 4a. Make up unit for kitchen range hoods (hydronic systems). Click for larger image. Note: when selecting range hoods understand there are two distinct functions. The first is the capture efficiency of the hood. This is independent of the fan capacity, i.e.: it's best not to think of the hood as a "vacuum". The extraction process relies on the natural convective plume of heat and contaminant to rise up into the hood, once there it can be removed from the hood with the exhaust fan.
For additional
support on this topic visit our
visitor services
page.
Additional hood
design information Figure 5. Schlieren images at different exhaust rates per
linear foot (lf)20
Figure 6. Range
hoods: kitchen pollutants hazardous to your health
Logue et al., 2013. Pollutant exposures from
unvented gas cooking burners: A simulation-based assessment for
Southern California. Environ Health Persp; Provisionally
accepted
Singer, B.C. et al, 2011, Pollutant Removal
Efficiency of Residential Cooking Exhaust Hoods, Environmental
Energy Technologies Division, Lawrence Berkeley National
Laboratory, LBNL Paper LBNL-4902E,
http://www.escholarship.org/uc/item/16f896xz
Singer, Brett C., 2011, Experimental Evaluation
of Installed Cooking Exhaust Fan Performance, Lawrence Berkeley
National Laboratory, LBNL Paper LBNL-4183E
http://escholarship.org/uc/item/8pv0317x
Brown, S., Dedicated Outdoor Air System for
Commercial Kitchen Ventilation, ASHRAE Journal, July 2007
Kitchen Exhaust and Make Up Air, Technical Papers
and Seminars, ASHRAE Winter Conference, Dallas, Texas, 2013
