ASHRAE Journal - July 2013 - 44

BUILDING SCIENCES

Figure 3: Satisfaction. We satisfy 60% to
70% of the makeup air requirement with
the modified backwall approach introducing air at floor level (the “blue arrows”
at the lower grilles on each side of the
appliance). We now provide a second
system that meets the rest—plus a little
bit (around 350 to 400 cfm), and this air is
typically preconditioned and introduced at
the ceiling at the perimeter of the kitchen
area (the “blue arrows” coming from the
two ceiling registers). We “suck” at the
center of the space at the appliance and

“blow” at the perimeter. The exhaust
hood and the two makeup air systems
are interlocked. When they are operating the rest of the house does not know
they are there. We, in essence, have
uncoupled the kitchen from the rest of
the building HVAC systems (the “black
dashed arrows” entering from the floor
registers and leaving up high on the
wall). We condition the kitchen cooking area with the house HVAC system
pretending that the cooktop/oven and
range hood are not there.

Figure 4: Smaller Hoods Are Easier.
With a 200 cfm kitchen range hood, we
interlock the range hood exhaust with a
supply fan that provides makeup air at
roughly a 10% greater flow rate than
the hood exhaust rate. This makeup air is
typically provided in a separate zone like
a basement that provides ample tempering of the makeup air by mixing. It then is

introduced via floor registers adjacent
the cook surface. The zone around the
appliance needs to “suck” but the rest
of the space blows or is neutral. The
10% greater flow rate works because
this air is not directly introduced under
the appliance but into the adjacent
zone and moves into the cooking area
as transfer air.

running exhaust fan—but really? Are we prepared to go that
far to compensate for the inherent deﬁciencies of exhaust-only
ventilation?
If you don’t suck on the house, storing bad stuff in the garage
is now better than storing bad stuff under the kitchen sink or in
a closet or in the basement. Of course, if you have a machine
shop in your garage, you need to provide a ventilation system
for the garage even if you have a balanced ventilation system
or supply ventilation system in the house.‡‡
Kitchen range hoods are beautiful things and ugly things at
the same time. They can backdraft anything and everything.
They need makeup in today’s houses or yesterday’s houses
that we have tightened. Can’t be any more straightforward
than that. So how to do it? Now would be a good time to
read Chapter 33, “Kitchen Ventilation,” of the 2011 ASHRAE
Handbook—HVAC Applications. I will confess that I didn’t
when I ﬁrst started dealing with these issues and learned the
lessons the hard way.
Let’s say that you have a 1,000 cfm kitchen exhaust hood
going into a tight house. Happens all of the time. When the
exhaust hood is operating, you can suck a golf ball down a
garden hose with the resulting negative pressure. The ﬁrst
thing we make sure of is that the exhaust hood actually
44

ASHRAE Journal

works to capture the efﬂuent. The hood must overhang the
cooking surface big-time. The absolute best approach is to
use a backshelf hood with side panels and large overhangs
on both sides and the front (Figure 2). Nothing new here.
Read Chapter 33.
Now, we need makeup air. When dealing with large hoods
(greater than 400 cfm), we recommend providing two kinds
at the same time but at different places. First, we provide
direct makeup air using a modiﬁed backwall approach such
that this makeup air is introduced at ﬂoor level (Figure 2).
This direct makeup air introduced at ﬂoor level should never
provide more than 60% to 70% of the hood exhaust. So, for
our 1,000 cfm exhaust our direct makeup airﬂow should
be between 600 cfm and 700 cfm. This air is rarely conditioned—except when we deal with rich clients in Aspen,
Colo., or Naples, Fla.§§ “If you can’t stand the heat, get out
of the kitchen” is based on reality.
Note that at this point, we have only satisﬁed 60% to 70% of
the makeup air requirement. We now provide a second system
that meets the rest, plus a little bit, and this air is typically
preconditioned. In our example we now provide around 350
cfm to 400 cfm of conditioned makeup air at the perimeter
of the kitchen area (Figure 3). We “suck” at the center and
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July 2013

ASHRAE Journal - July 2013

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