ASHRAE Journal - September 2021 - 59

COLUMN BUILDING SCIENCES
PHOTO 1 Hotel Rooftop Exhaust. Air is extracted from each bathroom with rooftop
exhaust fans. Each rooftop fan exhausts multiple hotel rooms. The rooms are
typically " paired " to a neighboring room and the pairs " stacked " over each other
with a vertical shaft connecting the bathrooms to the rooftop exhaust fan.
FIGURE 1 Typical Hotel Ventilation. Bathroom exhaust without ducted suite
makeup air leads to suite negative pressures.
Rooftop
Exhaust Fan
Bathroom
Exhaust
Corridor
Outside Air
Supply
Rooftop
Exhaust Fan
Bathroom
Exhaust
26, Table 6.1 It cites Kumar Kumaran's work in 2002
(National Research Council of Canada legend, yeah
Kumar!). The second source is from the Wall Coverings
Association1 with no formal publication date, but no
references were cited after 2002-so, I am guessing from
2002.
There are two wildly generic types of wall covering:
Type I (light duty) and Type II (medium/heavy duty).
Note that wall coverings are sold by weight per linear
yard, not square yard, and a roll is 27 in. (686 mm) wide.
Arithmetic is required. Kumar being Kumar gives us
values in metrics that are useful, such as 3.5 lb/100 ft2
(171 g/m2) -not so with the Wallcoverings Association.
Anyway, check out the values:
* Type I vinyl wallcovering (5 oz [142 g]) 4 perms at
50% relative humidity (RH);1
* Type I vinyl wallcovering (12 oz [340 g]) approximately
1 perm, ASTM E96-95, method B " wet cup " ;2
* Type II vinyl wallcovering (20 oz [567 g]) less than 1
perm, ASTM E96-95, method B " wet cup " ;2
These are wet cup values. Installing this type of material
on the inside of an air-conditioned building located
in a hot-humid, mixed-humid, marine or cold climate is
not a good idea.
Pressure
Now for the fun. First the obvious, and then the not
so obvious. The least expensive ventilation approach in
many hotels is rooftop exhaust (Photo 1) with air being
extracted from each bathroom with the intent of supplying
makeup air from the corridors via undercut doors
(not legal everywhere) (Figure 1) or supplying makeup air
directly to each room from " through wall " AC (packaged
terminal air conditioners [PTACs] or packaged terminal
Bathroom
Exhaust
Bathroom
Exhaust
Bathroom
Exhaust
Bathroom
Exhaust
Hotel Room/Bath
Suite
Corridor
Undercut
Door
Hotel Room/Bath
Suite
heat pumps [PTHPs] (Photo 2). Sometimes, not often,
there is ducted makeup air from central systems to balance
the pressures.
Except for the ducted makeup air from central systems
approach, the hotel goes negative pretty much all
the time, even with the " through wall " PTACs or PTHPs.
How many rooms? Say 100. The central exhaust fans run
all the time while the PTACs and PTHPs rarely run. The
duty cycle of the PTACs and PTHPs are different then the
on-time of the central exhaust system. You could sort it
out with effort, but it rarely happens.
But let's say you got it to work, and the exhaust air
is more than balanced by the supply air-by supplying
10% to 15% more makeup air than exhaust air, and
you do this everywhere in the hotel, including every
room. Yeah! The hotel is pressurized. Or is it? You have
addressed one problem but are not dealing with the big
problem, which is not so obvious.
Check out Figure 2. You typically end up with wall
cavities that are negative when the hotel is neutral or
positive, relative to the exterior. Huh? No way! Yup.
Walls typically suck. The culprit is in Photo 3: the ceilingsuspended
air-handling unit (AHU) in the dropped ceiling
area in the short hallway to the suite bathroom. Note
S E P T E M B E R 2 0 2 1 ashrae.o rg ASHRAE JOURNAL
59
Central Exhaust
Duct
Corridor Air
Supply Duct
Central Exhaust
Duct
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ASHRAE Journal - September 2021

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