Environmental Engineering Reference
In-Depth Information
movement such as trees, shrubbery, and other buildings. Such barriers
induce turbulence that reduces wind speed and alters wind direction.
The effect of wind on building infiltration is a squared function of wind
velocity or speed (mph, m/s). The modeled effect of wind speed on house
air exchange rates can be seen in
Figure 11.3
for a
T of 0°F (0°C) and 40°F
(22°C). Though the effect of wind speed on air exchange rates is exponential,
significant increases in infiltration-induced ventilation are only seen at high
wind speeds (>8 mph, 3.56 m/s). As can been seen in
Figure 11.3
,
the
combined effect of stack effect and wind speed on building air exchange
rates (ventilation) can be significant.
Figure 11.3
describes the effect of indoor/outdoor temperature differ-
ences and wind speed on a single house. Because of differences in building
tightness, distribution of leakage areas, and orientation to the wind, air
exchange rates in other houses under similar stack effect and wind speed
conditions are likely to be different (though the general form of the rela-
tionship will be much the same). These curves are based on the following
linear model:
∆
I = A + B(
∆
T) + C(v
2
)
(11.1)
where I
= infiltration rate (ACH)
A
= intercept coefficient, ACH (
∆
T = 0, v = 0)
B
= temperature coefficient
C
= wind velocity coefficient
∆
T = indoor/outdoor temperature difference, (°F)
v
= wind speed (mph, m/sec)
Both the temperature and wind speed coefficients are empirically derived
and differ for each building. Coefficient differences among buildings are
relatively small.
C. Infiltration and exfiltration air exchange rates
As indicated above, building air exchange rates associated with infiltra-
tion/exfiltration-induced airflows vary with indoor/outdoor temperature
differences (which vary considerably themselves), wind speed, and tightness
of the building envelope. They may also be influenced by pressure changes
associated with the operation of vented combustion appliances, bath-
room/lavatory fans, and leaky supply and return air ducts. Each of these
can increase infiltration and air exchange rates above those associated with
combined stack effect and wind infiltration and exfiltration values. Leaky
supply/return air ducts may be responsible for upwards of 30+% of infil-
tration/exfiltration-related air exchange in residential buildings.
In response to energy concerns in the late 1970s and early 1980s, the U.S.
Department of Energy supported several studies to evaluate infiltration and
exfiltration rates in U.S. housing stock. The average air exchange rate for
