
measured recirc ulation rates are compared in Figure 0.3. These are seldom the
same. Even worse: as shown in Figure 0.4, out of 27 units planned without
recirculation, 30 per cent have shown a recirculation rate of more than 20 per
cent.
Exfiltration
In supply and exhaust units, both airflow rates are either balanced, or the
supply flow rate is increased a little to make the building slightly overpressured.
When the envelope is not airtight, and when the balance between supply and
exhaust air is too large, air leaks through the envelope. This doe s not have
much influence on indoor air quality, but may strongly decrease the
efficiency of the heat recovery. In some buildings, as much as 100 per cent of
the supply air is lost in this way, as illustrated in Figure 0.5, which compares
0%
25%
50%
75%
100%
0% 25% 50% 75% 100%
Measured
Design recirculation rate
Figure 0.3 Comparison of design and measured recirculation rate in 34 air
handling units
Source: Roulet et al., 1999.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
0% 10% 20% 30% 40% 50%
Measured recirculation rate
Figure 0.4 Histogram and cumulated frequencies of measured
recirculation rates in air handling units designed without recirculation
xvi Ventilation and Airflow in Buildings
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