Environmental Engineering Reference
zero air change
4 air changes
8 air changes
15 air changes
50 air changes
room temperature at
increasing air change
Date in August 2003
Figure 3.11 Simulated room temperatures as a function of air exchange at constant heat transfer
Using the monitored window opening times from the window contact switch in-
formation, the effect of early evening room temperature increase due to the window
openings can be clearly seen in Figure 3.11. A maximum nightly cooling energy of
about 9 kWh can be theoretically achieved if the air exchange started at 21:00 and
a very high air exchange rate of 50 h − 1 were available (see data on 5 or 6 August).
However, as the windows are manually opened by the user at 18:00, the room first
heats up through the high ambient temperature and 2 . 7 kWh infiltration gains occur!
So, effectively, only 6 . 3 kWh of effective cooling energy can be provided by the night
ventilation as a maximum.
At a more realistic lower air exchange rate of 8 h − 1 , a maximum of 5 . 5 kWh can
be removed if night ventilation starts at 21:00 or 4 . 7 kWh effectively if the win-
dows are opened at 18:00. For similar measured ambient air temperature profiles (see
Table 3.2), and similar measured average air exchange rates (8 . 6h − 1 ), the room tem-
perature decreased less than in the building simulation and the cooling energy of
9 . 3 kWh per night was about double the simulated value. This shows that the air
exchange contained not just ambient air, but was partly composed of air from the
If the user does not close the windows during the daytime, which was the case for
the first ten days in August, there is a very strong increase in daytime temperature.
This additional heat load of more than 5-9 kWh during the day must first be removed
by night cooling.
The heat fluxes fromall internal surfaceswere then calculated to evaluate the thermal
discharging during the night: 40% of the total energy delivered by the room surfaces at
night came from 39m 2 of internal walls, although these walls are light constructions