The main reason ventilation is required to maintain a reasonable level of indoor air quality is the fact that people are in buildings. Their bio-effluents have to be removed and diluted. The required flow rates are therefore normally expressed in volume per person. Sensors may also directly control the demand based on the CO2
concentration, and can improve the air quality while saving energy. The operation control regime should be considered for the control of ventilation rates and air distribution to maintain acceptable indoor air quality. Typically, minimal ventilation rates for air quality control (background ventilation, i.e., of the order of 1 ACH during building occupancy) will be less than those required for direct ventilative cooling (see § 2.2.4). The minimal ventilation rate is reasonably controlled automatically, as personal detection of air quality conditions is generally too subtle to be considered [49]. The control is critical in winter conditions and can have significant energy consequences.
Thus, air quality control will normally not be an issue during direct ventilative cooling. To reduce heat losses to a minimum without heat recovery in winter, it is favourable to restrict the ventilation intervals (to a few minutes). Higher ventilation rates can supply the total required amount of air in a shorter time period. During these short time periods, the building heat losses are smaller because most of the heat is stored in the building fabric, and the entering fresh air quickly heats up again.
In a number of cases with the so-called ‘sick building syndrome’ symptoms, people rely on the strategy to ventilate more because of high emissions from the building, the furniture materials and the badly maintained mechanical ventilation systems. This is not very energy efficient. The goal must be to keep emissions as low as possible.
The strategy therefore is source and product control, but not ventilation.
Indoor air quality is usually evaluated by the CO2 concentration indicator.
Unfortunately, there is no agreement on the limit values for good air quality (see Figure 2.23). According to EN 13779 [55] high indoor air quality (IAQ) is achieved with less than 400 ppm above the level of outdoor air, medium IAQ in a range between 400 to 600 ppm, moderate IAQ from 600 to 1000 ppm, and low IAQ above 1000 ppm. The German Federal Ministry for the Environment, Nature Conservation
and Nuclear Safety considers 1000 ppm in schools as hygienically good, from 1000 to 2000 ppm as hygienically noticeable, and above 2000 ppm as not acceptable.
EN 15251 [41] uses 1000 ppm as the upper limit for the design of ventilation systems. According to the Commission Delegated Regulation (EU) No 244/2012 [56], ‘energy efficiency measures … shall be compatible with air quality … levels according to CEN standard 15251 on indoor air quality or equivalent national standards’.
Figure 2.23: Monitored and allowed CO2 concentrations [21,41,55,57-59].
2.3.2 Thermal comfort
Thermal comfort is seen as a state of mind that expresses satisfaction of the occupants. It is assessed by subjective evaluation [60] since the occupants will desire differently based on their physiology and psychology. Besides the psychological parameters such as individual expectations, thermal neutrality is maintained when the heat generated by human metabolism is in thermal equilibrium with the surroundings. The main factors that influence thermal comfort are the metabolic rate, the clothing insulation, the air temperature, the mean radiant temperature, the air velocity, and the relative humidity.
0 200 400 600 800 1000 1200 1400 1600 1800 2000
CO2 concentration in ppm
400 ppm external concentration
assumed for city centres
The Predicted Mean Vote (PMV) model [60] is one of the most recognised thermal comfort models, and has been incorporated into a number of standards and design codes (e.g., ISO-7730 [61]). But the PMV method should be applied only to air-conditioned buildings, while the adaptive model can be generally applied only to passively operated buildings where no mechanical systems have been installed. The adaptive model was developed with the idea that outdoor climate influences indoor comfort as occupants dynamically interact with their environment. The operative room temperature is allowed to increase in naturally ventilated, non air-conditioned buildings with rising ambient air temperatures. Occupants in the warm period control their thermal environment by means of clothing, controllable natural ventilation, fans, and shading elements [62]. Extensive field studies showed that the occupants of naturally ventilated buildings do accept and even prefer a wider range of temperatures than in air-conditioned buildings because their preferred temperature depends on outdoor conditions.
Adaptive comfort models are implemented in standards such as European EN 15251 and ISO 7730 standard, and slightly different in the American ASHRAE 55 standard.
Contrarily to the ASHRAE 55 standard [63], and in accordance with the EN 15251 standard [41], the adaptive approach can be applied to hybrid (mixed-mode) buildings whenever the mechanical systems are not running. In contrast to the PMV model, the adaptive model does not reflect the influence of humidity. According to the Commission Delegated Regulation (EU) No 244/2012 [56]: ‘energy efficiency measures … shall be compatible with … indoor comfort levels according to CEN standard 15251…. In cases where measures produce different comfort levels, this shall be made transparent in the calculations.’
The temperature excess method cumulates the hours with room air temperatures above a given setpoint and compares them with limiting values, e.g., 5% of all office hours.
In this thesis, the acceptable temperature setpoints were calculated following the adaptive comfort limits, which are defined in the European standard EN 15251 [41].
Depending on the exponentially weighted running mean of the daily mean ambient air temperature series of the previous week, recommended operative temperatures are calculated for different comfort categories (for details see § 2.6.3). The criteria were obtained through investigations in office buildings with user operated windows [48].
2.3.3 Energy consumption
The HybVent project investigated that ‘in well-insulated office buildings, which are becoming more and more common in IEA countries, ventilation and cooling account for more than 50% of the energy requirement’ [3]. Due to passive cooling and controlled natural ventilation, there is no energy consumption for cooling and ventilation. It is assumed that if thermal comfort can be guaranteed without air conditioning, then significant cooling and ventilation energy conservation can be achieved [64]. Energy savings by natural ventilation can mostly only be evaluated when simulation tools are used, as two identical buildings with different ventilation or climatisation strategies are rarely available for monitoring. The savings on venting, heating, and cooling energy can be determined by comparing natural ventilation strategies (while maintaining thermal comfort) with an identical office building for which mechanical ventilation is used (e.g., utilizing building energy simulation tools).
A 30% reduction of the cooling energy consumption and 40% reduction of the installed cooling capacity was predicted for a UK low energy office building with a stack driven night ventilation air change rate of 10 per hour [51]. 40% reduction of the daily cooling demand was simulated for a high thermal mass office building in Belgium [65]. Blondeau investigated that night ventilation with air change rates of 8 per hour can reduce cooling requirements by 12 to 54%, depending on the temperature setpoint [18].
The primary energy consumption of naturally ventilated office buildings in Denmark was compared with that of mechanical ventilation systems [66]. The naturally ventilated buildings consumed 40 kWh/m² per year, whereas the consumption of mechanical ventilation systems varied from 50 kWh/m² per year (VAV system) to 90 kWh/m² per year (CAV system). The primary energy conservation for naturally ventilated office buildings in Belgium was calculated to be 8 kWh/m² per year [23].
Studies conducted on the 23 storey Liberty Tower of Meiji University in Tokyo [3]
showed that about 17% of energy consumption for cooling is saved by using the natural ventilation system.