OPEN ACCESS
In residential buildings, air renewal is usually entrusted to the occupants, who open windows at will. On the other hand, a controlled mechanical ventilation system (MV) may provide fresh outdoor air by mechanical means, thus diluting indoor pollutants and improving the indoor air quality (IAQ).
The aim of this paper is to evaluate the technical features of controlled MV systems and their energy and financial appropriateness in residential buildings. Several configurations of mechanical ventilation system are designed for a reference residential unit located in various locations of Italy, covering a wide range of climates. For each configuration (singleflow, doubleflow) the overall cost for installation is calculated.
Therefore, for all the configurations of MV system, the electric energy to feed the fan and the thermal energy to counterbalance the heating needs due to the ventilation are determined, and compared to the energy needs in a building without any mechanical ventilation system.
The results show that mechanical ventilation introduces considerable primary energy savings, with an attractive payback time of the investment especially in cold climates. This makes mechanical ventilation systems an appealing technology for reaching the target of Zero Energy Buildings.
Mechanical ventilation, Residential buildings, Heat recovery, Primary energy, Costs.
Building ventilation, that is to say the natural or mechanical inlet of outdoor air, is one of the terms which contribute to the energy consumption for space heating. Natural ventilation usually comes from the intentional opening of the windows by the occupants, pushed by the desire to improve the indoor air quality (IAQ). A minor contribution occurs through cracks and vents, and this depends by the air tightness of the envelope. In both cases, the air flow is a function of the difference of temperature and pressure between indoor and outdoor environment [1].
Now, it is necessary to highlight that new buildings are characterized by low heat losses through the envelope, and natural ventilation has come to represent a noteworthy share of the energy needs for space heating.
However, houses have also become more and more tightly sealed, mainly by adopting windows with very low permeability to air.
The airtightness of the envelope is defined by the parameter n_{50}, which is the number of air changes per hour under a pressure differential of 50 Pa. Passivhaus standard, suggests that n_{50} must be below 0.6 h^{1}; on the other hand, Italian regulations recommend n_{50} < 2 h^{1} for singlefamily houses and n_{50} < 1 h^{1} otherwise.
Air tightness implies the difficulty to ensure enough outdoor air, necessary to dilute pollutants and odors, provide good IAQ and control the relative humidity. This question cannot be neglected, since insufficient ventilation can cause adverse health effects for the occupants [2].
To meet these two requirements, namely to ensure air renewal and to minimize heat losses, the adoption of controlled Mechanical Ventilation (MV) systems provides a possible solution.
MV plants can be basically classified in singleflow and doubleflow systems. Singleflow MV systems foresee air extraction through terminals installed in wet rooms (kitchen, bathrooms) and connected to a fan through conduits, while air supply to living rooms is achieved by means of vents installed on the outer envelope that operate thanks to the negative pressure gradient. This kind of MV systems must be carefully designed, as the permeability of the envelope may significantly affect their performance [3].
Doubleflow MV systems foresee air extraction from wet rooms and air supply to living rooms through two separate conduits. In this case two fans are necessary, which implies higher electricity needs for their operation than in singleflow systems; difficulties may also arise with the allocation of the ventilating unit [4, 5]. However, this configuration also allows to preheat supply air from the exhaust air implementing a heat recovery (HR).
In this sense, Hekmat et al. reported that, by using a MV system equipped with a HR, the total energy consumption can be reduced up to 20 %, and that the choice of the ventilation strategy can significantly reduce the energy consumptions [6]. Fukushima et al. reported that the primary energy consumption of a doubleflow MV system can be reduced by 25 % with respect to singleflow MV systems, by adopting lowenergy fans and a highefficiency heat exchanger [7]. However, the results of this comparison are strongly influenced by the efficiency of the heating system: indeed, the higher the efficiency, the lower the importance of the HR [8].
Nevertheless, despite all these attractive features MV systems are still not very widespread especially in residential applications. In Italy, just 1 % of the total number of dwellings in the existing building stock is provided with a MV system, either with or without heat recovery, whereas in France this rate raises to 35 % [9]. According to a survey, the main reason for not adopting a MV system, even if available, is the high cost of operation for about 58 % of responders; other main concerns are the difficulty of operation (21 %) and the noise produced by the fan (8 %) [10]. Another remarkable study [11], conducted in UK over 20 buildings equipped with heatrecovery doubleflow ventilation, points out that less than 10 % of the occupants keep their MV system continuously in operation throughout the year; about 50 % of them keep the system constantly switched off, and prefer to open windows at will.
Finally, Beko et al. highlighted that MV systems with heat recovery are associated with additional capital and maintenance costs, and are supposed to have potential health implications if the maintenance is not adequately conducted. They also often require a change in user habits [12].
In the present study the above mentioned issues will be further investigated, with the aim to cast light on the technical, energy and financial suitability of MV systems in residential applications.
2.1 Description of the dwelling
The plans of the residential building considered in this study are shown in Fig. 1. It is a singlefamily house with three floors: the ground floor hosts the living room and the kitchen, whereas the first floor contains three bedrooms and a bathroom. The second floor is an attic, which must not be equipped with mechanical ventilation, since there is no need for air renewal.
Figure 1 identifies the rooms where air has to be supplied or extracted by the controlled mechanical ventilation system. The overall net horizontal surface of the building is A_{b} = 161.7 m^{2}; however, the surface interested by the mechanical ventilation system, i.e. ground and first floor, is A = 109.4 m^{2}, with a corresponding volume V = 306.4 m^{3}.
The building has a reinforced concrete structure, very common in Mediterranean countries. The outside walls are composed of a single layer of lightweight clay blocks (30 cm), insulated from the outer side (4 cm). The transmittance is U = 0.35 W m^{2} K^{1}, and it is below the recommended threshold for new buildings in Italy. As for the floor slabs, they consist of a 20cm slab made of reinforced concrete and hollow bricks, covered with a concrete screed to fall (5 cm) and a tiled floor.
The windows are provided double 4mm sealed glazing filled with argon and thermalbreak aluminum profiles; the inner pane is treated with a lowemissive coating, with an overall U = 3.1 W m^{2} K^{1}.
This building described in this section corresponds to the design of a real building that will be built in Catania, a town on the Eastern coast of Sicily, in Southern Italy. Here, the climate is warm in winter, as witnessed by the low Heating Degree Days (HDD = 833 °C·day), defined with reference to a base outdoor temperature of 12 °C.
Figure 1. Plans of the dwelling selected as a case study.
Table 1. Useful floor area and net volume for the main rooms
Room 
A [m^{2}] 
V [m^{3}] 
Kitchen 
17.4 
48.7 
Living room 
27 
75.6 
WC #1 
6 
16.7 
Bedroom #1 
15.8 
44.4 
Bedroom #2 
12.1 
33.9 
WC #2 
5.2 
14.4 
Bedroom #3 
16.8 
47.0 
Corridors 
6 
16.7 
2.2 Configurations for controlled mechanical ventilation
With reference to the building previously described, four possible configurations of the controlled mechanical ventilation systems will be investigated, namely:
In particular, hygroadjustable terminals are equipped with a nylon membrane, which modifies its shape when exposed to moisture. This allows to automatically modulate the aperture size of the terminal as a function of the presence of occupants, resulting in a modulation of the rate of ventilation when combined with a constant pressure / variable volume fan unit. As an example, most hygroadjustable extract units available on the market can modulate the rate of ventilation between 6 m^{3}/h at RH ≤ 30 % and 45 m^{3}/h at RH ≥ 70 %. The extract units usually installed in the kitchen may manage, for a short time lapse (usually 30 minutes), a peak air flow rate. The occupants can control the activation of this peak regime by operating on a switch or a remote control.
On the other hand, constantflow terminals are equipped with a membrane that can modify its shape according to the pressure made available from the fan, thus allowing for an almost constant rate of ventilation. With this kind of terminals, rooms are ventilated by a constant rate of outdoor air (e.g. 15, 22, 30 or 45 m^{3}/h, according to the commercially available size). As for hygroadjustable terminals, the extract unit in the kitchen can boost the ventilation rate on demand.
Constantflow terminals are slightly cheaper than hygroadjustable terminals; however, hygroadjustable terminals allow to save thermal energy [13]. A study carried out in France by the CSTB has shown that the energy savings associated with hygroadjustable terminals range between 25 % and 60 % of the heat losses for ventilation, depending on the type of dwelling and the conditions of occupancy. Furthermore, additional savings of electricity arise. However, there is very poor literature to confirm these figures; the results of this paper will cast light on this issue.
3.1 Required ventilation rate for air renewal
The calculation of the rate of ventilation needed to dilute pollutants and assure indoor air quality in residential units can be carried out according to different approaches.
The Italian standard UNI 10339:1995 prescribes to ventilate residential units with at least 11 L/s per person [14]. In this case study, the dwelling is designed to host N_{p} = 4 people, hence the nominal rate of ventilation is:
Q_{UNI} = N_{p}·11 [l/s] = 55 [l/s] = 158.4 [m^{3}/h](1)
On the other hand, a performanceoriented approach is suggested by the European technical report CEN TR 14788 follows [15]. Here, CO_{2} is identified as the tracer pollutant, and the ventilation rate is calculated in order to dilute CO_{2} concentration below a threshold of acceptability (y_{i} = 800 ppm). The calculation considers the CO_{2} concentration outdoors (y_{o} = 400 ppm) and its rate of release due to human respiration (q_{D} = 18 L/h per person in the daytime and q_{N} = 12 L/h per person at night). If one considers t_{D} = 16 h and t_{N} = 8 h as the diurnal and nocturnal time of occupancy, respectively, the required mean rate of ventilation is:
$\mathrm{Q}_{\mathrm{CEN}}=\frac{\mathrm{N}_{\mathrm{p}} \cdot\left(\mathrm{q}_{\mathrm{D}} \cdot \mathrm{t}_{\mathrm{D}}+\mathrm{q}_{\mathrm{N}} \cdot \mathrm{t}_{\mathrm{N}}\right)}{\mathrm{y}_{\mathrm{i}}\mathrm{y}_{\mathrm{o}}} \cdot \frac{1}{24}=160\left[\mathrm{m}^{3} / \mathrm{h}\right]$(2)
Finally, a much more detailed approach to the definition of the ventilation rates is available in the French regulations [16]. Here, the exact rate of ventilation depends on the type of mechanical ventilation system and on the number of main rooms in the dwelling. Table 2 reports the values prescribed for a residential unit with four main rooms.
It is interesting to underline that with constantflow terminals it is possible to envisage only two modes of operation: in the base regime, minimum ventilation rates are extracted from all wetrooms, whereas in the peak regime just the extract terminal in the kitchen operates at its peak. On the other hand, in the base regime hygroadjustable terminals may modify their ventilation rate within a quite broad range, according to the indoor relative humidity.
It is also necessary to remark that the three different regulations lead to very similar results, since the nominal ventilation rate ranges between 158 and 165 m^{3}/h. In this paper, the recommendations of the French regulation are retained to size the MV system and to assess the energy needs, due to their higher degree of detail.
To simplify the calculation, when dealing with hygroadjustable systems three main regimes are introduced, as described in Table 3, where the corresponding frequency of occurrence (f) is also reported, both for singleflow (S) and doubleflow (D) systems. Basically, the base regime holds when occupants are not at home, while the peak regime occurs when people are at home and the peak flow in the kitchen is activated. The intermediate regime corresponds to an intermediate situation.
Table 3 also provides information about the electric power absorbed by the ventilating unit in all regimes. The values are gained from the technical sheets provided by manufacturers.
Table 2. Prescribed ventilation rates in France (m^{3}/h) [16]
Type 
Kitchen 
WC #1 
WC #2 
Total 

Constant flow 
Peak: 120 
30 
15 
Peak: 165 

Base: 45 
Base: 90 

Hygro adjustable 
Peak: 120 
5/30 
5/15 
Peak: 165 

Base: 10/45 
Base: 20/90 
Type 
Regime 
G [m^{3}/h] 
f [%] 
P_{el} [W] 

(S) 
(D) 

Constant flow 
Peak 
165 
10 % 
21 
95 
Base 
90 
90 % 
12 
40 







Hygro adjustable 
Peak 
165 
10 % 
21 
95 
Intermediate 
80 
60 % 
13 
38 

Base 
30 
30 % 
7.5 
25 
Starting from the data in Table 3, it is possible to assess the average daily air change rate according to Eq. (3):
$\overline{\mathrm{n}}=\frac{\sum_{\mathrm{k}}\left(\mathrm{Q}_{\mathrm{k}} \cdot \mathrm{f}_{\mathrm{k}}\right)}{\mathrm{V}}$(3)
As a result, $\overline{\mathrm{n}}$= 0.32 h^{1} for constant flow and $\overline{\mathrm{n}}$= 0.24 h^{1} with hygroadjustable flow.
3.2 Calculation of the initial costs
For all the MV systems proposed in the previous section (constant or hygroadjustable ventilation rate, single or double flow), a list of components to be installed in the reference building has been made.
To this aim, detailed technical documentation provided by some manufacturers were consulted; the manufacturers are well known on an international scale. The list includes all components needed for a functioning and complete installation, such as:
Hence, the overall cost is calculated. The costs include the purchase of the components (VAT included), the installation by skilled workers and any other masonry work to install and conceal the ducts.
Figure 2 reports the results of this research for all the proposed MV configurations, and the percentage distribution of the costs for equipment and installation. The costs range from around 1150 € for constant singleflow systems to around 3950 € for hygroadjustable doubleflow systems. Overall, for a given solution (i.e. constant or hygroadjustable flow) the cost of a doubleflow system is more than twice as high as for a singleflow system.
Figure 2. CMV systems: costs for equipment and installation
4.1 Final and primary energy consumption
The financial suitability of mechanical ventilation systems in residential buildings depends on whether the savings of thermal energy for space heating determined by a controlled ventilation rate are sufficient to counterbalance the electricity consumption to operate the fan.
The results presented hereafter try to answer this question, by evaluating the overall energy consumption for all the proposed mechanical ventilation schemes applied to the building presented in Section 2.1.
In particular, the thermal energy needs for space heating in winter, associated with the introduction of fresh outdoor air for ventilation purposes, can be determined through Eq. (4):
$\mathrm{E}_{\mathrm{th}}=\frac{\rho \cdot \overline{\mathrm{n}} \cdot \mathrm{V} \cdot \mathrm{c}_{\mathrm{p}} \cdot \mathrm{HDD} \cdot 24}{3.6 \cdot 10^{6}} \cdot\left(1\eta_{\mathrm{hr}}\right)$(4)
Equation (4) implies that the thermal energy needs are a function of the Heating Degree Days (HDD). Here, ρ = 1.25 kg m^{3} and c_{p} = 1005 J kg^{1} K^{1}. The values of the average ventilation rate are those already presented in Section 3.1.
The term h_{hr} is the efficiency of the heat recovery unit, which is relevant only for doubleflow mechanical ventilation. This parameter ranges between 0.86 and 0.94, according to the manufacturers, and may depend on the air flow rate and the operating conditions. In this paper, an average value
h_{hr} = 0.9 is considered, while h_{hr} = 0 applies to singleflow mechanical ventilation. On the other hand, the annual electricity consumption of the fans can be determined by Eq. (5), starting from the data reported in Table 3:
$\mathrm{E}_{\mathrm{el}}=\frac{\sum_{\mathrm{k}}\left(\mathrm{P}_{\mathrm{el}, \mathrm{k}} \cdot \mathrm{f}_{\mathrm{k}}\right) \cdot 24 \cdot \mathrm{ND}}{1000}$(5)
Here, ND is the number of days included in heating season. As an example, according to the national regulations about energy savings in residential buildings, in Catania the space heating system can operate only from the 1^{st} of December to the 31^{th} of March, which means ND = 121 days. Obviously, a mechanical ventilation system may operate all year round; however, in this work attention is paid to the only heating season, when the effects of ventilation on the energy balance are more evident.
In case of natural ventilation, the only relevant contribution comes from Eq. (4). Here, the average rate of ventilation is set to $\overline{\mathrm{n}}$= 1 h^{1}, which takes into account both the air infiltration and the occasional opening of the windows by the occupants. Of course, the air change rate in natural ventilation depends on several issues, such as the occupants’ behaviour and the air tightness of the envelope. However, some literature suggests that $\overline{\mathrm{n}}$= 1 h^{1} is reasonable, especially in mild climates and in not very tight buildings, both circumstances being common in Italy [17, 18]
After the calculation of the final energy needs (thermal energy plus electric energy), the following step has consisted in the evaluation of the overall annual primary energy consumption. This is determined in terms of kWh per unit net surface of the dwelling by means of Eq. (6):
$\operatorname{PE}\left(\overline{\mathrm{n}}, \eta_{\mathrm{hr}}\right)=\left[\frac{\mathrm{E}_{\mathrm{th}}\left(\overline{\mathrm{n}}, \eta_{\mathrm{hr}}\right)}{\eta_{\mathrm{th}}}+\frac{\mathrm{E}_{\mathrm{el}}(\overline{\mathrm{n}})}{\eta_{\mathrm{el}}}\right] \cdot \frac{1}{\mathrm{A}}$(6)
In Eq. (6), the overall efficiency of the space heating system is $\eta_{\mathrm{th}}$ = 0.80, including the performance of the heat generator and the losses for heat distribution and emission.
On the other hand, the conversion factor from primary energy to electricity distributed to the grid is set to 1.95 kWh/kWh_{el}, according to a recent Italian standard [19]. This value refers only to nonrenewable primary energy sources, and can be translated into an overall efficiency $\eta_{\mathrm{el}}$= 1 / 1.95 = 0.513 for electricity production and distribution.
The results of Eq. (4) and Eq. (5) are shown in Fig. 3. Here, one can observe that controlled mechanical ventilation systems introduce a consistent reduction in the overall energy consumption, if compared to natural ventilation. In particular, the thermal energy requirement associated with natural ventilation is E_{th} = 2138 kWh/year; this value can be drastically reduced with constantflow (680 kWh/year) and hygroadjustable singleflow mechanical ventilation (515 kWh/year). Moreover, both doubleflow configurations make thermal energy needs almost negligible (i.e. below 100 kWh/year), thanks to the highefficiency heat recovery unit.
On the other hand, the electricity consumption is not negligible in mechanical ventilation systems, especially in doubleflow configuration, due to the operation of two fans and to the pressure losses in the heat recovery unit. The highest value of electricity consumption pertains to the constant doubleflow mechanical ventilation system (132 kWh/year), whereas the singleflow hygroadjustable system only consumes 35 kWh/year of electricity.
Figure 3. Final energy needs: natural and mechanical ventilation
Figure 4. Primary energy consumption: natural and mechanical ventilation
When looking at the primary energy needs, calculated through Eq. (6), all MV configurations enable outstanding savings in comparison with natural ventilation, see Fig. 4. Indeed, primary energy savings range from 65.4 % for the constant singleflow system to 89.2 % for the doubleflow hygroadjustable system.
However, the difference between the two doubleflow configurations is not high. Hence, given that the cost of hygroadjustable terminals is significantly higher than for constantflow terminals, their adoption may be questionable under a merely financial point of view. This issue is further developed in the next section.
4.2 Financial issues: the payback time
The calculation of the annual operating costs of mechanical ventilation systems is the following step to evaluate their financial suitability in residential buildings, if compared to the practice of natural ventilation. Hence, provided that these costs are lower than for natural ventilation, it will be possible to make a balance between the annual savings and the initial investment for their installation.
If looking at the operating costs, these are calculated through Eq. (7). The costs are due to the fuel (natural gas) needed to feed the heat generator – limited to the energy needs for ventilation – and to the electricity consumed by the fans.
$\mathrm{C}_{\mathrm{op}}=\frac{\mathrm{E}_{\mathrm{th}}}{\eta_{\mathrm{th}} \cdot \mathrm{LHV}} \cdot \mathrm{c}_{\mathrm{f}}+\mathrm{E}_{\mathrm{el}} \cdot \mathrm{c}_{\mathrm{el}}$(7)
In Eq. (7), the Lower Heating Value of natural gas is LHV = 9.9 kWh/m^{3}. Moreover, the average unit cost of electricity and natural gas for residential clients is respectively set to c_{el} = 0.24 €/kWh and c_{f} = 1.15 €/m^{3} [20].
However, the maintenance costs must also be included among the annual costs for the MV systems. Maintenance mostly consists in cleaning the extract and inlet terminals with suitable products, and in cleaning or substituting the filters in the ventilating unit, only for the doubleflow systems. Upon consultation with the manufacturers, the maintenance costs are set as C_{ma} = 20 €/year for singleflow systems and C_{ma} = 40 €/year for doubleflow systems.
Finally, the simple payback time (SPT) for the proposed mechanical ventilation systems is assessed through Eq. (8):
$\mathrm{SPT}=\frac{\mathrm{C}_{\mathrm{in}, \mathrm{mv}}}{\mathrm{C}_{\mathrm{op}, \mathrm{nv}}\left(\mathrm{C}_{\mathrm{op}, \mathrm{mv}}+\mathrm{C}_{\mathrm{ma}}\right)}$(8)
The annual costs resulting from this analysis are shown in Fig. 5. As one can observe, the annual savings provided by mechanical ventilation systems range between 183.1 €/year for constant singleflow ventilation to 235.5 €/year with hygroadjustable doubleflow ventilation. In fact, doubleflow ventilation systems are penalized by the relative high price of electricity in the residential sector. Consequently, the annual costs for doubleflow ventilation are not too far from those calculated for hygroadjustable singleflow, in spite of their much higher initial cost.
Figure 5. Annual costs: natural and mechanical ventilation
This result influences the simple payback time, which is around 16 years for doubleflow mechanical ventilation systems. On the other hand, the simple payback time is between 6 and 7 years for singleflow mechanical ventilation systems.
Figure 6. CMV systems: simple payback time
According to these results, singleflow mechanical ventilation systems seems to be much more appealing than doubleflow in residential applications, at least in the warm climate considered so far. The role of the climate on the outcomes of the study will be examined in the next section.
The outcomes discussed in the previous section are likely to be different in other climatic contexts. Indeed, as highlighted by Eq. (4), the thermal energy needs for space heating caused by outdoor ventilation is proportional to the Heating Degree Days. Even the electricity demand of the ventilating unit during the heating season is dependent on the climate: in particular, in Italy the duration of the heating season, defined by national regulations, changes according to the number of Heating Degree Days, as reported in Table 4.
Table 4. Definition of the heating season in Italy
HDD 
Dates 
ND [days] 

from 
to 
from 
to 

0 
600 
01/12 
15/03 
105 
601 
900 
01/12 
31/03 
121 
901 
1400 
15/11 
31/03 
137 
1401 
2100 
01/11 
15/04 
166 
2101 
3000 
15/10 
15/04 
183 
3001 
 
01/10 
30/04 
212 
Thus, in order to understand how the severity of the climate in winter – measured by the HDD – can modify the outcomes discussed in the previous section, the calculations have been repeated by varying the HDD in the range between 750 and 3500. This range has been chosen because it includes the great majority of the Italian territory, except a few municipalities in the Alpine regions.
The results of this investigation are reported in Fig. 7 (primary energy consumption) and Fig. 8 (simple payback time). To make the results more comprehensible, the position of four representative cities is underlined, namely:
From Fig. 7, one can see that in all climates the use of hygroadjustable terminals introduces significant primary energy savings if compared to constantflow, especially in singleflow ventilation systems
On the other hand, from a financial point of view there is little difference between hygroadjustable and constant flow systems. The reason is that the initial extracost for hygroadjustable systems compensates the annual savings for fuel and electricity, thus making the two solutions almost equivalent in terms of SPT (Fig. 8).
However, the main outcome of this analysis is that in cold climates (HDD > 2750) the simple payback time for mechanical ventilation converges asymptotically below 2 years for singleflow mechanical ventilation, and below 4 years for doubleflow mechanical ventilation (see Fig. 8), thus making them very interesting in financial terms.
Figure 7. Primary energy consumption vs HDD
Figure 8. Simple payback time vs HDD
As suggested by the outcomes of the paper, controlled mechanical ventilation systems in residential buildings undoubtedly contribute to significantly reduce the primary energy consumption for space heating, while also allowing to provide good indoor air quality.
However, the installation of these systems is not always easy to promote under a financial point of view, especially in warm climates and in case of doubleflow ventilation systems. In fact, the simple payback time in localities with HDD < 1000 is above 12 years, which justifies the scepticism reported by several surveys in the literature. On the other hand, as the climate gets colder (HDD > 1500), the simple payback time of controlled mechanical ventilation systems becomes more interesting, and finally converges below 2 years for singleflow systems and below 4 years for doubleflow systems.
These results are encouraging, if one considers that warm climatic conditions only occur in the coastal area of the Mediterranean basin, while higher HDD are much more frequent in Italy.
This study has only focused on the energy needs in the heating season. In the cooling season, the thermal energy needs for airconditioning would be relevant only in warm climates. Here, the ventilation through outdoor air can even play a positive role in terms of passive cooling, if properly exploited when the outdoor temperature is lower than the indoor temperature [21, 22]. In this sense, a mechanical ventilation system can assure an appropriate ventilation rate, without the need to keep windows open through the night, which may generate noise and privacy issues.
In midseasons, mechanical ventilation systems might not be used, allowing the building to be freerunning during the rest of the year [23].
In any case, the results presented so far are likely to be influenced by several parameters:
It is then evident that the financial attractiveness of controlled mechanical ventilation systems in comparison with natural ventilation has to be evaluated with care case by case.
More detailed studies are being carried out to consider more precisely the air ventilation patterns [25].
A 
net surface area, m^{2} 
c 
unit cost, €.kWh^{1}, €.m^{3} 
C_{p} E 
specific heat capacity, J kg^{1} K^{1} final energy, kWh.year^{1} 
f 
fraction of operation, % 
HDD 
heating degreedays, °C.days 
LHV 
lower heating value of the fuel, kWh.m^{3} 
n 
air change rate, h^{1} 
ND 
number of days, days 
P 
power, W 
PE 
primary energy consumption, kWh.year^{1} 
q 
rate of CO_{2} production, L.h^{1} 
Q 
air flow rate, m^{3}.h^{1} 
RH 
relative humidity, % 
SPT 
simple payback time, years 
t 
time lapse, h 
V 
net volume of the building, m^{3} 
y 
CO_{2} concentration, ppm 
Greek symbols 

ρ 
density, kg.m^{3} 
η 
efficiency,  
Subscripts 

el 
electricity 
f 
fuel 
hr 
heat recovery 
i 
indoor 
in 
initial 
ma 
maintenance 
mv 
mechanical ventilation 
nv 
natural ventilation 
o 
outdoor 
op 
operation 
th 
thermal 
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