Natural Ventilation is More Energy Efficient Than Its Reputation
Houses with natural ventilation systems can be more energy efficient than those with mechanical supply-exhaust ventilation, when considering the different uses of ventilation systems and a building’s heat source.
The energy consumption of mechanical and natural ventilation systems was last studied in Finland in the early 2000s. At that time, natural ventilation systems proved to be more energy efficient in low-rise dwellings. Mechanised systems have developed significantly since then, so it would be time for a new comparative study. Nevertheless, calculations show that natural ventilation can still save energy.
According to established understanding, natural ventilation wastes heating energy, for the heat contained in the exhaust air cannot be recovered. In mechanical systems, heat recovery is possible, but energy is needed to move the air. Because heating the air consumes more energy than moving it, it is easy to think that the heat recovery equipment always saves more energy than the fans consume. In reality, however, it is not that simple.
The energy used for heating and fan operations is directly comparable only in buildings with direct electric heating. In buildings heated with a heat pump system, significantly less electricity is used for heating than when heated with direct electricity, so the benefit from heat recovery decreases accordingly, and the importance of fan electricity increases. The situation is also similar in houses with district heating, when the issue is considered from the point of view of the environmental effects of energy production. In the energy calculations, this is taken into account with energy form coefficients that reflect the relative environmental effects of different forms of energy. Since district heating is often obtained from the excess heat from electricity production, the coefficient for district heating is 0.5 and the coefficient for electricity is 1.2. The higher the coefficient of the energy source, the less energy the house to be built must consume.1
Different Systems are Used in Different Ways
The energy consumption of buildings is also affected by how they are used. In 2005, researchers at Tampere University of Technology (TUT) were surprised to discover that the total energy consumption in buildings equipped with natural ventilation was on average lower than in buildings equipped with mechanical ventilation, regardless of the type of mechanical system or whether the system included heat recovery. Based on the results of the research, lifestyles and living habits seem to be the largest factor influencing the total energy consumption of buildings.2
On the other hand, the characteristics of ventilation systems affect how they are used. In a natural ventilation system, the cool incoming air leads the user to adjust the vents to a smaller opening in the wintertime, which results in adequate ventilation and the saving of heating energy. In a mechanical supply-exhaust ventilation system, unduly large air flows are not as easily noticed, because the intake air is heated and does not cause draft problems. In the above-mentioned TUT study, the average ventilation coefficients of natural ventilation systems in wintertime was 0.3 times per hour, but 0.41 times per hour for mechanised intake-exhaust ventilation systems. If, in a mechanical system, the ventilation in wintertime is reduced as greatly as in a natural ventilation system, the importance of heat recovery decreases.
The advantage of natural ventilation compared to mechanical supply-exhaust ventilation is the possibility of room-specific adjustment. In mechanical ventilation, one can usually only adjust the power of the machine, which affects the ventilation of all rooms. In a natural ventilation system, for example, the ventilation of a single bedroom can be adjusted to be higher at night and lower during the day. If the building has few residents compared to the number of rooms, the ventilation of rooms with little use can also be reduced without affecting the ventilation of other rooms. Such possibilities for adjustment bring practical energy efficiency to the natural ventilation system, which is normally not taken into account at all in energy calculations.
The benefit obtained from the heat recovery equipment is also significantly affected by the airtightness of buildings. The importance of unintentional ventilation through leakage points is amplified in the wintertime, when the large difference between indoor and outdoor temperatures increases air flow at leakage points. The more air exchanged through the leakage points of the building envelope, the smaller the benefit from heat recovery. This is especially important when it comes to older wooden buildings, which are difficult to make sufficiently airtight for mechanical ventilation.
The E-Value Calculation Favours Mechanical Ventilation
The energy consumption of buildings is often compared with the help of E-value calculations – after all, the E-value, that is, the calculated energy performance reference value, is a tool specifically intended for evaluating energy efficiency. However, simplifications have been made in the E-value calculation method, as a result of which the actual energy consumption of buildings often differs significantly from the calculated ones. The simplified calculation makes mechanical ventilation equipped with heat recovery seem better than it actually is, and natural ventilation correspondingly worse. The reason is, among other things, that in calculating the E-value, it is assumed that the air volumes remain constant throughout the year. Ventilation according to need may only be considered if it has been implemented with building automation.
The aforementioned points can be illustrated with calculations that take into account the energy consumption of ventilation and the efficiency of heat production. Take, for example, a 120 m2 single-family house with geothermal heating. The seasonal coefficient of performance of the geothermal heat pump for the heating season is assumed to be 3.5. The prescribed level of the specific fan power of ventilation must in a new building be 1.8 kW/(m3/s) maximum.
In a new building, the benchmark for a ventilation machine’s heat recovery, in accordance with regulations, is an annual efficiency ratio of 55 percent. Table 1 shows the results of the calculations with the normal baseline values for the E-value calculation, while Table 2 shows the results of the calculations when the necessary use, in accordance with the seasons, is taken into account, but the air volumes are assumed to be the same in both systems. In Table 3 the calculations are made according to how ventilation systems are actually used. Positive numbers mean a higher consumption for natural ventilation, and negative numbers mean higher consumption for mechanical ventilation.
There is a large difference between a conventional E-value calculation and the calculation model that describes the actual usage as accurately as possible. When the actual usage is considered, the mechanical ventilation that meets the minimum requirements of building regulations consumes more energy than natural ventilation. If the specific fan power of the mechanical system is low and the annual efficiency ratio of heat recovery is high, then there seems to be a saving in energy. However, the calculated difference in energy consumption between ventilation systems is rather small. Even in the best-case scenario, mechanical ventilation only brings enough energy savings to cover the annual purchase costs of air filters. When the price of filter replacement work is also considered, natural ventilation becomes more affordable. ↙
The tables below compare the total amount of energy that is used to move and heat the air in a 120 m2 single-family house with geothermal heating, when the coefficient of performance for the heating season is 3.5. Positive numbers mean a higher consumption for natural ventilation and negative numbers mean a higher consumption for mechanical ventilation.
Engineer in the field of energy and HVAC technology, who runs his own company, LK Energiaratkaisut.
Restoration architect, author and partner in Livady Architects.
1 788/2017 Valtioneuvoston asetus rakennuksissa käytettävien energiamuotojen kertoimien lukuarvoista; em. asetuksen perustelumuistio 21.11.2017, 3.
2 Juha Vinha et al.: Puurunkoisten pientalojen kosteus- ja lämpötilaolosuhteet, ilmanvaihto ja ilmatiiviys, Tampereen teknillinen yliopisto, rakennustekniikan osasto, talonrakennustekniikan laboratorio. Tutkimusraportti 131, 2005, 37; Lauri Jääskeläinen: ”Rakennusfysiikka vielä lapsen kengissä”, Rakennettu ympäristö 4/2002, 6–7.