In modern era, we strive to achieve the most efficient use of energy, which also applies to the energy required to maintain a suitable living environment in buildings. In order to achieve this, ventilation systems with heat exchangers are used. These exchangers are meant to transfer the heat from the outgoing air that is leaving the building to the fresh incoming air that is entering the building. It is possible to calculate the efficiency of the ventilation system with a heat exchanger using the first or second law of thermodynamics and the primary energy conversion factor. The calculated results are compared using different energy balance shells of the building. We find that the calculated efficiency of the heat exchanger and the entire ventilation system depends on whether the first or the second law of thermodynamics is considered in the calculation. It is demonstrated that the increase of the mass flow rate through the ventilation system increases the efficiency of the heat exchanger and ventilation system. When using fossil fuels instead of renewable energy sources, the amount of primary energy required to operate the ventilation system is higher.
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