Purpose - The purpose of this paper is to compare two different ventilation strategies, displacement and mixing, in heat, ventilating and air conditioning (HVAC) systems with recourse to computational fluid dynamics (CFD). Design/methodology/approach - The flow and the heat and mass transfer are numerically predicted inside an air-conditioned room with a desk and an occupant for the cooling and heating periods in moderate climate regions, like Mediterranean countries. Focus is placed on energy efficiency, thermal comfort and internal air quality (IAQ), evaluated from the simulations of the three-dimensional, turbulent, non-isothermal and buoyant flow of moist air. Findings - For the cooling period, displacement exhibits higher energy and ventilation efficiencies promoting simultaneously better comfort for the occupant. For the heating period, mixing performs better due to the short-circuit phenomenon occurring with the displacement flow. Overall, mixing behaves better for air-conditioning of typical office rooms in Mediterranean-climate countries, where heating and cooling climatization modes have to be alternated according to the season. Research limitations/implications - Room, desk and occupant are designed as parallelepipeds. No experimental work is performed but models used are previously validated by other authors against experimental data. Practical implications - The results indicate a short-circuit flow phenomenon that must be avoided when designing HVAC systems. Originality/value - Use of grilles layout typical for the cooling period to study the air-conditioning of a typical office room during the heating period, incorporating in the model a transport equation for the moisture. IAQ is simulated together with the flow, the heat and the comfort conditions: velocities, temperature, predicted mean vote (PMV), predicted percentage of dissatisfied (PPD), draught rating (DR), PPD due to air quality (PDQ) and air moisture content are calculated simultaneously. © Emerald Group Publishing Limited.
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