On the optimisation of age of the air in the breathing zone of floor heating systems: The role of ventilation design

For a mechanically ventilated space, the nominal age of the air can be obtained by the reciprocal of the air change rate. However, values of the local mean age of the air in practice may differ to some extent from the nominal one since the nominal time constant employs as reference the theoretical optimum model. This discrepancy could become more prominent in spaces conditioning with both ventilation and heating system where the indoor air pattern is affected by turbulent mixed convection flow. Given importance of knowledge on the distribution of age of the air in these spaces, the present study provides insights on how ventilation design in floor heating systems can optimise the delivery of fresh air into the breathing zone. In this context, by establishing a computational fluid dynamic (CFD) model validated against experimental data, the local mean age of the air as well as the indoor air quality (IAQ) indices in the breathing zone of the floor heating system are examined under different ventilation modes. Six different ventilation scenarios are regarded in order to find the optimal ventilation design in terms of the delivery of the fresh air and ventilation effectiveness in occupied space. Furthermore, the integrated effects of the floor temperature and inlet supply temperature along with the ventilation design on the distribution of local age of the air are addressed. The obtained results indicate that the up-supply ventilation strategy is more efficient than down-supply one in the ventilation effectiveness and delivery of fresh air into the breathing zone. Moreover, it is shown that the mean age of the air in the breathing zone of the up-supply ventilation increases with increment of the Ri (Richardson number), whereas an increase in Ri improves the delivery of fresh air in down-supply mode. For a given floor temperature, the similar trend is also observed in the variation of age of the air with the characteristic temperature of supply inlet, namely the temperature difference between inlet supply and surrounding walls.