The exponential limit to the cooling of buildings by natural ventilation

Abstract

In manual heat gain and heat loss calculations the standard linear ventilation and infiltration heat loss formula (/)NV(t_ai t_ao), (with a suitable factor to relate to comfort temperature), is used. In the 1986 CIBSE Guide Section A8, Summertime temperatures in buildings, a limiting ventilation heat loss formula is used as the air change rate, N, increases, but, as with older guides, there is no reference or explanation of its derivation,¹ although the 1970 Guide offers some explanation. This limiting equation is not used in the 1999 Guide A, Environmental design which is unfortunate. This paper analyses the ventilation heat loss based on a log mean temperature difference heat transfer approach which also shows a limiting heat loss effect like the A8 equation. It is also suggested that an admittance value should be used instead of a constant heat transfer coefficient which gives greater insight into the heat transfer process. Significant reductions in the heat transfer process are found at relatively low air change rates using the more correct formulae, which could have implications for summer natural ventilation, during both day and night.