Estimation of design ventilation rates for buildings

Abstract

Building fabric heat loads have decreased in recent years as standards of thermal insulation have improved. This in turn has led to an increase in the relative importance of the infiltration component of the total heat loss. The quantification of this component requires the accurate assessment of air infiltration rates under relevant environmental design conditions. Unfortunately, techniques for such assessments are much less well developed than those for calculating fabric heat loss and in many cases building services designers still rely upon the use of empirical estimates which are often unrealistic. This paper presents a case study in which a combined measurement and modelling procedure is applied to provide the designer with a more realistic estimate of air infiltration rates for an existing building. The basis of this work involves the use of the fan pressurisation measurement technique to determine actual leakage characteristics for the building envelope. This information provides real input data for a nodal airflow computer model, the results of which can then be used to estimate appropriate design ventilation rates. The case study involves application of the technique to a large secondary school undergoing refurbishment, and shows clearly that empirical estimates could have led to the installation of a severely undersized heating system. The measurements, in fact, highlighted a problem of excessive infiltration rates which required remedial action. The effectiveness of this remedial action was then confirmed by further measurements and remodelling was carried out to establish the final infiltration rates to be applied by the design engineers.