A Simplified Model for Predicting Thermal Losses in Duct-work Systems

Air is usually the final method for thermal distribution in a building. The air distribution system (duct-work) may be located at several places inside or outside the building. The location of the ducts leads to thermal losses. These losses consist of: leakage, conduction, and thermal cycling. Traditionally, quantifying these loses requires temperature and flow data from many places in the system. This paper presents a model for quantifying these losses requiring less input data. The model breaks the delivery efficiency down into supply and return effectiveness terms, each of which may be further broken down into individual sections. This allows pinpoint identification of sections of the duct system contributing to poor performance. The inputs to the model are the physical properties of the ducts, the cold deck (supply plenum), room, and ambient temperatures, and the amount the system leaks (as a fraction of the total flow). The physical properties are combined into a conduction efficiency term. The temperatures are combined into a conduction potential term.