Simple model for an optimiser

Abstract

A simple model of a building is developed to analyse the preheating curve for optimised heating. The model considers an effective thermal mass of the building so that internal air temperature can be used rather than a mean fabric temperature. With this model a curve similar to that of Birtles and John is derived. At low outside temperatures and with small heating plant this curve is found to bend over. The model results can be explained in terms of a time constant and a plant size ratio. Variable heat emitter output is also considered and the characteristic curve retains the same form. Birtles and John's empirical constants are related to building parameters of the simple model. The response factor for a building as used in the CIBSE Energy Code is found to relate approximately to the model. It is pointed out, however, that the CIBSE admittance method used in deriving the response factor is slightly erroneous. A number of other points relating to optimiser performance are explained by the model.