Using modified multiple heating-degree-day (HDD) and cooling-degree-day (CDD) indices to estimate building heating and cooling loads

Abstract

There is substantial confusion in the literature concerning the appropriate reference temperatures to use for the computation of heating-degree-day (HDD) and cooling-degree-day (CDD) indices, with the former ranging from 10 to 20 °C and the latter ranging from 10 to 28 °C. Here, three reference temperatures and corresponding indices are proposed for both HDD and CDD. The first reference temperature is the indoor thermostat setting, the second is the outdoor temperature at which heat loss balances the internal heat gain, and the third is the outdoor temperature at which internal and solar heat gains are balanced by heat loss. The second and third reference temperatures vary with the envelope properties and clustering of units in multi-unit residential buildings. Differences between the reference temperatures can be used to compute the usable internal and solar heat gain for a given building. Differences between HDDs and CDDs as computed using daily mean and hourly data serve to highlight the importance of thermal mass. Usable internal heat gain increases slightly as window-to-wall ratio increases from 0.1 to 0.8, while usable solar heat gain drops substantially. For multi-unit residential buildings with a high-performance thermal envelope, minimizing solar heat gain is important even in the winter in cold climates.