In current practice, HVAC designers oversize systems inorder to provide the theoretical benefit of perfectoccupant comfort in the consideration of all possibleextreme conditions. Routine use of the autosize option ofsimulation tools and the assigned or implied safetyfactors leads to the potential oversizing that have beenreported in literature. Even though the most importantmandate is to reduce the risk of uncomfortable hours,current design methods and simulation tools offer only alimited way to do this. Indeed, unwanted outcomes canresult from uncertainty in the parameters of the buildingenergy model that is used.
How to overcome the negative effects of oversizedHVAC systems in terms of energy use, equipment life,maintenance and financial penalties by rightsizingstrategies with various perspectives has been subject ofprevious studies. This study focuses on an incrementaldownsizing approach backed up by an uncertaintyanalysis. A system consisting of packaged heat pumpunits in commercial buildings is used to demonstrate theapproach. Even though a heat pump unit has both heatingand cooling coils, only the capacity of cooling coils isanalyzed in this paper. Acceptable risk magnitudes interms of unmet hours, impact of modeler’s ignorance,and energy & cost savings as the result of downsizing areinvestigated. The proposed method opens the door toselecting the appropriate system size with moreflexibility and confidence in cases where comfort criteriacan be relaxed.