Combination hydronic systems meet space heating and domestic hot water (DHW) loads in residential buildings. However, the current practice of labeling these appliances for just one function (typically space heating) fails to indicate their actual annual performance and to identify the energy saving potential possible with a system upgrade. New research shows that a linear input/output relationship can be used as a simple model of the performance of a combination hydronic heating system over heating only, DHW only, and combined loads. This simple model can be used to study the impact of system parameters on the annual performance.