Air-water heat pumps suffer from reduced thermal output and poor efficiency in cold conditions. As a consequence, they are usually vastly overdimensioned for most of the heating season. These inherent disadvantages are largely mitigated in hybrid systems, in which a second heat generator provides heating support when required. In this work, a hybrid heat pump system for existing buildings consisting of a retrofitted air-water heat pump and a gas boiler is modeled and examined in full-year dynamic numerical simulations. It is benchmarked with comparable monovalent systems for a 1970s' single family home as well as a renovated variant of the same building. The nominal thermal output of the AWHP as well as the volume of the buffer storage tank are varied in order to study their impact on system performance. With the renovated building model, significantly higher efficiencies (SPF 3.88 vs. 3.34) and load factors (0.57 vs. 0.36) are achieved. Medium-sized heat pumps attain the highest SPF values, the reason for which is rooted in the alternative-parallel bivalent operation scheme and the dependency of the bivalence point on the heat pump characteristic. The volume of the buffer storage tank has very limited impact on system performance. (C) 2013 Elsevier B.V. All rights reserved.