A heat pump is one of the most popular energy transformation devices to provide the building with the necessary heating and cooling energy during the cold and warm seasons. Air source heat pumps (ASHP) in building heating and/or hot water systems are becoming more and more attractive these days because they can use renewable energy as an energy source instead of fossil fuels and thus contribute to the fight against climate change. By using an evaporator heat exchanger, ASHP takes the low-potential heat from the ambient air and transforms it into higher-potential heat for building heating and/or hot water preparation. The main problem with this type of energy transformer is the freezing of the evaporator at high outdoor humidity and a temperature close to 0 °C when the condensed moisture of the ambient turns to frost on the surface of the evaporator heat exchanger. This phenomenon significantly reduces the efficiency (COP) of the ASHP. Thus, its performance strongly depends on the climatic conditions of the environment in which it operates. This study presents a numerical model of the heat pump under investigation developed with the TRNSYS software. The type of heat pump used in TRNSYS has been adjusted according to the heat pump characteristics provided by the manufacturer. The validated model is used to model the heating system of a building in the three Baltic States. Modeling results under different climatic conditions are presented.