District heating companies have ambitious targets for lowering carbon emissions in production. Large heat pumps offer an interesting alternative for district heating production allowing utilization of various heat sources. The primary objective of this study is to examine the viability of large heat pumps in existing district heating systems. The study uses three types of systems to simulate how increasing the share of heat pump production influences district heating systems when optimized for the lowest production costs. The second objective of this study is to understand the district heating companies' perspective on increasing amounts of heat pumps in their systems. Based on the simulations, the largest potential for heat pumps is in small district heating systems, where they reduce the use of fossil fuels. In medium and large systems with economical combined heat and power production, the potential of heat pumps is smaller. The findings of the simulations together with insights from the interviews imply that the viable amount of heat pump based heat production in DH systems would be around 10-25% in Finland, which is much higher than the current 3%. (C) 2018 Elsevier Ltd. All rights reserved.