The aim of this paper is to study the cost-effectiveness of using waste hot water as an energy source for heat pumps. The research was made in one part of the hotel complex on Kopaonik, in which the whole system is implemented with a heat pump and piping system for collecting hot water from showers and sinks (gray-water). Tap water that is collected in package (reservoirs) has a certain average temperature, which is more than enough for a very high level of profitability of the heat pump. In the paper, a statistical analysis of the test results of two parallel systems for space heating with the difference in energy consumption and emissions of CO2 into the atmosphere. The first system uses fuel oil as an energy source to heat water which is further distributed to the heaters. Another system uses a heat pump water-water, which operates using electricity with high-efficiency thanks to the energy that is generated in the sanitary wastewater.