Abstarct: Waste energy recovery plays an important role in resource management. about 40% of produced energy in the country, consumes in domestic and commercial consumption that 21% of this energy expended in heating of water that has a specific portion in consumption of produced energy. in this project, it is tried with making a double-pipe heat exchanger, waste heat in bathroom recovered and forward to consumption cycle. hot water that is used in bathroom, after pouring to sewage, passed through the copper pipe and another copper pipe that is wrapped around the prior one and cold water crossing through it, absorbs heat from sewage water and reheats water entering to building. reheated water can enter the bathroom directly and also enter the heating system; or forward to both bathroom and heating system that every case is investigated separately in this project. heat exchanger efficiency in different mass flow rates is between 20% to 45%. and maximum pressure drop is 40 bar that will not influence on total pressure drop. in economic study, payback period is calculated with net present value method. since energy prices have not already reached to its actual prices, it is tried to study by export gas prices. finally, for different connections of heat exchanger in building, payback period is calculated 7 to 12 year for internal gas prices and 2 to 3 for export gas prices.