**4. Summary**

This article described the state of the art of several existing LTDH systems. The advantages of LTDH networks over the traditional district heating networks have been discussed. Reviewed cases and studies intensified the energy efficiency potential of LTDH. This system provides a unique opportunity to integrate renewable heat sources such as geothermal and solar as much as possible. The capital costs of LTDH is generally less than the high-temperature DH. In a very low-temperature district heating concept, heat pumps are required mainly for domestic hot water at the end-users. Since the LTDH technology is relatively new, the regulations, including policies and incentives and design standards, are not well established. Further investigations are needed in order to identify the design criteria and develop regulations, including the transition from the exiting hightemperature DH to LTDH. The behavior of the LTDH under different operating conditions and various design configurations should define by holistic models that are available yet. The LTDH system has the potential to receive full market attention as the technology evolves.
