Heat exchangers can also be installed in wastewater pressure pipes. The energy yield is always very high in these pipes. The only disadvantage compared with normal sewers, known as gravity sewers, is that the installation of the heat exchangers requires some excavation work. Wastewater pressure pipes must be uncovered, and usually a whole section of a pressure pipe is replaced by a new pipe section with an integrated heat exchanger system. If a pressure pipe is installed for the first time or rehabilitated, this approach is still very attractive economically. But if no new building or rehabilitation is required, the extent of the excavation work is decisive for an evaluation of the economic viability. The extent of this work is different for every project. Because gravity sewers can be walked through, from a particular dimension, the installation of the heat exchangers does not require any excavation work. This makes the system essentially very attractive, especially in urban areas where, because of the lack of open space, many renewable energies cannot really be considered as supply options.
