Heat recovery unit from gray water

A heat recovery unit comprising:

• an inner tube;
• an outer tube being larger than the inner tube, the outer tube externally enclosing the inner tube so as to create a conduit between the inner tube and the outer tube;
• a first end-piece configured to seal a first end of the inner tube, and a first end of the outer tube, wherein the first-end piece comprises two nozzles, one for the inner tube configured for input of a first liquid, and one for the outer tube configured for output of a second liquid;
• and
• a second end-piece configured to seal both a second end of the inner tube, and a second end of the outer tube, wherein the second end-piece comprises two nozzles, one for the inner tube configured for the output of the first liquid, and one for the outer tube configured for the input of the second liquid, wherein:
• the heat recovery unit further comprises an intermediate annular space within the conduit and between the inner wall of the outer tube and the outer wall of the inner tube, the intermediate annular space being configured to contain a third liquid for providing a protective barrier between the inner and outer tubes.