Abstract: The invention relates to a waste water heat recovering unit for preheating cold fresh water with already heated water, comprising a cleaning liquid tank (9) for collecting warm cleaning liquid, an intermediate tank (10) which is connected via a pipe (101) with the cleaning liquid tank (9), a pump (8) arranged in the pipe (101) which conveys at least part of the warm cleaning liquid from the cleaning liquid tank (9) into the intermediate tank (10), and a heat exchanger (4) which is connected, on the one hand, via a fresh water pipe (14) equipped with a volume controlling valve (1) to a fresh water connection for cold water and, on the other hand, to the intermediate tank (10) containing the warm cleaning liquid, a drain valve (12) being provided at the outlet of the heat exchanger (4) in the cleaning liquid guidance so that there flows through the heat exchanger (4) either substantially the same volume of cold fresh water as warm cleaning liquid from the intermediate tank (10), or only warm cleaning liquid wit