The water demand of private houses in ancient Roman towns was never estimated with specific reference to an articulated group of premises within a settlement, and in relation to specific types of installations and their well- defined connections to the public water supply network. The remains of the piping still preserved in some private buildings in Pompeii, and the structures which they supply, are the object of a two-year (2023-2025) study funded through the Small Research Grant programme of British Academy. The authors have been surveying various private residences along Via di Mercurio in Regio VI, an area which is believed to have been disconnected from the public water mains after the 62 CE earthquake. From the study of the remains located in the upper areas of the road, it will be possible to comment further on the phases of the water supply, and estimate the quantities of water needed to operate various types of garden pools and fountains. This paper presents the results obtained for two houses, Casa della Fontana Grande and Casa dei Dioscuri, discussing the connections to the original public water towers and the final possible modes of operation.