The majority of the domestic water supply projects developed until now is based on very simply Domestic Hot Water (DHW) systems like electrically heated storage cylinder and gas or oil heating boilers. For Civil Engineers project designers the calculations were a quite simple task because they used to select the equipment without specific and demanding criteria. Nowadays, it is necessary to choose well the equipments to minimize its life-cycle cost because a minimum of three types of equipments must function together: the solar collector; the storage tank or cylinder

and the backup equipment (e. g. condensing oil or gas heating boilers). The system operation is also controlled by an automatic electronic unit. Not forgetting other system accessories, like pipes, pumps and valves [7,8]. The passive systems are commonly used on single residential buildings and they are more compact and easily mounted. But in multi residential buildings, there are several different consumers and it is necessary to think the most advantageous system installations option. Design professionals are dealing today with this dilemma because: they have little experience; they do not have much technical information to support their choices and there are not yet many multi­residential buildings with this technology in Portugal to learn with. Aware of this situation, a previous analysis for six system possible options (Table 1, Fig 1.) is presented next.

Each of these design options has, in a preliminary perspective, positive and negative points.

Systems 1 and 2 — All equipments are individual — Advantages: self management of the system by each family (they adapt the system to their consumptions needs); minimal problems and conflicts with neighbours; the solar panels could be mounted with different tilt angles for each apartment adapted to their consumptions needs. Disadvantages: great number of water pipes and accessories, more building interior space needed and more heat losses; more home interior space to install equipments; high maintenance expenses for each family; if one home produces solar energy

in excess it is not possible to redistribute it; in seasonal or not occupied spaces there is wasteful production of energy; more complexity on the system mounting. System 3 and 4 — Centralized solar collector — Advantages: less initial investment ; better optimization of the collector area and of the solar south-oriented roof area; optimization of the captured solar energy and a rational distribution for all consumers; management and maintenance in charge of the building condominium administration; the energy of seasonal occupied or not occupied homes can be used by others; each individual consumer must adapt their consumptions needs to a collective system. Disadvantages: more home interior space to install equipments; heat exchanger in inverted system; the householders must pay a service not total adapted to their consumptions needs; some homes can take more profit from the solar collector system then others, it depends of the consumption needs schedule and habits. System 5 and 6 — All equipments are sheared by all householders — Advantages: less pipes and accessories; more reduced initial investment; no individual system maintenance; householders pays a total service of hot water supply; better optimization of the collector area and of the solar south-oriented roof area. Disadvantages: initial part of the system with inverted supply system (water supply starts from the first floors); complex management of the return water of the circuit into the water storage tank; variations on hot water temperatures can occur; lack of preparation of the condominium administrations firms to manage this service; system regulated to a constant level of water temperatures not adapted to consume households schedule with less energy efficient management; requires an adapted collective interior space for a high capacity water storage tank and backup equipments; the building structure must resist to higher loads due to the heavy equipments installed; the building architecture must be prepared for maintenance, repair and substitution of big equipments; reduced preparation of the local authorities to deal with this collective systems (taxes to pay etc); big size systems are more vulnerable to operation problems and damages.

The first two systems require some initial investment and in some cases can lead to collector areas that are not effectively used. The second group of systems seems to have more vantages due to the fact that can result in a better flexibility in the system management, made by both condominium administration and individual consumers. The last systems also have some advantages but represent a more vulnerable operating system.