Domestic-type solar hot water systems incorporating evacuated collectors have been shown to function with up to twice the efficiency of flat plate collectors[l, 2], but considerable cost reductions are required to make systems incorporating evacuated collectors economically competitive. Single ended all-glass concentric tubular evacuated collectors, consisting of an absorber tube and envelope in a thermos flask configuration, have been intensively researched and developed and such collectors are now produced by two companies[3]. Production costs less than U.S. $4 per ft 2 of absorber tube have been estimated for large volume production[4]. However, further research into low cost methods of transferring heat energy from the collector tubes to a storage tank is necessary. An extremely simple manifold for low pressure fluid circulation between collector tubes and storage tank has recently been studied[5, 6]. The manifold consists of a single header pipe (encased in insulation), with connectors attached by O-ring seals to the open ends of collector tubes (Fig. 1). The collector tubes are usually inclined in a north-south direction with open ends up. Fluid completely fills the header pipe and the inner volume of each absorber tube and heat is transported with high efficiency from the absorber tube to the header pipe by thermosiphon flow. Hot fluid in the header pipe is transferred to a storage tank by means of pumped or thermosiphon circulation. A measure of the efficiency of heat extraction from the absorber tube of each evacuated collector tube for particular operating conditions is given by the difference (~T) between the average temperature of the selective absorbing surface on an absorber tube and the temperature of fluid in the header pipe at the point of connection of the collector tube. A detailed experimental and theoretical study of the heat extraction efficiency of collector panels incorporating evacuated tubes and various heat extraction manifold designs has shown that a value of ~T 8°C in full solar flux will degrade the efficiency of the collector panel by at least 0.01 due to increased heat losses from the evacuated collector tubes[7]. A method has recently been developed for accurate measurement of STfor absorber tubes in conjunction with various heat extraction manifolds and a wide range of collector operating conditions[59]. The method involves simulating solar energy input to an absorber tube by electrically powered heater strips mounted above and below a glass tube identical to the absorber tube within an evacuated collector (see Fig. 1). Thermocouple temperature sensors attached to the surface of the glass tube and to the manifold, allow determination of ~T for various simulated solar fluxes (obtained by adjusting the power dissipated in the heater elements), and various fluid temperatures and flow rates. These experiments have shown that a water-in-glass manifold of the type shown in Fig. 1 operates with ~T 6°C for a simulated solar flux of 1 kW m-215] while other manifold designs incorporating metal tubes and fins within the absorber tube operate with 8T > 20°C[9]. The high heat extraction efficiency and the simplicity of the water-in-glass manifold design suggests its suitability for a low cost domestic hot water system. In particular, because no part of the manifold protrudes into the absorber tubes (the O-ring seal between collector and manifold may be made on the collector envelope), the manifold should not be subject to extreme temperatures even in the event of collector stagnation. Consequently low cost moulded plastic with service temperature in excess of 100°C may be suitable for construction of the manifold. Figure 2 illustrates an extremely simple low pressure hot water system design in which the tubes are connected directly into the tank. In this design, hot fluid thermosiphons directly from each collec-