Hermetic primary circuit for thermal solar system

A thermal solar installation with draining by gravity, comprising at least:

• one absorbing solar collector (1) raised relative to the remainder of the installation and in which a heat-transfer liquid circulates,
• one hermetic primary circuit (2), comprising a supply circuit (15,29,30,11) and a return circuit (12,26,27, 25,14) towards and from the collector (1) respectively, the primary circuit being provided with a pump (5,19) as well as with a thermally insulated buffer tank (31) and initially filled with heat-transfer liquid at an imposed level (32), having the function of a liquid/air separator,
• at least one heat exchanger (4) and
• one secondary circuit (8) transporting the recovered energy towards a location of use (3), wherein:
• the return circuit of the primary circuit (2) is subdivided into two conduits (25;26,27), a first conduit (26,27) that brings the heat-transfer liquid back to a low point of the buffer tank (31) when the pump (19) is operating and a second conduit (25) opening at a high point of said tank (31), and provided with an anti-return valve (20), which allows air to be returned to the collector (1) and hence the primary circuit (2) to be drained by gravity when the pump (19) no longer operates;
• the first conduit (26,27) is separated into a conduit (27) opening into the buffer tank (31) and a bypass conduit (28) opening into said supply circuit (30) downstream from the pump (19), and provided with a normally open valve (21) that closes as soon as the pump (19) operates;
• the buffer tank (31) is connected to the exchanger (4) through a connection conduit to the exchanger (14), the return conduit (29) connecting the exchanger (4) to the pump (19) and comprising an anti-return valve (22), being located upstream from the connection of the supply circuit (30) to the bypass circuit (28).