Abstract: The present invention relates to a thermal solar installation with draining by gravity, 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 exchan