A rotary valve comprising: a housing defining a fluid flow path; a valve disk disposed in the flow path, the disk arranged to block the flow path when the disk is in a first rotational position; a shaft extending through a bore in the housing and connected to the disk such that rotation of the shaft causes rotation of the disk within the flow path; and a seal assembly arranged to extend around an outer circumferential surface of the shaft and to inhibit fluid flow from the flow path through the bore, and wherein the seal assembly comprises: a first seal ring having a first ring angled surface; a second seal ring having a second ring angled surface; and an elastic ring disposed between the first seal ring and the second seal ring and in contact with the angled surfaces; wherein the elastic ring is arranged to act on the angled surfaces to bias the first seal ring and second seal ring radially inwardly and apart from one another; wherein an annular shoulder is formed in the housing around the bore, and wherein the first seal ring is arranged to load against the annular shoulder when the first seal ring and second seal ring are biased apart from one another; and wherein the seal assembly is arranged to provide a sealing pressure between the first seal ring and the annular shoulder that is less than a sealing pressure between the first seal ring and the shaft so that, in use, fluid that leaks from the flow path into the bore passes between the first seal ring and the annular shoulder to increase the pressure acting on the radially outward surface of the elastic ring and the radially outer surfaces of the first seal ring and second seal ring, which in turn increases the force biasing the first seal ring and second seal ring radially inwardly and apart from one another. a housing defining a fluid flow path; a valve disk disposed in the flow path, the disk arranged to block the flow path when the disk is in a first rotational position; a shaft extending through a bore in the housing and connected to the disk such that rotation of the shaft causes rotation of the disk within the flow path; and a seal assembly arranged to extend around an outer circumferential surface of the shaft and to inhibit fluid flow from the flow path through the bore, and wherein the seal assembly comprises: a first seal ring having a first ring angled surface; a second seal ring having a second ring angled surface; and an elastic ring disposed between the first seal ring and the second seal ring and in contact with the angled surfaces; wherein the elastic ring is arranged to act on the angled surfaces to bias the first seal ring and second seal ring radially inwardly and apart from one another; a first seal ring having a first ring angled surface; a second seal ring having a second ring angled surface; and an elastic ring disposed between the first seal ring and the second seal ring and in contact with the angled surfaces; wherein the elastic ring is arranged to act on the angled surfaces to bias the first seal ring and second seal ring radially inwardly and apart from one another; wherein an annular shoulder is formed in the housing around the bore, and wherein the first seal ring is arranged to load against the annular shoulder when the first seal ring and second seal ring are biased apart from one another; and wherein the seal assembly is arranged to provide a sealing pressure between the first seal ring and the annular shoulder that is less than a sealing pressure between the first seal ring and the shaft so that, in use, fluid that leaks from the flow path into the bore passes between the first seal ring and the annular shoulder to increase the pressure acting on the radially outward surface of the elastic ring and the radially outer surfaces of the first seal ring and second seal ring, which in turn increases the force biasing the first seal ring and second seal ring radially inwardly and apart from one another.