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The Hot Valve Seat and the Cold Valve Seat are parts of the slide valve assembly. On failure of the cold water supply, the Hot Valve Seat closes onto the Hot Valve Face- the flat surface inside the valve body - and stops the hot water flow. On failure of the hot water supply, the Cold Valve Seat closes on the Cold Valve Face, the flat surface on the underside of the valve cover assembly.
This applies to point of use TMVs and also the TSV1 shower valve.