Changing Hot Water tap washer in Kitchen

[Kris43]

Can someone just put me right and let me know wether changing a washer in a hot water tap means draining the system by tying up the ball cock in the loft or ( as my missus is trying to assume ) a much quicker way.

[Agitated]

lookin the aring cupboard where the cold feed goes into the bottom of the hot water cylinder
there is normally a gatevalve wich has a RED Handle too shut off water from CWST shut this and turn on hot tap to drain below top of HWC replace washer. turn valve on again WITH hot tap open to clear air out of pipework

[Kris43]

Looked in Airing cupboard and no sign of Gate valve.

[Agitated]

Ok Look in loft space around base off tankpipe going down to airring cupboard if not its a tie up ballvave job

[squeaky]

Silly question but is there an isloation valve under the sink?. This is sometimes overlooked.