Hot water question

[Riley]

Called to a flat today tenant complaining that their hot water is only coming out at a trickle. Says was having a shower and all of a sudden went stone cold. My first assumption was cwsc not filling properly but get up in loft and full tank. Now I'm questioning the set up in general. she says the shower used to work well only on a gravity system and a metres head so thought it could work but surely not well all other outlets are running hot at a trickle. No pumps or anything like that I'm assuming blockage but unsure where to start any thoughts

 

[Killy Bing]

Blocked valve or airlock. Any valves on the cylinder or hot take off?

 

[Reg Man]

Scaled up outlet to the hot water cylinder.

 

[Riley]

No valves. All heat from immersion. Hmmm hunt the scale it is then.

 

[Reg Man]

Top outlet pipe on the hot water cylinder, every time.