URGENT Loft room shower problems

[Nicklm]

Hi everyone
Hoping someone has an idea what's going on with my loft conversion shower.
It runs ice cold after 5 mins and cant be the thermostatic bar as we've now changed it 3 times. The hot water in the tap also runs cold after 5 mins. The water us fed from a combi boiler on the ground floor and the first floor shower has never had this problem.
Thanks in advance for any ideas

 

[ShaunCorbs]

Dead leg of cold water in the pipes once hot does it stay hot etc ?

 

[Ben-gee]

Not enough flow to fire boiler, try using a low pressure shower and see it that helps (eg Mira Coda)

 

[Nicklm]

hot water runs out after 5 mins. Same in tap, if you turn off for 5 mins then start again hot water returns but goes again.

 

[ShaunCorbs]

Do you have low pressure in your house eg around 1-1.2 bar ?

 

[Nicklm]

water pressure is very good.

 

[SimonG]

Mixer tap or shower passing cold to hot water feed.