Shower water temp unstable hot first then sudden cold and it continues

[Thunderexpert]

Hi guys, I have this water temp issue which has lasted for years. Basically what happened is the shower water temp has never been stable in the bathroom, when we turn on the shower water it comes out hot as expected but after a few seconds it becomes ice cold, and then it becomes hot again. This then carries on for ever.

We have 3 floors, the bathroom shower is on the 1st floor and its temp not stable, there is another small shower cubic on the top (2nd floor), and it also has a same problem, but just even worse, because we can't even get hot water from that shower cubic at all.

just wondering if anyone can help advise what is likely wrong, do we need to change a boiler systems. The one we have right now is a conventional one with a boiler in the kitchen and a big tank on the top floor. The current boiler system is about 10 years old.

can anyone help?

 

[Paulus]

Do you have a thermostatic shower valve on the first floor or a mixer?

Where are the feeds to the shower taken from?

The problem with the higher shower is probably that you are relying on gravity feed and the hot water pressure is insufficient for the type of shower.