Plumbing and maintenance
Shower runs lukewarm or cold: where to look
The short answer
A shower that has lost heat is usually a valve, a supply balance or a scale problem, the source of hot water is often blameless.
How the system behaves
Point 1
Thermostatic cartridges scale up over time and start limiting the hot side; symptoms come on gradually.
Point 2
Where hot works elsewhere but not at the shower, the problem is almost certainly local to the valve or its inlet filters.
Frequently asked
Why does it go cold when a tap is used?
That is classic non-thermostatic behaviour or an unbalanced supply.
Can a cartridge be cleaned?
Sometimes descaled successfully; badly scaled cartridges are usually replaced.
Talk through shower runs cold with the person doing the work
Send a short description of the room and what you want to change. We'll come back with practical options rather than a brochure.