Hot at the Basin, Cold at the Shower

What it rules out immediately

If a basin two feet away runs hot, the water heater is working, the hot supply reaches that bathroom, and nothing about the heating system is at fault. The problem is at or inside the shower valve.

Say that on the call. It is the difference between a visit that starts at the valve and one that starts in the basement.

The usual causes, roughly in order

A failed or scaled cartridge, which can block the hot side while passing cold. A limit stop that has moved or been set to its coldest position. A blocked hot inlet at the valve body. A crossed connection, where hot and cold were reversed at installation and a mixing path is confusing everything.

All four are valve-side and all four are ordinary.

The one that is not the valve

A failing mixing arrangement elsewhere can feed cooled water to one branch. It is uncommon, and it is worth mentioning only because the test for it is easy: check whether more than one fixture is affected.

If the answer is yes, this is not the page you need and the fault is upstream.

And what not to do

Do not open the valve up to see. A shower valve is behind a wall, under pressure, and frequently the only thing holding a tiled installation together at that point. What is actually back there explains why this job is priced the way it is.

• Cartridge or valve
• Behind the wall