The Part That Moves Versus the Part That Does Not

Why the distinction decides the job

Changing the cartridge is work done from the front, through the trim, with the water off. Changing the body means opening the wall and working on the pipework, and on a tiled shower that is a different order of job.

Which is why any quote for a shower valve problem is worth asking one question about: is this the cartridge, or the body?

What points to the cartridge

Temperature control that has become vague or reversed. A handle that has become stiff, or one that turns freely with less effect than it used to. A drip from the showerhead that continues after the handle is fully off. Hot missing while cold is fine or the reverse.

These are all the moving assembly, and on most valves they are all the same repair.

What points to the body

Water appearing inside the wall, in a ceiling below, or at the base of the wall on the other side. Visible corrosion at the connections when the trim is off. A valve so old that no cartridge is made for it any more.

The last of those is the most common reason a sound valve gets replaced, and it is the same parts-availability problem that decides faucet jobs.

The thing to have ready on the call

The manufacturer name, which is usually on the trim plate or the handle. Cartridges are specific to a maker and often to a range, and the difference between having the name and not having it is frequently the difference between one visit and two.

A photograph of the trim helps more than a description.

And the scale question

In hard water areas a cartridge can be seized by mineral rather than worn out. That changes how the old one comes out — sometimes with a puller, sometimes not at all — and it is worth flagging if the household already knows its water is hard.

• Replacing one
• Behind the wall