Why is the pressure low at one tap but fine elsewhere?

If one tap is weak and the others are fine, the problem is at that tap, not in the supply. Three causes account for nearly all of it, in this order.

1. The aerator

The screen in the end of the spout collects limescale and grit. It unscrews — often with a small key supplied with the tap — and either cleans in descaler or costs two euros to replace. This is the cause more often than everything else combined.

2. A valve that is not fully open

The isolating valve under the basin, or the one behind the appliance, may have been left part-closed after a previous job. A screwdriver-slot valve gives no clue from its appearance; open it fully and try again.

3. A filter or restrictor in the line

Angle valves with a built-in filter catch grit and eventually block. New hand showers often carry a moulded flow restrictor in the inlet. Both are designed to be removed and looked at.

When it is not local

If every outlet is weak, or the weakness follows the hot side only, the cause is upstream — the supply, a filter on the main, or the water heater — and that is a different question.

This does not replace the manufacturer's own instructions. Every dimension, thread size, flow rate, operating pressure, water seal depth and load figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A flow rate is reported with the pressure the manufacturer measured it at, and is never converted to another pressure, because no conversion exists.