Why does my hot water run out so quickly?

The answer depends entirely on which kind of system you have, and the two common types fail in completely different ways.

If you have a hot water cylinder, you have a fixed reservoir, and running out means either the tank is too small for your usage or it is not reheating properly. Check the heating schedule first — many cylinders are set to heat only at certain times, and a schedule that made sense for a previous household will not suit a new one. Then check the thermostat setting on the cylinder. Then consider the immersion heater or boiler coil: a failed immersion element, or a coil heavily coated in limescale in a hard-water area, heats far more slowly than it should.

A very common and easily missed cause is a failed cylinder jacket or insulation, which lets the stored water cool much faster than it should between uses.

If you have a combi boiler, there is no reservoir — water is heated on demand as it flows through. Running lukewarm rather than running out is the usual symptom, and the causes are different: a blocked or scaled plate heat exchanger, a diverter valve that is not switching fully from heating to hot water, or simply a flow rate higher than the boiler can heat. Combi output is quoted in litres per minute at a given temperature rise, and a powerful shower head can exceed it, especially in winter when incoming mains water is colder.

In both systems, check for a hot water recirculation issue or a mixing valve failing and diluting hot with cold.

A sudden change from normal behaviour is the useful signal. Gradual decline over years usually means limescale; an abrupt change usually means a component has failed.