Too little hot water
Cold, lukewarm or short-lived hot water
No hot water, water that stays lukewarm and a supply that runs out sooner than it used to are separate clues. Tell us whether every tap behaves the same, how long the change has been happening and whether it comes and goes. A problem at one fixture may not involve the heater at all.
On an electric heater, a heating element, the thermostat or the circuit can be behind lost heat. On a gas heater, the pilot or thermocouple may be involved. The same split applies to a heater that keeps shutting off, which is why the unit is checked before any part is replaced.
