Water Not Hot Enough or Runs Lukewarm

The water heater is working — you're getting water, just not as hot as it should be, or it starts hot and fades to lukewarm partway through. This spans a wider range of causes than a full shutdown, from a simple dial setting to a genuine sensor fault.

Most likely causes, ranked

• Thermostat set lower than expected. Easy to overlook after a power outage reset a digital thermostat, or after someone else in the house adjusted it — check the actual setting before anything else.
• Scale build-up reducing heat transfer efficiency. On tankless units, mineral scale coating the heat exchanger makes it work harder for less output, and most brands flag this directly: Rinnai LC and Rheem 1L are both scale-monitoring warnings that can precede or accompany noticeably weaker heat output.
• Outgoing water temperature sensor fault (tankless). If the sensor that measures water leaving the unit is reading incorrectly, the control board can under-heat the water while thinking it's hitting the target — see Rinnai 32.
• Demand exceeding the unit's rated flow capacity (tankless). Running two showers and a dishwasher at once on a single tankless unit spreads its fixed heating capacity across more flow, which drops the output temperature at every fixture — this is expected behavior, not a fault, once you're past the unit's rated GPM.
• Sediment layered in the bottom of a storage tank. Sediment insulates the water from the burner or lower element, reducing effective hot water output and, in bad cases, shortening how long hot water lasts before running out.
• Failed or partially failed lower heating element (electric tank). If only the lower element has failed, you may still get some hot water from the upper element alone, just much less of it.
• Undersized unit for household demand. Not a fault at all — a unit sized correctly for a smaller household or a smaller original fixture count can simply run out of capacity as demand grows.
• Cross-connected or leaking mixing/tempering valve. If a mixing valve is letting in more cold water than intended, or hot and cold lines are crossed somewhere in the plumbing, output temperature drops regardless of what the unit itself is doing.

Check this first

1. Confirm the actual thermostat setting on the unit — don't rely on memory, check the dial or display directly.
2. Note whether every fixture in the house is affected equally, or just one — one-fixture-only points at a local mixing valve or crossed line, not the water heater.
3. On a tankless unit, note whether the drop happens only when multiple fixtures run at once — that points to exceeding rated flow capacity, not a fault.
4. Check whether the unit is showing a scale-warning code or has gone past its recommended flushing interval (most manufacturers recommend an annual descaling flush).
5. On a tank unit, note how quickly hot water runs out compared to when it was new — a much shorter duration than the tank's rated capacity points at sediment or a failed element.

Which fault code are you seeing?

If a code or blink pattern is showing, look it up directly: code lookup, or jump to a brand: Rinnai, Navien, Rheem, AO Smith.