Hot water runs out too quickly / not hot enough
There's some hot water but it runs out fast or never gets properly hot — often a partially-failed element, a thermostat set low, or a system being asked to do more than its size.
Safety first
Isolate before testing elements/thermostats. Don't simply turn the thermostat up high — scald risk; tempering requirements apply.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
One of two elements failed (dual-element systems)
Most likelyLarger systems have upper and lower elements; if one fails you get reduced/partial hot water.
- 2
Thermostat set low / partially faulty
#2A low or drifting thermostat under-heats the tank.
- 3
Off-peak window too short for demand
#3An off-peak system not fully reheating overnight runs out during the day.
- 4
Sediment / scaling reducing capacity
#4Sediment build-up reduces effective tank capacity and heating.
- 5
System undersized for the household
Least likelyDemand has outgrown the tank size.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Check the thermostat setting and, on dual-element systems, whether both elements are working.
Sensible setpoint; both elements heating.
Both fine — consider off-peak window / demand / sediment.
Low setpoint or a failed element — rectify (mind tempering).
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Not enough hot water
→ step 2 - 2decision
Is the thermostat sensible and both elements working?
Yes→ step 3No→ step 4 - 3decision
Does the off-peak window fully reheat for the demand?
Yes→ step 5No→ step 6 - 4result
Low setpoint or failed element — rectify (mind tempering).
- 5result
Consider sediment/scaling or system sizing.
- 6result
Window/demand mismatch — consider boost / controlled-load options.
Common mistakes apprentices make
- Cranking the thermostat up without regard to scald/tempering.
- Not realising a dual-element system has two elements.
- Blaming the system when one element has quietly failed.
- Ignoring off-peak window vs demand.
When to stop & escalate
Element/thermostat work is licensed electrical; tempering valves and sediment flushing may involve a plumber. Genuine undersizing is a system-sizing decision.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
No hot water (electric storage system)
An electric storage hot water system has gone cold — no hot water at the taps. Usually the element, thermostat, supply, or (for off-peak) the tariff/timing.
Off-peak / controlled-load hot water not heating
A controlled-load (off-peak) hot water system isn't being energised during its tariff window — the element and thermostat may be fine, but power never arrives at the right time.
Heater tripping the RCD / earth leakage
A heater circuit trips its RCD/earth-leakage protection — often when cold and first switched on, or once it's been damp — pointing to leakage to earth from the element.