Instantaneous / heat-pump hot water — no power or fault light
An instantaneous electric or heat-pump hot water unit shows no power or a fault indicator and isn't heating — different from a simple storage element fault.
Safety first
These units have their own controls and (heat pumps) refrigeration circuits. Isolate, prove dead, and follow the manufacturer's documentation. Don't open sealed refrigeration components.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
No supply / tripped protection
Most likelyLost supply or a tripped breaker/safety switch leaves the unit dead.
- 2
Unit fault code / lockout
#2The unit's controller is in a fault/lockout state (sensor, flow, refrigeration, etc.).
- 3
Controlled-load timing (heat pump on off-peak)
#3Some heat pumps run on controlled-load and only operate in a window.
- 4
Internal component fault
Least likelyA failed control board, sensor, or (heat pump) compressor.
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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeConfirm supply at the unit and check its protective device; note any fault code/indicator.
Supply present; any fault code recorded.
Powered — work the unit's fault code per its documentation.
No supply / tripped — restore (find why if tripped).
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Instant/heat-pump HWS fault
→ step 2 - 2decision
Is supply present at the unit (protection on)?
Yes→ step 3No→ step 4 - 3decision
Is it within its operating/controlled-load window?
Yes→ step 5No→ step 6 - 4result
Restore supply (find why if tripped).
- 5result
Work the unit's fault code per documentation (escalate if needed).
- 6result
Outside window / controlled-load not energised — chase that.
Common mistakes apprentices make
- Treating it like a simple storage element (it isn't).
- Ignoring the unit's own fault code.
- Not realising a heat pump may run on controlled-load.
- Opening sealed refrigeration components.
When to stop & escalate
These units are licensed electrical work and often need the manufacturer/qualified service for coded faults and refrigeration (heat pump) issues. Work to the unit's documentation.
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.
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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.
Hot water system tripping the safety switch
The hot water system trips the safety switch — typically when it heats — pointing at an element leaking to earth or moisture in the system.