QualifiedHigh risk

Ducted air-con system has no power / won't start

A ducted system (roof/cupboard fan unit + zones/controller) is dead or won't start — pointing at the dedicated supply/isolator, the controller, zone motors, or the unit's PCB.

Start fault-finding

Safety first

Roof-space/cupboard units involve height/access and a dedicated supply — isolate and prove dead. Refrigeration internals need a refrigeration tech.

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. 1

    Dedicated supply/isolator off or tripped

    Most likely

    The ducted unit's dedicated circuit/isolator (often in the roof) is off/tripped.

  2. 2

    Zone controller / wall controller fault

    #2

    The controller has no power or has failed, so the system won't start.

  3. 3

    Unit PCB / transformer fault

    #3

    The unit's control board or its low-voltage control transformer has failed.

  4. 4

    Safety/condensate switch tripped

    Least likely

    A condensate overflow or safety switch has shut the unit down.

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.

Test 1 of 3
1

Check the dedicated supply/isolator (often in the roof space) and confirm power at the unit.

Expected reading

Isolator on, circuit healthy, power at the unit.

If it passes

Powered — check the controller and any safety/condensate switch.

If it fails

Isolator off / tripped — restore (find why if tripped).

Often leads toSplit-system air-con not running at allAir-con tripping the power
View all expected readings at once
1. Check the dedicated supply/isolator (often in the roof space) and confirm power at the unit.
Isolator on, circuit healthy, power at the unit.
2. Check the wall/zone controller has power and responds; check for a tripped condensate/safety switch.
Controller responsive; safety switches not tripped.
3. Isolate and check the unit's control transformer/PCB per the unit docs.
Healthy control transformer and responsive PCB.

Fault-finding flowchart

The same logic as a decision tree.

  1. 1
    start

    Ducted system no power

    → step 2
  2. 2
    decision

    Is the dedicated isolator on and power at the unit?

    Yes→ step 3No→ step 4
  3. 3
    decision

    Does the controller respond and are safety/condensate switches OK?

    Yes→ step 5No→ step 6
  4. 4
    result

    Restore isolator/circuit/supply.

  5. 5
    result

    Suspect the unit transformer/PCB — rectify / qualified service.

  6. 6
    result

    Dead controller or tripped safety/condensate — rectify.

Common mistakes apprentices make

  • Forgetting the dedicated isolator is often up in the roof space.
  • Overlooking a tripped condensate-overflow switch.
  • Not checking the controller has power.
  • Working at height without safe access.

When to stop & escalate

Electrical supply/controller/transformer work is licensed electrical; refrigeration internals need a refrigeration technician. Condensate/drainage issues may also involve mechanical work.

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

HVAC & air-con

Split-system air-con not running at all

A split-system (head unit + outdoor condenser) is completely dead — no response from the remote/controller — pointing at supply, isolator, controller, or the indoor PCB.

4 causes3 test steps
QualifiedHigh risk
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Air-con tripping the power

An air-con unit trips its breaker or safety switch — on start, when the compressor kicks in, or randomly — pointing at the compressor, an earth fault, or the supply/protection.

5 causes3 test steps
AdvancedHigh risk
HVAC & air-con

Air-con condensate pump not working / unit shut down on overflow

An aircon has shut down on a condensate overflow/float switch, or its condensate pump isn't running — water isn't being removed, so the unit stops to prevent overflow.

4 causes2 test steps
QualifiedMedium risk