Ideal H n fault codewhat it means and how to fix it
An Isar, Icos, Mexico HE or Esprit has detected that its live and neutral are the wrong way round. Ideal's entire remedy is to check the wiring to the boiler — an electrician's job.
Take care
Get a Gas Safe registered engineer to look at your Ideal H n
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What H n means
H n is Phase Reversal Error. The boiler's flame detection works by measuring a small current between the detection electrode and earth, and that measurement assumes the burner side of the circuit is at neutral potential. If the supply to the boiler has live and neutral swapped — at the fused spur, in a plug that has been rewired, or where an electrician has extended a circuit — the board sees a reference it cannot use and reports H n rather than misread a flame.
Ideal's fault-finding frame is a single box: check the wiring to the boiler for reversed live and neutral. It is not a boiler fault so much as an installation one, and it appears most often just after electrical work in the house, or on an old installation that has been wrong since the day it was wired and has only begun to matter because a replacement board is stricter than the original.
The Logic generation does not publish a code for this. Nothing about H n is for the householder to correct, because the fault is in fixed wiring, but knowing what it means saves calling a heating engineer to a job that belongs to an electrician.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Live and neutral reversed at the spur or plug | Ideal's one check. A fused connection unit wired with the cores swapped, or a boiler on a plug that has been rewired the wrong way. | Common |
| Recent electrical work upstream | A consumer unit change or a circuit extension that has reversed polarity on the boiler's circuit. The boiler is often the only thing in the house that notices. | Occasional |
| External controls wiring | A programmer or wiring centre installed with the switched live returned on the wrong terminal can put the boiler's live where its neutral should be. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£60–£150
Time on site
30 minutes
Correcting polarity at the spur is a short electrical visit, £60–£120. Tracing a reversal further back in the house wiring £100–£150. No boiler parts.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Ideal fault codes
- F9Get it checkedThe control board has not been told what boiler it is in, or has failed. L9 is the same fault locked out. The 2011 Logic Combi row also covers a gas valve short circuit.Related: electronics and power
- dUUsually simple to fixNot a fault. The diverter valve has been parked in its mid-position for servicing and the boiler is waiting to be told the service is over. Ideal publish the way out.Related: electronics and power
- Blank displayGet it checkedNothing on the screen at all. Ideal's user guides send you to the boiler's switch and the consumer unit first; the installer charts then work from the terminal block to the board.Related: electronics and power
- F7Get it checkedThe mains voltage reaching the boiler has dropped below what the board will run on. Ideal's advice is an electrician or your electricity supplier, not a heating engineer.Related: electronics and power
- C0Get it checkedThe board cannot activate the Boiler Chip Card that tells it what model it is fitted to. Ideal's engineer check is whether the small plastic card is in its slot at all.Related: electronics and power
- L9Get it checkedOn a Logic, L9 is the locked-out form of the unconfigured-board fault F9. On an Isar HE it is something else entirely: a heat exchanger overheat read by the dry-fire thermistor.Related: electronics and power
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions about Ideal H n
Is H n dangerous?
Reversed polarity is a wiring fault that should be put right regardless of the boiler, because it can leave parts of an appliance live when its switch is off. The boiler has done the right thing by refusing to run. Do not open anything yourself.
Who do I call — a gas engineer or an electrician?
Ideal's remedy is to check the wiring to the boiler, and a Gas Safe engineer will normally have the meter and the competence to do that at the spur. If the fault is further back in the house wiring, they will tell you it needs an electrician.
It started after we had work done. Is that a coincidence?
Rarely. H n appearing after a consumer unit, socket or programmer job is the classic presentation, and the person who did the work should come back to check polarity on the boiler circuit.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.