Ideal H 3 fault codewhat it means and how to fix it
The hot water thermistor on an Isar HE combi is reading outside its range. Heating usually carries on; the taps suffer. Ideal test the sensor on the plate heat exchanger, then wiring, then board.
Take care
Get a Gas Safe registered engineer to look at your Ideal H 3
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 3 means
H 3 is DHW Thermistor Error, and it belongs to the Isar because the Isar is the combi of this generation: the Icos is heat-only, the Mexico HE is heat-only, and neither has a hot water sensor to complain about. The thermistor sits on the plate heat exchanger where mains water is heated on demand, and it tells the board when the tap water has reached the temperature set on the hot water knob — Ideal limit it to 65°C.
Without a reading the board can trust, it cannot control that temperature, so it stops making hot water rather than risk scalding or running cold. Ideal's frame is the thermistor frame again with the location changed: remove the DHW thermistor from the plate heat exchanger, disconnect it, measure it at 25°C, 60°C and 85°C against the standard table, replace it if wrong, and otherwise check continuity to the board before replacing the PCB.
The Logic Combi2 and i-mini2 report the same sensor as F 0C; the Logic and Logic ESP1 have no separate code for it at all. On an Isar, then, H 3 is a useful specific: it tells the engineer the fault is on the hot water side before the casing comes off.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| DHW thermistor out of tolerance | The plate heat exchanger sensor sees the widest temperature swings in the boiler, from cold mains to 65°C in seconds, many times a day. It ages faster than the heating sensors. | Common |
| Scale on the plate heat exchanger | Not in Ideal's chart, and this page's inference rather than theirs: a scaled plate can deliver water in surges of temperature that the board reads as a sensor fault, which would explain a sensor that tests fine and keeps tripping. More likely where the water is hard. | Occasional |
| Wiring or PCB | The chart's last two boxes. Continuity from the board to the sensor, then the board. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£380
Time on site
40 minutes
A DHW thermistor £80–£150 fitted. If the plate heat exchanger is scaled, a descale or replacement adds £150–£350. A PCB, rarely, £200–£380.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Ideal Logic flow or return thermistor (clip-on NTC)
£12–£35Authorised Ideal parts distributor
Price checked 28 September 2026
Ideal Logic primary PCB
£150–£320Authorised Ideal parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Ideal fault codes
- F4Get it checkedThe flow thermistor — the sensor reading the temperature of water leaving the heat exchanger — is giving a value the board will not accept. L4 is the same fault as a lockout.Related: sensors and thermistors
- F5Get it checkedThe return thermistor on the pipe bringing water back from the radiators is reading outside its range. On most Logic boards L5 is the locked-out form of the same fault.Related: sensors and thermistors
- L5Get it checkedTwo different faults share this code. On the two-window Logic ES boards it means five resets in fifteen minutes; on the 2011 Logic, the ESP1 and the Combi2 it is a return thermistor lockout.Related: sensors and thermistors
- F6Get it checkedThe outside sensor for weather compensation is reading wrongly or is not connected. It only appears on boilers that have one, or have been told they have one.Related: sensors and thermistors
- FAGet it checkedThe return pipe is reading hotter than the flow, which cannot happen in a boiler that is plumbed and sensing correctly. Ideal call it a negative differential; the Vogue screen says Flow/Return Reversed.Related: sensors and thermistors
- F0Get it checkedOn the Logic Code Combi ES and first i-mini, F0 is the sensor that detects water flow reporting a fault. It is not the same as L3, which is the flow itself having stopped.Related: sensors and thermistors
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions about Ideal H 3
Why do I still have heating?
Because the heating side is controlled by the flow thermistor, which is a different sensor. H 3 only concerns the reading from the plate heat exchanger, so radiators can run while the taps do not.
Can an Icos or Mexico HE show H 3?
Not according to Ideal's manuals. Both are heat-only boilers with no plate heat exchanger and no hot water thermistor, and neither installer manual lists H 3. Their hot water comes from a cylinder controlled by its own thermostat.
Is it the same as F 0C on a Logic?
The same sensor and the same test, on a newer boiler. If your display shows F 0C in three characters you have a Logic Combi2 or i-mini2 and that page is yours.
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.