Ideal H 1 fault codewhat it means and how to fix it
The control thermistor on the flow pipe of an Isar, Icos, Mexico HE or Esprit is reading wrongly. Ideal test its resistance and the wiring to the board, then replace the sensor or the PCB.
Take care
Get a Gas Safe registered engineer to look at your Ideal H 1
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 1 means
H 1 is Control Thermistor (Flow) Error on the LED display, the temporary code for the sensor the boiler steers by. It sits on the central heating flow pipe and tells the board how hot the water leaving the heat exchanger is; when its reading falls outside what the board will believe, the boiler shows H 1 and will not run properly until the reading comes back.
Ideal's chart removes the thermistor from the flow pipe, disconnects it and measures it — 9,700 to 10,300 ohms at 25°C, 2,400 to 2,600 at 60°C, 1,000 to 1,100 at 85°C — replacing it if the values are wrong; if they are right, the engineer checks continuity between the board and the sensor, repairs the wiring if there is none, and replaces the PCB if there is.
The same sensor on a Logic produces F4, and the same chart, but the consequences run further on the older boiler: Ideal's L A overheat frame sends the engineer to this thermistor when the flow passes 90°C before the overheat thermostat trips, because a control thermistor reading low is how an Isar comes to overheat in the first place.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Thermistor out of tolerance | Ideal's three-temperature test decides it. An NTC of this age that reads high at 25°C tells the board the water is cooler than it is. | Common |
| Wiring or connector between sensor and board | The continuity check in the chart. A corroded terminal on a sensor that has lived beside a hot pipe for fifteen years adds resistance the board reads as temperature. | Occasional |
| PCB input failed | Correct resistance, good continuity, H 1 still showing: the last box in Ideal's chart is 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 thermistor £80–£150 fitted. Wiring repaired £70–£110. A replacement PCB, the last resort in Ideal's chart, £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 1
Can I keep using the boiler with H 1 showing?
It will not let you, or not for long. H 1 is temporary in the sense that it clears if the reading recovers, but a boiler that cannot trust its flow sensor does not fire normally, and Ideal give the householder nothing to try for it.
Is this why my Isar overheated?
Possibly. Ideal's overheat chart points the engineer at the control thermistor when the flow exceeds 90°C before the overheat thermostat trips. A sensor under-reading lets the boiler run hotter than it thinks it is, and the independent thermostat catches it as L A.
What is H 2, then?
The same test on the other sensor. H 2 is the control thermistor on the return pipe, on Isar models that have one; H 3 is the hot water thermistor on the plate heat exchanger. Each has its own page.
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.