Ideal H 4 fault codewhat it means and how to fix it
On an Isar HE, H 4 is the dry-fire thermistor on the heat exchanger; on an Icos HE it is the flue thermistor. Both guard against running hot with no water.
Take care
Get a Gas Safe registered engineer to look at your Ideal H 4
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.
Which boiler is showing H 4?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
An Isar HE, or an Esprit or Elise on the same platform
Dry-fire thermistor error: the sensor on the heat exchanger body.
An engineer's check of pressure, pump and sensor, ending at the heat exchanger. If the display alternates H 9 or L 9 instead, the exchanger has actually overheated — see the L9 page.
An Icos HE heat-only boiler
Flue thermistor error: the sensor in the flue outlet.
The same sequence on the flue sensor, and Ideal's instruction to inspect the heat exchanger flueways for blockage. Engineer only.
What H 4 means
H 4 is the code that most rewards knowing which boiler you own. The Isar HE installer manual calls it Dry Fire Thermistor Error: a sensor on the heat exchanger body that detects the casting heating with no water passing through it. The Icos M3080 manual calls the same code Flue Thermistor Error: a sensor in the flue outlet that detects combustion products leaving too hot, which happens for the same reason.
The two frames are otherwise nearly identical and unusually thorough. Can the fault be reset by switching off the mains? Is the system filled, vented and its isolation valves open? Is the pump operating? Then the resistance test at 25°C, 60°C and 85°C, replacing the thermistor if wrong; and if the thermistor is right, "inspect heat exchanger for blockage or damage in the flueways — clean or replace as necessary". That last box is the point.
H 4 is the sensor saying the heat exchanger has been hotter than it should, and Ideal want the exchanger looked at even when the sensor turns out to be sound. On the Isar the same dry-fire thermistor also produces H 9 and L 9, Heat Exchanger Overheat, when the reading is not merely wrong but high.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Boiler run with little or no water | Low pressure, an airlock or a failed pump lets the heat exchanger heat with nothing to carry the heat away. Ideal's chart asks about filling and the pump before it touches the sensor. | Common |
| Thermistor out of tolerance | The resistance test decides it. On the Isar the sensor sits against a very hot casting and ages accordingly. | Common |
| Heat exchanger flueways blocked or damaged | The end of Ideal's chart and the finding nobody wants: on an Icos the flue thermistor reads hot because the products cannot get out of a fouled exchanger. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£900
Time on site
1 hour
A thermistor £80–£150 fitted. Cleaning the heat exchanger flueways £120–£200 as part of a service. A replacement heat exchanger on a boiler of this age, £500–£900, is usually the point at which a new boiler is discussed instead.
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.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Ideal Logic flow or return thermistor (clip-on NTC)
£12–£35Authorised 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
- L1Get it checkedThe boiler is not seeing enough water flow through it to run safely, so it has stopped. Usually a pump, an air lock, or a blocked filter.Related: overheat and circulation
- FDGet it checkedNo water flow has been detected. The pump is the prime suspect and this will become an overheat if it is left.Related: overheat and circulation
- L3Get it checkedThe boiler has stopped because it cannot detect water moving through it. Ideal's own steps are to fill the system to 1 bar with it cold, bleed the radiators and try again.Related: overheat and circulation
- L AGet it checkedThe overheat thermostat on an Isar, Icos, Mexico HE or Esprit has tripped and the boiler has locked out. Ideal say hold reset for two seconds; if it recurs, switch off and call the installer.Related: overheat and circulation
- FUGet it checkedThe water leaving the boiler is more than 50°C hotter than the water coming back, which means very little of it is moving. Check the valves are open before anyone condemns the pump.Related: overheat and circulation
- F12Get it checkedThe pump rotor is jammed. Ideal's remedy is to clear the blockage — on a modern Logic that is an engineer freeing or replacing a sealed pump head, not a screwdriver in a bleed screw.Related: overheat and circulation
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions about Ideal H 4
Is H 4 the same as an overheat?
It is the sensor for one. On an Isar the dry-fire thermistor gives H 4 when its reading is out of range and H 9 or L 9 when it is genuinely too hot. Either way the question Ideal ask first is whether there was water in the boiler.
Can I switch it off and on to clear it?
Ideal's chart does ask whether the fault can be reset by switching off the mains, and a code that clears and does not return may have been a one-off. But H 4 is a heat exchanger warning, and a code that comes back is not one to keep clearing.
Why does the chart end at the heat exchanger?
Because the sensor exists to protect it. If the thermistor is sound and still reports too hot, the exchanger has either run dry or cannot get its combustion products away, and Ideal want it inspected, cleaned or replaced.
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.