Ideal FU fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Ideal FU
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 FU means
Ideal write this one as FU on the Logic and F 0U on the Combi2, and their table reads "Flow/Return Differential greater than 50°C". Water that leaves at 75°C and returns at 20°C has either gone round very slowly or barely gone round at all, and the board treats a gap that wide as proof that the heat exchanger is not being cooled.
Ideal's fault-finding chart for it is a checklist rather than a flowchart, and it is mostly about the system, not the boiler: are the isolation valves under the appliance open, are the radiator valves open, is the pump wired and running, is there a blockage somewhere in the circuit. It is the code you get when a boiler is switched back on after a summer in which somebody closed a valve, or when a system filter has finally filled with the sludge it was fitted to catch.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Isolation or radiator valves closed | Ideal put this first. The two valves under the boiler, a closed zone valve or a house full of thermostatic valves all turned down leave the pump pushing against a wall. | Common |
| Pump slow, seized or not receiving power | Ideal's next step is pump wiring, then the pump. A rotor that has stuck over the summer moves enough water to avoid a no-flow code and not enough to close the gap. | Common |
| Blockage in the circuit | A choked system filter, a blocked heat exchanger or a plate heat exchanger scaled shut. Ideal's last instruction is simply "check system blockages", which in practice means a flush. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£600
Time on site
30 minutes
Nothing if it was a closed valve. A pump £180–£300 fitted. A system flush and filter clean £300–£600 if the circuit is sludged.
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
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
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
- 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
- F13Get it checkedThe board has lost proper communication with or control of the pump. Ideal group F13, F15 and F17 as one Pump Fault: check the pump wiring, and if that is sound replace the pump.Related: overheat and circulation
Own an Ideal boiler? Save it and its codes come first next time.
Related guides
Common questions about Ideal FU
Is FU the same as L1?
They are neighbours. L1 is a flow temperature overheat, the point at which slow circulation has already made the water too hot. FU is the board noticing the symptom earlier, from the gap between flow and return. Left alone, a boiler showing FU often shows L1 next.
Which boilers print it as F0U?
The Logic Combi2, System2 and Heat2 from 2023, and the i-mini2, whose displays have three characters. The 2011 Logic, the ESP1 and the i-mini show FU. Ideal's FAQ writes it as "Flashing F AND U". They are one code.
Could a system filter cause this?
Yes, if it is full. A magnetic filter that has not been emptied at a service becomes the blockage it was meant to prevent, and it sits on the return, exactly where a restriction opens the differential.
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
- Manufacturer support portal
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.