FUIdeal
Ideal FU: flow and return temperature difference of more than 50°
Checked against Ideal's own documentation: Ideal Heating: what does my boiler fault code mean? (page 2). Last reviewed October 2026. How we check
Check these first, then call an engineer
Ideal's general fault code guide describes FU as: flow and return temperature difference of more than 50°. It means the water coming back to the boiler is more than 50° cooler than the water leaving it.
What you can safely do
- Bleed your radiators.
- Check the pressure gauge reads between 1 and 1.5 bar, and top up if it is low.
- Restart the boiler.
Call a Gas Safe engineer if
- FU comes back. Ideal says there may be a blockage and an engineer visit is needed.
Find a Gas Safe engineer near you
Opens the official Gas Safe Register search.
Applies to Ideal boilers (Ideal's general fault code guide). Codes can differ between models, so check your manual.