BoilerFaultCodesIdeal H4 Fault Code
The Ideal H4 fault code means: your boiler has detected low water pressure in the system, which means it cannot circulate enough water to heat your home or provide hot water safely.. It appears on 2 Ideal models in our database.
Technical description: ALTERNATING 'H' '4' GO TO FRAME 93
What causes the Ideal H4 fault?
This fault occurs when the water pressure inside your heating system drops below a safe operating level, usually because water has escaped through a small leak or air was bled from the radiators. Without enough pressure, the boiler cannot move water around your pipes to heat your home, so it shuts down to prevent internal damage. The most common reasons are a leaking radiator valve, a failing expansion vessel, or a dripping pressure relief valve.
This issue is more common in autumn when homeowners turn their heating back on for the first time, as the system settles and reveals any pressure lost during the summer months.
DIY fix possible
Locate the filling loop (usually a silver braided hose with small black valves underneath or near the boiler).Slowly open the valve(s) until you hear water flowing and the pressure gauge rises to 1.5 bar (the green section).Close the valves tightly and press the 'Restart' or 'Reset' button on the control panel.
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Ideal models that show the H4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal H4 fault code information on this page is sourced and cross-checked against the official Ideal service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026