BoilerFaultCodesTechnical description
ALTERNATING 'H' '4' GO TO FRAME 93
What causes this 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.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.