Ferroli A09 Fault Code
The A09 fault means the flue gas thermostat or high limit thermostat has tripped, indicating a potential overtemperature condition or sensor failure. In South London hard water areas, A09 is frequently caused by limescale accumulation on older Ferroli Modena and DOMIproject units.
What is the Ferroli A09 fault code?
The Ferroli A09 fault is triggered when the flue gas thermostat or the overheat high limit thermostat detects a temperature exceeding the safe operating threshold, or when one of these sensors fails. The boiler locks out as a safety measure to prevent flue overtemperature or system overheating. On Ferroli boilers, A09 can arise from a genuine overtemperature condition — most commonly limescale on the heat exchanger restricting water flow — or from a failed sensor reporting falsely. In South London hard water areas, heat exchanger limescale is the most common root cause on units over 7 years without servicing.
What causes the A09 fault?
The most common cause in South London. Thames Water's calcium-heavy supply deposits scale on the Ferroli heat exchanger, creating an insulating layer that causes flow temperatures to spike and the high limit thermostat to trip.
If the pump is not moving water at the correct rate, heat builds up inside the boiler rapidly and the overheat thermostat trips.
Heavy sludge restricts water flow — the boiler generates heat faster than it can distribute it, leading to A09.
The flue gas thermostat can fail in the tripped position, displaying A09 even when flue temperatures are normal. This is a sensor fault rather than an overtemperature condition.
The high limit thermostat on the heat exchanger can similarly fail, generating false A09 lockouts. Diagnosis requires temperature measurement to distinguish a genuine trip from sensor failure.
On Ferroli combi models, a stuck diverter valve can trap water in the primary circuit and cause localised overheating.
What you can safely check yourself
Wait 15–20 minutes before resetting. Resetting a still-hot boiler will trigger the thermostat again within seconds and repeat the A09 lockout.
Repressurise to 1.2–1.5 bar via the filling loop if below 1 bar before attempting a reset. Low pressure reduces water circulation and can contribute to overheating.
If A09 returns within the same heating cycle, do not reset again. Call a Gas Safe engineer — repeated resets on an overheating boiler can cause damage.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Allow boiler to fully cool before opening — safety first
- Test flue thermostat and high limit thermostat resistance at ambient temperature
- Power flush to remove limescale and sludge from the heat exchanger
- Descale heat exchanger if scaling is concentrated and severe
- Test pump output pressure and current draw — replace if underperforming
- Check and free diverter valve on combi models
- Fit a magnetic filter to prevent sludge recurrence
- Assess repair vs replace on units over 8 years — heat exchanger replacement on older Ferroli boilers can make replacement the better financial decision
Ferroli boilers in South London hard water areas develop limescale faster than higher-spec units. Modena and DOMIproject models in Wandsworth, Lewisham, and Lambeth frequently see A09 faults on units over 7 years without servicing. Thames Water supplies some of the highest calcium concentrations in England — without annual servicing and a magnetic filter, limescale-related overheating on Ferroli boilers is predictable. Reion sees A09 as one of the two most common Ferroli faults in South London (alongside A01).
How much does a A09 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Ferroli boiler showing A09
Switch off and allow the boiler to cool for 15–20 minutes.
Check system pressure and repressurise to 1.2–1.5 bar if needed.
Press the reset button for 3 seconds.
If A09 returns on the first heating cycle, call a Gas Safe engineer.
Do not attempt repeated resets on an overheating boiler.
If the A09 diagnosis is severe heat exchanger scaling or a cracked heat exchanger on a Ferroli unit over 8 years old, repair cost can approach the installed price of a budget replacement. Ferroli heat exchanger parts can also carry a 3–7 day lead time. Reion gives an honest repair-vs-replace comparison at the diagnostic visit, including a quote for an equivalent new boiler if replacement is the better option.
Common questions about the Ferroli A09 fault
What does a Ferroli A09 fault code mean?
Is a Ferroli A09 fault dangerous?
Can I fix a Ferroli A09 fault myself?
How much does a Ferroli A09 repair cost in London?
Should I replace my Ferroli boiler if it keeps showing A09?
Other Ferroli fault codes
Need a Ferroli engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
