Ferroli A15 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A15 fault code mean?
The A15 code is a manual-reset lockout that appears when your Ferroli boiler has triggered the F05 blocking fault five times within a single 24-hour period. The F05 fault itself relates to the air pressure switch, a safety device that confirms the fan is generating sufficient draught to safely expel combustion gases through the flue. When that switch fails to register the correct pressure reading repeatedly throughout the day, the boiler escalates the situation to a full A15 lockout rather than continuing to attempt automatic recovery. Because this code represents a pattern of failures rather than a single one-off event, it almost always points to an underlying mechanical or component problem that needs a Gas Safe engineer to diagnose properly.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Blocked or obstructed flue terminal Common
Debris, bird nesting material, or a build-up of ice and snow at the external flue outlet can restrict the airflow needed for the air pressure switch to close correctly. Because the boiler has already faulted five times, an intermittent blockage that clears and returns throughout the day is a strong candidate.
- Failing or worn fan Common
The fan must spin at the correct speed to generate the draught that the pressure switch monitors. A fan with worn bearings, a failing motor, or a weak tachometer signal may produce inconsistent airflow, causing the pressure switch to drop out repeatedly and trigger F05 each time.
- Faulty air pressure switch Common
The switch itself can develop a fault — a ruptured diaphragm, corroded contacts, or a failed seal — that causes it to misread pressure even when the fan and flue are functioning correctly. Given that F05 has occurred five times, a failing switch is a very plausible cause.
- Blocked or partially blocked venturi Sometimes
The venturi is a small tube that channels air pressure to the switch. Condensation, debris, or limescale can partially block it, causing the switch to receive inaccurate pressure readings. This is often overlooked but is a relatively quick fix for an engineer once identified.
- Frozen or blocked condensate pipe Sometimes
In cold weather, the condensate drain pipe can freeze, causing condensate to back up into the flue system or sump. This can restrict airflow enough to affect the pressure switch reading, particularly in boilers where the condensate trap and flue share a common area.
- PCB or wiring fault Rare
A fault on the printed circuit board or damaged wiring between the PCB and the air pressure switch can cause the boiler to misinterpret the switch signal, generating false F05 faults. This is less common but should be investigated if the switch, fan, and flue all test as serviceable.
How to fix it
- Check your external flue terminal from ground level DIY safe
Look for any obvious blockages at the point where the flue exits the building — debris, bird nesting material, heavy snow, or ice. Do not climb on ladders or access the roof. If you can see a clear obstruction and safely remove it without working at height, you may do so, but do not touch the flue pipe itself.
- Check whether the condensate pipe is frozen (cold weather only) DIY safe
If outdoor temperatures have dropped below zero, locate the white plastic condensate pipe that runs outside your property. If it feels solid or has visible ice around it, you can thaw it carefully using warm (not boiling) water poured over the pipe or a warm damp cloth wrapped around it. Do not use a heat gun or open flame.
- Attempt a single manual reset DIY safe
Once you have checked the above, press and hold the reset button on your boiler — typically marked with an 'R' or a flame symbol — for around five to ten seconds. Allow the boiler a full two minutes to attempt a restart before concluding it has failed. Do not reset more than two or three times in total; repeated forced resets on a recurring fault can stress the heat exchanger and fan.
- Do not continue resetting if the fault returns DIY safe
Because A15 means the boiler has already faulted five times today, if the code reappears shortly after your reset it confirms a persistent underlying problem. Stop resetting and arrange for a Gas Safe engineer. Forcing repeated restart attempts risks causing additional component damage.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
A qualified engineer will inspect the flue system end to end, test the air pressure switch with a manometer, check fan speed and tachometer output, examine the venturi and condensate trap for blockages, and test PCB outputs. They will carry out any necessary repairs — whether that is replacing the air pressure switch, servicing or replacing the fan, clearing a blocked venturi, or investigating the PCB — and verify safe combustion before handing the boiler back to you.
Parts you may need
- Air pressure switch · from £35
- Flue fan / fan motor assembly · from £95
- Venturi assembly · from £25
- Condensate trap · from £20
- PCB (printed circuit board) · from £130
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£320, depending on the underlying cause.
Frequently asked questions
What is the difference between Ferroli F05 and A15?
F05 is a blocking fault that the boiler can attempt to recover from automatically — it means the air pressure switch has not confirmed correct flue pressure on a single occasion. A15 is a lockout that requires a manual reset, and it only triggers after F05 has occurred five times in the same 24-hour window. In practice, if you are seeing A15 you have already been through five F05 events, which means the boiler has been struggling with this issue all day and the fault is unlikely to be a random one-off.
Can I fix the Ferroli A15 fault myself?
There are a small number of checks a homeowner can safely carry out: looking for obvious flue blockages from ground level, thawing a frozen condensate pipe in cold weather, and performing one or two manual resets. However, because A15 is an escalated recurring lockout, the underlying cause almost always requires a Gas Safe registered engineer to diagnose. Working on the fan, air pressure switch, flue system internals, or PCB is not safe or legal for an unqualified person.
How much does it cost to repair a Ferroli A15 fault in the UK?
Most people with this fault pay somewhere between £120 and £320 including parts and labour. A straightforward air pressure switch replacement typically falls around £140–£200 all in, while a fan replacement tends to cost £180–£250. An engineer call-out to diagnose and reset without needing a new part usually runs £80–£150. If the fault turns out to involve the PCB, costs can rise to £250–£350 or more, but that is a less common outcome — worth bearing in mind for older Ferroli models where parts availability can extend lead times and add to the bill.
Why does the Ferroli A15 fault keep coming back after I reset it?
Each reset clears the lockout temporarily, but if the root cause — a failing fan, a faulty air pressure switch, a partially blocked venturi, or a restricted flue — has not been fixed, the boiler will fault again as soon as it tries to run through its pre-ignition checks. Repeatedly resetting without addressing the cause puts additional strain on components and can lead to secondary damage. If the fault returns within a short time of resetting, treat this as a sign that a Gas Safe engineer is needed rather than attempting further resets.