Ariston 111 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 111 fault code mean?
Fault code 111 signals that your Ariston boiler has detected water pressure in the heating system has dropped below the minimum safe operating level. What makes this code distinct from the similar code 108 is that it applies specifically to Ariston models equipped with an automatic filling (auto-replenishment) system. In theory, these boilers can top themselves up without any manual intervention — so when code 111 appears, it means the auto-fill mechanism has either attempted to restore pressure and failed, or cannot raise pressure to the required level at all. Most Ariston boilers need a cold system pressure of between 1.0 and 1.5 bar; a reading below 1.0 bar will typically trigger this fault. The underlying problem may be a leak somewhere in the system, a fault with the auto-fill device itself, or pressure lost gradually over time.
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
- Gradual pressure loss over time Common
All sealed heating systems lose small amounts of pressure naturally over months of use — through micro-seepage at joints, valve glands, and radiator bleed points. On auto-fill models this should be corrected automatically, so if code 111 has appeared the auto-fill has not kept pace, or the pressure has dropped faster than usual.
- System leak at a pipe, radiator, or fitting Common
A more significant drop in pressure — especially one that recurs quickly after topping up — often points to a leak somewhere in the pipework, at a radiator valve, or at a compression fitting. Even a slow drip can deplete system pressure over a few days and overwhelm the auto-fill system.
- Faulty or blocked automatic filling unit Common
Because this fault is unique to auto-fill models, the automatic filling valve or unit itself is a key suspect. It may be stuck closed, blocked with debris, or have failed electronically, meaning it simply cannot do its job of restoring pressure when levels drop.
- Pressure relief valve (PRV) weeping or open Sometimes
The PRV is a safety device designed to release water if pressure gets too high. If it has started to weep or has not reseated properly after a previous overpressure event, water slowly escapes through the discharge pipe outside, causing the system pressure to fall.
- Radiators bled recently without re-pressurising Sometimes
Bleeding radiators releases trapped air but also lowers system water pressure. If someone bled one or more radiators and the auto-fill system has not compensated fully, the pressure can fall far enough to trigger this fault.
- PCB or wiring fault affecting auto-fill control Rare
In rarer cases the boiler's printed circuit board may be misreading the pressure sensor signal, or a wiring issue may be preventing the auto-fill command from reaching the filling valve — causing the boiler to report a pressure fault even if the underlying pressure is acceptable.
How to fix it
- Check the pressure gauge reading DIY safe
Locate the pressure gauge on your boiler — usually a dial on the front or underside. With the system cold, the needle should sit between 1.0 and 1.5 bar, ideally near 1.2–1.5 bar. If it is below 1.0 bar, this confirms low system pressure is the trigger for code 111.
- Top up system pressure using the filling loop DIY safe
Find the filling loop — typically a short silver braided hose with one or two valves, located beneath the boiler. With the system cool and the boiler off, slowly open the valve(s) one at a time and watch the gauge rise. Bring the pressure up to around 1.2–1.5 bar, then close both valves firmly. Do not overfill past 2.0 bar. If your model has the auto-fill function and topping up manually is not straightforward, check the boiler manual for the correct procedure for your specific unit.
- Reset the boiler once DIY safe
Press and hold the reset button for approximately 5 seconds until the boiler acknowledges the reset. Allow the boiler to complete its ignition sequence — this usually takes up to a minute. If code 111 clears and the boiler fires normally, monitor the pressure gauge over the next few days to make sure pressure holds steady.
- Check for visible leaks around the boiler and radiators DIY safe
While the boiler is running, have a look around accessible pipework, radiator valves, and the boiler casing for any signs of water — damp patches, water stains, or corrosion marks. Pay attention to the small plastic discharge pipe that exits through an external wall (the PRV outlet): a steady drip from this pipe suggests the pressure relief valve is weeping and needs engineer attention.
- Do not reset more than two to three times DIY safe
If code 111 returns shortly after resetting, or if pressure drops again within a day or two of topping up, stop resetting. Repeated resets without resolving the cause can mask a worsening problem and potentially cause damage to the pump or heat exchanger from running at low pressure.
- Call a Gas Safe registered engineer to investigate further Gas Safe engineer
If pressure will not hold, if you cannot locate an obvious leak, if the automatic filling unit appears to be not working, or if the fault code keeps returning, a Gas Safe registered engineer should carry out a full system inspection. They can pressure-test the system to find hidden leaks, inspect and replace the automatic filling valve if it has failed, check the pressure relief valve, and test the PCB's control of the auto-fill circuit. Do not attempt to work on any internal gas or electrical components yourself.
Parts you may need
- Automatic filling unit / auto-fill valve · from £55
- Pressure relief valve (PRV) · from £25
- System pressure sensor / transducer · from £30
- Filling loop assembly (manual backup) · from £20
- Expansion vessel · from £45
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 £100–£320, depending on the underlying cause.
Frequently asked questions
What is the difference between Ariston fault code 108 and fault code 111?
Both codes indicate low system pressure, but they apply to different types of Ariston boiler. Code 108 appears on models where the filling loop is operated manually — you have to top the pressure up yourself. Code 111 is reserved for models fitted with an automatic water-replenishment system that is supposed to restore pressure on its own. When you see 111, it means that automatic system has been unable to bring pressure back up to the required level, so the fault could lie with the auto-fill unit itself, rather than simply needing a manual top-up.
Can I just keep re-pressurising the boiler myself each time code 111 appears?
Topping up once or twice is fine and may resolve the issue if it was caused by gradual pressure loss or recent radiator bleeding. However, if you find yourself re-pressurising every few days, that is a strong sign of a water leak somewhere in the system. Continuing to add water without fixing the leak dilutes your inhibitor, promotes corrosion inside the pipework, and can eventually damage the pump or heat exchanger. At that point you should call a Gas Safe registered engineer to locate and repair the leak.
My Ariston boiler has an auto-fill system — why has it not fixed the pressure by itself?
The automatic filling device can fail for several reasons: it may be blocked with limescale or debris, stuck in the closed position, or have developed an electrical fault. It is also possible that pressure is dropping faster than the auto-fill unit can compensate — for example, if there is a significant leak or the PRV is weeping. When the auto-fill system cannot keep up, the boiler locks out and displays code 111 to alert you that intervention is needed. A Gas Safe engineer can inspect and replace the auto-fill valve if required; parts and labour for this typically cost in the region of £150–£300.
Could a more expensive component like the PCB be causing fault code 111?
It is possible but uncommon. In rare cases the PCB can misread the signal from the pressure sensor, or a wiring fault can prevent the auto-fill command from reaching the filling valve, causing a false low-pressure fault. If an engineer has confirmed system pressure is actually correct and the auto-fill unit is functioning properly, they may then investigate the PCB and wiring. PCB replacement on Ariston boilers typically costs in the region of £400–£600 including labour — considerably more than the common repairs — which is why engineers work through all other causes first.