Ariston 109 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 109 fault code mean?
Fault code 109 on Ariston boilers indicates that the system water pressure has climbed above 3 bar, triggering a safety lockout to protect the boiler and heating circuit. At this level, pressure is genuinely dangerous — the boiler will not operate until the underlying cause is identified and resolved. Do not attempt to reset the boiler or continue using it while this code is showing. Affected models include the Clas Net One, Clas One Combi, and Clas System One ranges.
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
- Failed or under-charged expansion vessel Common
The expansion vessel uses an internal air-filled diaphragm to absorb the extra volume of water that forms as the system heats up. If the diaphragm ruptures or the vessel gradually loses its air charge over time, it can no longer cushion that expansion, so pressure climbs sharply — often pushing past the 3 bar threshold that triggers code 109. This is the single most frequent root cause of this fault.
- Over-filled heating system Common
If too much water has been added to the heating circuit — perhaps when topping up after a pressure drop — the system starts cold already above 1.5 bar. Once the boiler fires and the water expands with heat, pressure rises further still, easily exceeding the 3 bar limit. Check where the needle sits on the pressure gauge while the system is cold; above 1.5 bar at rest almost always means the system is over-filled.
- Pressure relief valve weeping or failing open Sometimes
The pressure relief valve (PRV) is designed to discharge water outside if pressure gets dangerously high. If the PRV has been weeping or has previously opened at high pressure, its seat can become damaged, causing it to behave erratically. A faulty PRV may also contribute to pressure readings that trigger code 109.
- Failed secondary heat exchanger (combi models) Rare
On double-circuit combi boilers, a crack or failure in the secondary plate heat exchanger can allow domestic hot water and central heating water to mix. This cross-contamination introduces additional water volume into the sealed heating circuit, gradually pushing pressure up until code 109 appears.
How to fix it
- Check whether the pressure relief valve is discharging water DIY safe
Look for the PRV discharge pipe, which typically exits through an external wall or terminates near a drain outside. If water is actively dripping or flowing from it, this confirms the system pressure is high enough for the PRV to be venting. Note this for the engineer — it is a useful diagnostic clue.
- Check the system pressure gauge DIY safe
Locate the pressure gauge on the boiler fascia or pipework. If the needle is above 3 bar even when the system is cold, the system is over-pressurised. Do not attempt to reset the boiler at this stage.
- Bleed a radiator to cautiously reduce pressure DIY safe
If you are confident the system is over-filled (gauge well above 2.5–3 bar cold), you can use a radiator bleed key to slowly open a bleed valve on a radiator. Allow a small amount of water to escape until the pressure drops towards 1–1.5 bar, then close the valve. Only do this if you are comfortable doing so — stop immediately if you are unsure. Do not bleed multiple radiators at once. This is a temporary measure and does not address the root cause.
- Do not reset the boiler repeatedly DIY safe
While a single reset attempt once pressure has been brought down may be tempting, repeatedly resetting a boiler showing code 109 without fixing the underlying cause risks further damage. Avoid resetting more than once, and only do so if the gauge reads in a normal range (1–1.5 bar).
- Call a Gas Safe registered engineer Gas Safe engineer
An engineer will test the expansion vessel to determine whether it has lost its air charge or the diaphragm has failed. If the air cushion has simply depleted, they can recharge it — a relatively inexpensive fix. If the diaphragm is ruptured, they will replace the vessel. They will also inspect the PRV for damage and, on combi models, check the secondary heat exchanger for signs of cross-contamination.
Parts you may need
- Expansion vessel (replacement) · from £85
- Expansion vessel Schrader valve / recharge kit · from £12
- Pressure relief valve (PRV) · from £35
- Secondary plate heat exchanger (combi models) · from £120
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 £90–£350, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my Ariston boiler while fault code 109 is showing?
No. Code 109 is a safety lockout triggered because system pressure has exceeded 3 bar — a level that can damage boiler components or cause the pressure relief valve to discharge. Leave the boiler switched off and call a Gas Safe registered engineer promptly. Do not reset the boiler and continue running it without addressing the cause.
Can the expansion vessel be recharged rather than replaced?
Yes, in many cases. If the vessel's internal diaphragm is still intact but the air cushion has gradually leaked away, a Gas Safe engineer can recharge it using a pump via the Schrader valve — much like topping up a tyre. This is significantly cheaper than a full replacement. If the diaphragm has ruptured, however, the vessel must be replaced. An engineer can usually determine which applies within minutes of testing.
How much does it cost to fix Ariston fault code 109 in the UK?
Most repairs fall in the £90–£350 range. Recharging the expansion vessel is at the lower end, often covered as part of a service visit. Replacing the expansion vessel typically costs £150–£300 including parts and labour. A pressure relief valve replacement averages around £100. On combi models where the secondary heat exchanger is at fault, costs rise considerably — often £300–£600 or more — and in that scenario it is worth comparing repair cost against a new boiler, especially if the appliance is over ten years old.
Why does my Ariston keep losing pressure and then going over pressure?
This pattern — dropping pressure between uses and then spiking high when the boiler fires — is a classic sign of a failing expansion vessel. As the diaphragm deteriorates, it can no longer buffer pressure changes effectively. Low pressure when cold and high pressure when hot almost always points to the expansion vessel needing attention. Have a Gas Safe engineer inspect and test it.