Ariston Fault Code 108: Low System Pressure
Fault code 108 on an Ariston boiler indicates that the water pressure inside the central heating circuit has fallen below the minimum operating threshold. The boiler will stop functioning and lock out until the pressure is restored to a safe working level, typically between 1.0 and 1.5 bar. This is one of the most frequently seen Ariston fault codes and, in many cases, can be resolved by the homeowner without calling an engineer.
General guidance only — not a substitute for professional advice. 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.
What does the Ariston 108 fault code mean?
Fault code 108 on an Ariston boiler indicates that the water pressure inside the central heating circuit has fallen below the minimum operating threshold. The boiler will stop functioning and lock out until the pressure is restored to a safe working level, typically between 1.0 and 1.5 bar. This is one of the most frequently seen Ariston fault codes and, in many cases, can be resolved by the homeowner without calling an engineer.
Common causes
- Gradual pressure loss over time Common
Central heating systems naturally lose small amounts of water through micro-leaks, evaporation through components, and routine radiator bleeding. Over weeks or months this causes the pressure gauge to creep below 1 bar and trigger the 108 code. This is by far the most common reason for seeing this fault.
- Leak in the heating circuit Common
A more significant water leak — whether from a radiator valve, a joint in the pipework, or a component inside the boiler itself — can cause a rapid or repeated pressure drop. If you find yourself needing to top up pressure more than once a month, a leak is the most likely culprit.
- Faulty or weeping pressure relief valve (PRV) Sometimes
The PRV is a safety device that releases water when pressure gets too high. If it develops a fault, it can weep or discharge water continuously, causing pressure to fall steadily. Check the overflow pipe outside your home — if water is dripping from it when the boiler is cold, a faulty PRV is a strong suspect.
- Failing expansion vessel Sometimes
The expansion vessel absorbs the increase in water volume as the system heats up. If its internal membrane deteriorates, it can no longer do this job properly, leading to repeated pressure fluctuations and eventual low-pressure lockouts even after re-pressurising.
- Air in the system or seized circulation pump Sometimes
Significant air pockets trapped in radiators or around the pump can interfere with correct pressure readings and circulation. A pump that has seized or is running poorly can also contribute to circulation-related pressure issues triggering the 108 code.
- Filling loop left closed or isolation valves shut Rare
If the filling loop valves are in the wrong position, or a system isolation valve has been inadvertently closed — for example after recent servicing — water cannot maintain the correct pressure in the circuit.
- Faulty pressure sensor Rare
In less common cases, the pressure sensor or transducer itself is giving an incorrect low-pressure reading even though the actual system pressure is normal. This tends to be a diagnosis of exclusion once all other causes have been ruled out.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Locate the pressure gauge on the front of the boiler or on the pipework below it. If the needle is below 1 bar, low pressure is confirmed as the immediate issue. If the gauge reads 1 bar or above, the fault may be sensor-related and you should move straight to a reset and then contact an engineer if it persists.
- Top up the system pressure using the filling loop DIY safe
Find the filling loop — usually a braided silver hose with one or two valves located beneath the boiler. Slowly open the valve(s) to allow mains water into the system. Watch the gauge carefully and stop once it reads approximately 1.2–1.5 bar. Do not overfill; going above 2 bar can trigger a different fault. Once done, close the filling loop valve(s) firmly to prevent further flow.
- Bleed radiators if you have not done so recently DIY safe
If you bled radiators shortly before the fault appeared, trapped air may have caused the pressure to drop. Use a radiator key to open each bleed valve in turn, starting from the ground floor and working upwards. Hold a cloth beneath the valve — when water rather than air begins to escape, close it. After bleeding, re-check the boiler pressure and top up again if it has dipped below 1.2 bar.
- Reset the boiler DIY safe
Once the pressure has been restored to the correct level, press and hold the reset button (typically marked with an R or a flame symbol) for around three seconds until the boiler attempts to restart. Allow the boiler to complete its ignition sequence. If it fires up and runs normally, monitor the pressure over the next few days.
- Monitor for recurring pressure loss DIY safe
Check the pressure gauge again after 24–48 hours and again after one week. If the pressure holds steady, the issue was likely just gradual natural loss. If pressure drops again within a few days, there is an active leak or a component fault — do not keep topping up indefinitely as this can affect water quality in the system and may mask a worsening problem.
- Inspect visible pipework and radiators for signs of leaks DIY safe
Walk around your home and look carefully at radiator valves, pipe joints, and the floor beneath the boiler for damp patches, limescale deposits, or corrosion staining. These are tell-tale signs of a slow leak. Do not attempt to tighten joints on live gas pipework — water-side pipework joints can sometimes be tightened with a spanner by a competent person, but if you are unsure, leave it to an engineer.
- Call a Gas Safe registered engineer Gas Safe engineer
If the pressure continues to drop, if you can see an active leak, if the external overflow pipe is dripping when the system is cold, or if the boiler returns the 108 code after a correct re-pressurise and reset, you need a professional diagnosis. An engineer can pressure-test the system to find hidden leaks, inspect the PRV and expansion vessel, check the circulation pump, and replace any faulty components safely. Do not continue to top up the system as a workaround — the underlying fault needs to be fixed.
Parts you may need
- Expansion vessel (replacement) · from £60
- Pressure relief valve (PRV) · from £25
- Pressure sensor / transducer · from £35
- Filling loop assembly · from £20
- Circulation pump · from £90
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–£400, depending on the underlying cause.
Frequently asked questions
Can I just keep topping up the pressure every time the Ariston 108 code appears?
Topping up the pressure once or twice a year is completely normal — systems do lose small amounts of water over time. However, if you are needing to re-pressurise more frequently than that, such as every few weeks or repeatedly within a short period, this is a sign of an underlying fault such as a leak, a weeping pressure relief valve, or a failing expansion vessel. Repeatedly adding water to the system dilutes the corrosion inhibitor and can accelerate internal corrosion, so the root cause should be investigated by a Gas Safe engineer rather than masked with top-ups.
How do I top up the pressure on an Ariston boiler?
Most Ariston combi boilers use a filling loop — a short braided hose with one or two valves, usually found underneath the boiler. Make sure the boiler is switched off and cool, then slowly open the valve(s) while watching the pressure gauge. Stop when it reaches around 1.2 to 1.5 bar, then close the valve(s) fully. Reset the boiler and check that the 108 code has cleared. If you cannot locate the filling loop or the pressure will not increase, there may be a blockage in the loop or an isolation valve closed elsewhere — an engineer can help.
Could a faulty expansion vessel cause the Ariston 108 fault code?
Yes. A deteriorating expansion vessel is a fairly common cause of repeated low-pressure faults on older boilers. As its internal membrane fails, the vessel can no longer absorb the expansion of water when the system heats up. This sometimes causes the PRV to discharge water to relieve excess pressure, which then leaves the system under-pressurised once it cools. The result is a cycle of the 108 code returning shortly after re-pressurising. An engineer can check the vessel's pre-charge pressure and replace it if needed — typically costing £180 to £350 including parts and labour.
My Ariston boiler keeps losing pressure but I cannot see any leaks — what should I do?
Not all leaks are visible. Leaks inside the boiler casing, behind walls, or under floors may leave no obvious trace. An engineer can carry out a system pressure test to identify where water is escaping. Other possibilities include a weeping PRV that discharges outside the property through the overflow pipe — worth checking on a cold day when the system has been off overnight. If the expansion vessel has failed, this can also mimic the symptoms of a leak. A full diagnostic visit will typically cost £80 to £150, which is often money well spent rather than repeatedly topping up a system with an undiagnosed fault.