Error 108 in Ariston boiler: causes, low pressure, and solution

The 108 warning on an Ariston boiler almost always points to the same underlying fault: insufficient pressure in the heating circuit. It is a code designed to stop operation before the machine runs dry, protect sensitive components, and end up leaving both heating and domestic hot water out of service.

In practice, the scene usually repeats itself with almost mechanical precision: the pressure gauge drops below 1 bar, the screen shows the error, and the boiler locks out. In most domestic models, the normal operating range is roughly between 1 and 1.5 bar when cold, although it can vary slightly depending on the installation and the height of the home. When that level drops, the unit interprets it as a lack of water or a leak in the circuit.

If you have a problem with your boiler, you can use our free error code finder. From there you can find out and fix all errors easily and effectively.

What warning 108 on an Ariston really means

Error code 108 does not describe a complex electrical fault or a combustion failure. Its message is much more direct: the water pressure is below the threshold the system needs in order to circulate normally. In a combi boiler or heating-only boiler, the water in the circuit acts as the thermal carrier; if flow is lacking, heat exchange becomes unstable and the unit protects itself.

That lockout should not be read as an isolated bad sign, but as a symptom. Sometimes a water top-up is enough; other times, the drop in pressure reveals a recurring problem, such as a small leak, a depressurized expansion vessel, or a filling valve that no longer closes properly. That is why the same warning can be resolved in minutes or become a more in-depth inspection.

It is also worth distinguishing between a one-off reading and a repeated drop. If the pressure gauge needle falls from time to time after bleeding radiators or with temperature changes, the cause may be normal. But if the circuit loses pressure every few days, the system is asking for a technical inspection because something is letting water or air escape.

How to read the pressure gauge before touching anything

The first clue is not in a manual or on the digital panel, but in the pressure gauge, the small scale that measures circuit pressure. When cold, many domestic boilers operate with a reading close to 1.2 bar. Below 1 bar, the system enters a delicate zone; around 0.5 bar, a lockout is already common. That margin is not arbitrary: without enough pressure, the pump moves water with difficulty and the heat exchanger works out of range.

Looking at that indicator before acting avoids unnecessary mistakes. In some homes, the user tries resetting the boiler several times, as if the warning were just a random electronic quirk, when in reality the problem is right there in plain sight. The display warns, but the pressure gauge confirms. If both match, there is no need to keep trying to restart it; the pressure needs to be restored.

In multi-storey installations or those with long pipe runs, the reading may behave somewhat differently when the system is hot. Water expands and pressure rises by a few tenths, so the useful check is usually done when cold, with the heating off and the system stabilized. That measurement is the most reliable snapshot of the circuit’s real condition.

The most common cause: too little water in the circuit

The most frequent situation behind warning 108 is as simple as too little water in the closed circuit. This may happen after bleeding radiators, after maintenance work, or because of a slow micro-leak that leaves barely any visible trace. In that scenario, the boiler cannot maintain the minimum flow and protects itself with a lockout.

The basic solution is to refill water using the filling valve until the pressure returns to the correct range. That action, although simple, requires calm. Opening the valve too quickly or exceeding the recommended range can raise pressure beyond what is prudent and trigger the safety valve. The domestic heating system is sensitive, almost like a glass balloon: it needs water, but not too much.

On many Ariston units, the sensible reference point is between 1 and 1.5 bar when cold. Once that value is reached, the filling valve should be closed immediately. Leaving it open can create a continuous water inflow and drive pressure to an unnecessary level, with the resulting safety drip or even damage to other parts of the circuit.

How to refill pressure without stressing the unit

The filling valve is usually located at the bottom of the boiler, although the exact position depends on the model. Some units show it clearly; others require a flashlight and a bit of patience. Once located, it is opened gradually until the pressure gauge needle starts to rise. The movement should be gradual, not sudden, to avoid pressure spikes.

As water enters, the reading should be monitored continuously. Ideally, you stop filling when the indicator reaches the safe zone, not after it has gone well beyond it. In compact units, the rise is noticed quickly; in larger installations, it may take a little longer to stabilize. That delay is not a fault, but the natural response of the whole circuit.

After filling, the boiler usually returns to operation without further action. If the error disappears and the heating starts normally, the fault has been resolved at its most basic level. Even so, it is worth observing behavior over the next few hours. A proper repair does not just clear the code; it also keeps pressure stable over time.

When the boiler will not take water or the pressure does not rise

Sometimes the filling valve opens, but the pressure gauge barely moves. That situation usually points to a problem different from simply emptying the circuit. There may be air in the system, a partially blocked valve, or a component that is no longer doing its job. In that case, insisting without a clear approach does nothing.

It can also happen that the filling valve is faulty. Repeated-use mechanical parts wear out, lose tightness, or stop opening properly. When that happens, the pressure does not rise even though the user follows the correct procedure. The key is not to confuse a component failure with a filling mistake; they are different problems and require different solutions.

If water enters with difficulty or intermittently, the condition of the whole installation deserves attention. A filling valve lasts for years, but it is not eternal. A simple replacement can restore balance to the circuit and prevent the error from appearing again as a background alarm that disrupts winter time and time again.

Small leaks that lower pressure without leaving an obvious trace

A repeated pressure drop usually hides a water leak, although it is often so subtle that the user cannot see it at first glance. It may be in a gasket, a radiator, a circuit connection, the heat exchanger, or the expansion vessel. Sometimes a minimal amount of moisture is enough for the system to lose just enough to trigger the warning.

The trap with these leaks is how discreet they are. They do not always form puddles. Sometimes they evaporate when they touch hot surfaces or hide behind panels, skirting boards, or pipes that are not in plain sight. The user notices the symptom before the cause: every so often they have to top up, the pressure drops again, and error 108 reappears like a persistent shadow.

When the loss is continuous, topping up again and again is not a real solution. It is like bailing water out of a boat without plugging the hole. The real fix is to locate the leak point and repair the affected component before the circuit keeps weakening or the water ends up damaging nearby parts.

The role of the expansion vessel in pressure drops

The expansion vessel is one of the great overlooked parts of domestic heating, but its role is crucial. It absorbs the expansion of the water when the boiler heats up and prevents pressure from rising suddenly. If it loses air charge or its diaphragm deteriorates, the circuit starts behaving erratically: pressure rises too much when hot and drops too much when cold.

That irregular behavior can eventually trigger the same warning 108, even if the cause is not simply a lack of filling. The unit needs the pressure to have a stable base; if the vessel does not do its part, the installation seems to breathe with difficulty, as if inhaling and exhaling unevenly. In such cases, the fix is not to add more water, but to check the vessel itself.

An expansion vessel that is depressurized or damaged usually shows up alongside other signs: marked pressure swings, discharge through the safety valve, or frequent need for refilling. When that pattern appears, the problem is no longer a minor detail, but a structural element of the circuit that deserves technical diagnosis.

Why bleeding radiators can trigger the warning

After bleeding radiators, it is normal for the boiler pressure to drop. Each time air is released from the system, the circuit loses a little water with it. That drop may be enough to take the installation below the minimum level and cause code 108 to appear right after an apparently routine maintenance task.

That is why, after bleeding, you need to check the pressure again. It is not enough to close the bleed valve and consider the job finished. If the needle falls below the recommended range, the boiler will ask for water replenishment before operating normally. That detail explains why a system that was fine in the morning may lock out in the afternoon after a well-intentioned domestic bleed.

The relationship between air and pressure is closer than it seems. Trapped air reduces thermal efficiency, makes some radiators heat poorly, and upsets the balance of the circuit. Removing that air improves performance, yes, but it also requires restoring the pressure lost along the way. The two actions go together.

When to stop and call a technician

There are signs that distinguish a simple household issue from a problem that already requires expert hands. If the unit shows 108 again shortly after being refilled, if pressure rises and falls sharply, or if there is visible dripping, the boiler needs a full inspection. It is not wise to keep insisting with repeated refills, because that only hides the symptom.

Professional intervention is also advisable when the user cannot clearly locate the filling valve, when the valve does not respond, or when the circuit loses water without an obvious cause. In those cases, a technician can check tightness, the expansion vessel, the safety valve, the pump, and the overall condition of the heat exchanger. The value of diagnosis is not only in repairing, but in preventing the fault from recurring.

Safety matters more than speed. A boiler works with hot water, pressure, and electrical components in a compact space. Intervening without knowing which part is being handled can create a bigger fault than the original one. That is why, when the warning stops being occasional and becomes a habit, the sensible thing is to move from immediate remedy to a proper inspection.

What signs indicate the problem goes beyond the code

108 does not always appear on its own. Sometimes it comes with cold radiators, air noises in the pipes, pressure drops after the heating is switched on, or small drips under the machine. Each of those symptoms adds context. They are not decorative elements of the fault; they are its physical trace in the installation.

If the house takes longer to warm up or one part of the circuit behaves worse than another, the problem may have been building for some time. Pressure does not always disappear all at once. Often it leaks away little by little until the system can no longer maintain balance and the boiler detects it automatically. The warning appears late, not early, and that is why it should be read as a background alarm, not as the start of the problem.

In older installations, the sum of small wear points matters more than a single broken part. Aged seals, radiators with air, a worn valve, and a just-sufficient expansion vessel can form a chain of symptoms that ends in the same message. A good reading of the fault does not stop at the code: it looks at the whole system.

The importance of maintenance to avoid recurrence

A well-adjusted Ariston boiler should not need refilling frequently. If it does, something is changing in the circuit and maintenance stops being a decorative option and becomes a preventive tool. Checking pressure, bleeding carefully, inspecting gaskets, and monitoring the filling valve’s behavior greatly reduces the chance of seeing the warning again.

Prevention has a simple advantage: it keeps the problem from arriving at the worst possible time. Pressure drops are usually noticed most when the cold weather arrives and the heating is working at full capacity. In those weeks, a minor fault becomes a major domestic inconvenience. The circuit works better when monitored in calm conditions, not when it is already locked out.

Maintenance also helps distinguish between an isolated episode and a structural fault. An occasional top-up can be normal; a periodic drop is not. That seemingly small distinction makes the difference between a simple task and a repair that affects the hydraulic heart of the boiler.

What to do after restoring the pressure

Once the correct level has been restored, it is worth checking that the heating starts without oscillations and that the pressure gauge remains stable. Ideally, you should observe the unit for a reasonable amount of time, because some faults take a while to show themselves. If the boiler stays between 1 and 1.5 bar when cold and the message does not return, the response has been the right one.

After that first relief, it is useful to watch for unusual noises, drops in performance, or new pressure drops in the following days. A heating system speaks in small signals. A whistle, a faint drip, or a reading that shifts little by little can warn of a bigger fault before the lockout returns.

The practical reading of error 108 is simple and at the same time very specific: the boiler is asking for water, balance, and stability. When one of those three elements fails, the unit protects itself. Solving it means restoring its correct range, and if the problem persists, discovering what is emptying the circuit. That difference determines the continuity of home comfort.

What this warning reveals about the real condition of the installation

Error 108 works as a thermometer for the whole system, not just the boiler. It shows whether the circuit is sealed, whether the expansion vessel is functioning, whether the valves close properly, and whether the user has bled or refilled with due care. That is why its meaning goes beyond a simple number on the screen: it is a clue to the health of the entire installation.

When the warning is cleared with a small refill, the fault was minor. When it comes back again and again, the problem has usually been growing silently. The difference between an incident and a fault is repetition. A healthy system may need a one-off correction; a worn one starts asking for it insistently.

Seen this way, 108 is not an enemy, but a useful warning. It points to the edge before the void. And although the immediate solution may be as prosaic as opening a filling valve, the correct reading of the symptom makes it possible to understand whether behind it there is simply too little water or a longer history of lost pressure, small leaks, and accumulated wear.