Ariston 108 error code: refill request for low system pressure
Ariston error 108 on Clas and Alteas boilers is a refill request for low system pressure. The fill-then-reset sequence, and how 108 differs from the pressure-sensor codes.
What this means
Ariston shows 108 when the heating circuit pressure has fallen below the level the boiler needs, and it is asking for a refill. The owner opens the filling taps under the boiler until the gauge reads 1.0 to 1.5 bar cold, closes them, then clears the code with the reset control. A drop again within days points at a leak.
Equipment and model context
Ariston Clas ONE, Clas System ONE, and Alteas ONE boilers showing 108
- Clas ONE
- Clas System ONE
- Alteas ONE
- Clas HE Evo
This applies to Ariston Clas and Alteas family boilers whose user documentation lists 108 as a filling request. Ariston uses separate codes for a faulty pressure transducer, so if the gauge is already in band and 108 stays, treat it as a sensor branch and call an engineer.
What you may notice
- The Ariston display shows 108, and heating or hot water is interrupted or will not start.
- The pressure gauge or bar reading on the boiler is below 1.0 bar with the system cold.
Likely causes
- Water has been lost from the sealed circuit through bleeding radiators, a weep at a fitting, or a discharge from the safety valve outside.
- The internal expansion vessel has failed, so there is no air cushion to keep the circuit pressure up once the heated water cools back down.
Diagnostic paths
108 goes away after the fill-and-reset sequence
The gauge was under 1 bar, opening the filling taps brought it to 1.5 bar, closing them and pressing reset cleared 108, and the pressure has held since.
Record the date and the pressure. Check the gauge every week or two. No further action is needed while the boiler holds pressure in the normal band.
108 comes back within a few days each time
The pressure falls below 1 bar again shortly after each refill, or there is visible dampness near pipes or a drip from the external safety-valve pipe.
Stop refilling and arrange a Gas Safe registered engineer to locate the loss. Keep a note of refill dates and amounts, since a fast loss and a slow one lead to different places.
Ariston holds 108 as a filling request until it is both refilled and acknowledged, so the pressure reading and the reset both have to be satisfied before the boiler resumes.
- Low pressure detectedReading falls below the operating threshold
- 108 displayedBoiler requests a refill and pauses
- Filling taps openedSystem brought back to 1.0 to 1.5 bar cold
- Reset pressedOwner acknowledges the fill so the boiler re-checks
- Normal operationBoiler resumes if the reading is now in band
| Step | What to do | If it does not work |
|---|---|---|
| Fill | Open both filling taps under the boiler slowly to 1.0 to 1.5 bar cold | No rise means the mains supply or the tap itself is blocked |
| Isolate | Turn both filling taps fully off so no mains water keeps feeding in | A tap left cracked open lifts the pressure over time |
| Reset | Press the boiler reset control once to clear 108 | 108 returning at once means the gauge is still low or a sensor fault is present |
The order of the steps matters
On many Ariston boilers 108 is worded as a request to add water rather than a full safety lockout, and clearing it takes two actions in order. First the pressure has to be raised: the filling taps are the two valves or knobs under the boiler, turned so they line up with the pipe, and left open only until the gauge reaches about 1.5 bar with the system cold. Then both taps are turned fully off, and the reset control is pressed once so the boiler re-reads the pressure and starts.
Skipping the reset leaves 108 on the screen even though the gauge is now correct, which sends people back to the taps and risks overfilling. Skipping the tap closure leaves mains water feeding in, so the pressure keeps climbing until the safety valve outside starts to drip. Doing both, in order, is what clears the code cleanly.
When 108 is not a simple refill
Ariston has separate codes for a faulty pressure transducer, the sensor that tells the board what the water pressure is. If 108 appears while the gauge already shows a healthy 1.2 to 1.5 bar, or it returns immediately after a correct fill and reset, the reading the board is acting on may be wrong rather than the water being genuinely low. That is a sensor and wiring check behind the boiler casing, which is engineer territory.
A pressure that falls repeatedly after correct refilling is the other non-simple case. Adding water again and again introduces oxygen that corrodes the system from the inside, and it does nothing about where the water is going. An engineer traces the loss, checks the expansion vessel with the system depressurised, and confirms the safety valve is not passing before recommending a fix.
What to give the technician
Tell the engineer the Ariston model, the gauge reading before and after filling, whether 108 cleared on reset, how many refills the system has needed recently, and whether the outside safety-valve pipe has dripped.
Safe homeowner checks
- With the boiler cold, open both filling taps under the boiler slowly until the gauge reads 1.0 to 1.5 bar, then turn both taps fully off and press reset once.
- Note whether 108 clears after the reset and whether the pressure is still in band the next morning.
Is a reset appropriate?
Press reset once after a correct refill to 1.0 to 1.5 bar. If 108 returns straight away, do not keep resetting; the reading is still low or a sensor fault is present.
When to call a technician
- A pressure that will not hold, 108 showing with the gauge already in band, a suspected transducer fault, or any check inside the boiler casing needs a Gas Safe registered engineer.
Questions people ask about this
Do I need to reset an Ariston boiler after topping it up?
Yes. Ariston holds 108 as a filling request until it is acknowledged, so after the gauge reaches 1.0 to 1.5 bar and the taps are closed, press the reset control once so the boiler re-checks and restarts.
The gauge reads 1.4 bar but 108 will not clear. Why?
The pressure transducer or its wiring may be misreporting, so the board still sees low pressure. Ariston uses separate codes for that, and it is an engineer check behind the casing rather than more refilling.
How often should an Ariston boiler need topping up?
A sealed system that is sound holds pressure for long periods. Needing to refill every few days means water is leaving the system or the expansion vessel has failed, and that needs a Gas Safe engineer.
Evidence recordHow this page was checked
operation manual, official manufacturer support article · checked 2026-09-09
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Ariston technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- operation manual, official manufacturer support article
- Scope of the definition
- Ariston Clas ONE, Clas System ONE, and Alteas ONE boilers showing 108
- Last checked
- 2026-09-09