Ariston 108 fault codewhat it means and how to fix it
The heating circuit does not contain enough water to run safely. Refill through the filling loop to about 1.2 bar and the code clears.
Before you start
How often have you had to do this over the past year?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
This is the first time, or the first in over a year
Completely normal. Fill, purge, and forget about it.
Note the date so you know the interval if it happens again.
Two or three times in the past year
More than a healthy system should need. Either there is a small leak or the expansion vessel is losing charge.
Raise it at the next service and specifically ask for the expansion vessel charge to be checked with the system drained — it is quick and it is the check most often skipped.
Monthly or more often
That is an active leak. Beyond the water damage risk, every refill brings fresh oxygenated water into the system, and that is what produces the black sludge that ruins pumps and blocks heat exchangers.
Book a leak investigation instead of topping up again. Check under the appliance, around radiator valves, and any ceiling below a pipe run in the meantime.
What 108 means
Ariston describe 108 as "insufficient water (request filling)", and on the Genus One the display spells it out as "Fault 108 — Filling Needed". That is a more accurate description than the plain "low pressure" most manufacturers use, because what the appliance cares about is having enough water in the primary circuit to carry heat away from the exchanger. The threshold is not a fixed number, which is worth knowing before anyone quotes one at you: Ariston make it an installer parameter, adjustable between 0.4 and 0.8 bar, set to 0.6 bar as it leaves the factory.
Below that the boiler first shows the 1P4 filling warning and then the 108 fault. The refill itself is undramatic — Ariston tell you to open the filling valve and close it once the gauge reads between 1 and 1.5 bar. The pressure loss is what deserves attention. A sealed system does not use water up, so if the gauge falls, water has left, and how fast it falls tells you whether that matters.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Ordinary slow loss over a long period | Every sealed system seeps a little at valve glands and air vents. An annual top-up is unremarkable maintenance rather than a fault. | Common |
| Radiators bled without refilling afterwards | Bleeding removes water along with the air. Work through a houseful of radiators and the gauge will be visibly down by the end. | Common |
| Leak somewhere on the system | Typically a compression joint beneath a floor or a weeping valve gland. The rate of pressure loss is the clue that separates this from normal seepage. | Occasional |
| Expansion vessel has lost its charge | Without a working vessel, pressure climbs sharply as the system heats and the safety valve discharges the excess outside. The gauge reads low once everything cools. | Occasional |
What to try yourself
With the system cold, read the pressure. On the Clas One and E-Combi One it shows on the LCD; on the older Clas HE it is an analogue gauge below the appliance.
Find the filling loop underneath the appliance and open it slowly. Watch the pressure climb and close it between 1 and 1.5 bar, which is the range Ariston give.
Do not overfill. The safety valve will dump the excess outside and leave you low again once everything cools, and above 3 bar an Ariston shows a separate 109 fault instead.Close the filling loop fully and check the code has cleared. If the system now sounds airy or a radiator has gone cold at the top, Ariston have a deaeration cycle for exactly that: with no heating or hot water being called for, hold the Mode button for five seconds and it runs for about seven minutes. Press Mode to stop it early.
This is not part of the routine top-up and it is not a fix for the fault — it clears air, nothing else. Ariston pair it with opening the manual air vent on the heat exchanger, which is inside the case and an engineer's job, so expect it to help less on its own.Write the date down. If you find yourself repeating this within a couple of months, the problem is a leak rather than the pressure, and topping up is treating the symptom.
Get a Gas Safe registered engineer to look at your Ariston 108
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£450
Time on site
15 minutes
Free if it is simply a refill. A pressure sensor is £120 to £200 fitted, a leak trace £90 to £160, and an expansion vessel £180 to £450 depending on access.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Expansion vessel (8 litre, sealed system)
EV-8L£35–£85Plumbing merchant
Price checked 28 September 2026
Pressure relief valve, 3 bar
PRV-3B£18–£42Plumbing merchant
Price checked 28 September 2026
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Related Ariston fault codes
- 109Get it checkedSystem pressure has gone above 3 bar. Usually this means somebody overfilled it, or the expansion vessel has failed.Related: water pressure
- 501Get it checkedNo flame was detected during ignition and the appliance has locked out. Check the gas supply and the condensate pipe before booking anyone.Related: same brand
- 101Get it checkedThe heating circuit has overheated and the appliance has shut down. The usual reason is that water has stopped moving, not that it is firing too hard.Related: same brand
- 103Get it checkedThe appliance is reporting insufficient circulation — water is not moving through it fast enough. Worth acting on before it becomes an overheat.Related: same brand
- 604Get it checkedFan speed is outside the range the appliance expects, so it will not allow the burner to light. This one needs an engineer.Related: same brand
- 303Get it checkedThe main control board has reported an internal fault. It needs an engineer, and it is worth checking the electrical supply before assuming the board is dead.Related: same brand
Own an Ariston boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.