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", which is a more accurate description than the plain "low pressure" used by most manufacturers, because what the appliance actually cares about is having enough water in the primary circuit to carry heat away from the exchanger. It stops the burner below roughly 0.5 bar. There is a detail specific to Ariston worth knowing before you start: on many models the refill procedure includes a purge cycle, entered by holding the Mode button for five seconds until the display reads PURGE. That runs the pump to clear air while you fill, and skipping it is why some people top up correctly and still end up with air locks and noise afterwards. The pressure loss itself 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.
Hold the Mode button for about five seconds until the display shows PURGE. This starts the air-clearing cycle Ariston specify as part of the filling procedure.
Find the filling loop underneath the appliance and open it slowly. Watch the pressure climb and close it at 1.2 bar.
Do not go beyond 2.5 bar cold. Overfilling makes the safety valve dump water outside and leaves you low again once it cools — and on Ariston appliances a reading above 3 bar triggers a separate 109 fault.Close the filling loop fully, then press the Mode button again to leave the purge cycle. The display returns to its normal view.
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
Gas Safe Register engineers only. No obligation.
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 August 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 5 August 2026
Pressure relief valve, 3 bar
PRV-3B£18–£42Plumbing merchant
Price checked 5 August 2026
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
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
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: water pressure
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: water pressure
- F1Get it checkedSystem pressure has dropped below the minimum the boiler will run at. Topping it back up to 1.2 bar usually clears it straight away.Related: water pressure
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: water pressure
- E10Usually simple to fixSystem water pressure has fallen too low for the appliance to fire. Top the system back up to 1.2 bar and it will restart.Related: water pressure
Related guides
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