What Ariston error 108 means
Error 108 is Ariston's low water pressure code. The boiler has measured too little pressure in the sealed heating circuit and has stopped firing until the water is topped up. The E-Combi One LPG design, installation and servicing instructions for Great Britain and Ireland describe it as "Insufficient water (request filling)", and the Clas One and Clas System One user manual from Ariston Thermo UK calls it "Insufficient water pressure".
The fix is usually simple, and a homeowner can do it. The part worth thinking about is why the pressure fell in the first place, because a boiler that keeps losing water will keep showing 108.
Which Ariston boilers show 108, and how it looks
We have found 108 with the same meaning in these Ariston documents:
- E-Combi One LPG 24 and 30, in the error table of the UK and Irish installation manual.
- Clas One and Clas System One, in the operation shutdown table of the UK user manual.
- Clas One 18 to 35, in the English and French installation and servicing instructions (reference 420000428401).
- Alteas One and Genus One, where Ariston's training manual describes water missing from the primary circuit.
- Alteas One Net, which shows "Fault 108" together with the message "Filling Needed".
The manuals print the code as "1 08". The 1 tells you the fault sits in the primary (central heating) circuit, and 08 is the fault itself, so 108 and 1 08 are the same code. Plenty of owners of E-Combi Evo models search for this code too, and the meaning is the same low pressure warning, though we have not seen an Evo manual to confirm the wording.
How to fix Ariston error 108 with the filling loop
The E-Combi One LPG manual says the pressure can be restored by filling the heating circuit. The Clas One user manual shows the filling loop underneath the boiler, with a flexible hose and two taps. Work with the heating off and cold so the reading is accurate.
- Look at the pressure reading on the display or gauge and note the starting figure.
- Check the flexible hose of the filling loop is connected at both ends.
- Open the first tap fully, then open the second tap slowly. You should hear water running into the system.
- Watch the pressure climb and close both taps when it reaches 1 to 1.5 bar.
- On the Clas One, the boiler runs an air purge while it fills and shows PURGE on the display for about seven minutes before returning to normal. Let it finish.
- If 108 is still showing once the pressure is back up, press RESET once.
Go slowly. If you overshoot, the boiler can show 1 09, which the manuals list as system pressure above 3 bar. If the gauge goes well past 1.5 bar, you can bring it down by bleeding a radiator.
If your filling loop looks different, or you cannot find it, our boiler filling loop guide explains the common types.
Why the pressure keeps dropping
A sealed heating system should hold its pressure for months. If you top up and 108 comes back within days or weeks, water is leaving the system somewhere. Ariston's manual is clear on this point: if refilling is needed often, turn the boiler off, switch off the external electrical switch, close the gas cock and have a qualified technician check for water leaks.
Things you can safely look for yourself:
- Recent radiator bleeding. Bleeding lets out air and a little water, so a drop afterwards is normal. Top up once and see if it holds.
- Damp under the boiler. Look, without opening the case, for drips or staining on the pipes underneath.
- Radiator valves and pipe joints. Check for wet carpet, green or white crust on fittings, or marks on ceilings below upstairs radiators.
- The pressure relief pipe outside. A small copper pipe often runs from the boiler through the wall. If it drips, or there are water marks on the wall below, the boiler may be discharging water.
Our boiler losing pressure guide goes through these causes in more depth.
When to call a Gas Safe registered engineer
Book a Gas Safe registered engineer if 108 returns soon after you top up, if you find water inside or under the boiler, or if the relief pipe outside is dripping. Do not open the boiler casing to look for the leak. The parts that control pressure, such as the expansion vessel, relief valve and pressure sensor, sit inside the sealed appliance.
An engineer will usually check the expansion vessel charge, test the pressure relief valve, look for internal leaks and check that the pressure reading is accurate. It helps to tell them:
- how often you have been topping up, and from what pressure to what pressure
- whether the pressure rises sharply when the heating is on
- whether any radiators, pipes or the outside relief pipe show signs of water
Typical repair costs in the UK
Many 108 cases cost nothing beyond a top-up. When a part is to blame, these are the typical ranges.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Internal leak or pipe joint repair | Typically £100 to £300 |
| Expansion vessel recharge | Typically £80 to £150 |
| Expansion vessel replacement | Typically £180 to £400 |
| Pressure relief valve | Typically £100 to £200 |
| Pressure sensor or switch | Typically £100 to £220 |
Other Ariston codes linked to pressure
Error 108 sits in the 1 xx group with the other central heating circuit codes. If the pressure is fine but the boiler still will not heat, the code may be about circulation instead, which the manuals list in the 1 03 to 1 07 range. See our Ariston fault codes overview for how the groups work and for the other Ariston codes we cover.