Ariston CLAS ONE 24 error 108: safe first checks

Error 108 on an Ariston CLAS ONE points to insufficient water pressure in the heating circuit. This guide is limited to the UK/IE CLAS ONE 24 combi boiler, G.C.N. 47-116-85 and product code 3301043, using Ariston user manual 420011124402, Revision 00, dated 08/23.

CLAS ONE controls and filling arrangements can differ from other Ariston boilers and from older installations. Confirm the exact model and manual before acting.

What does error 108 mean?

Ariston’s user manual describes code 108 as a safety shut-off caused by insufficient water pressure. The same manual tells the user to check the pressure gauge when the system is cold and gives a cold range of 0.6 to 1.5 bar.

A low reading can explain the code, but it does not explain why the pressure fell. A recurring drop, a visible leak or an unfamiliar filling arrangement needs professional attention.

First, confirm the boiler

Before touching any control, record:

1. The full code shown on the display.

2. The model name and G.C.N. from your paperwork or appliance label.

3. The pressure-gauge reading when the system is cold.

4. Whether heating, hot water or both have stopped.

5. Any visible water, staining or unusual noise around the boiler or radiators.

This article applies only to the CLAS ONE 24 combi identified above. It does not cover CLAS SYSTEM ONE or another Ariston model, even though some manuals group several products together.

Safe checks you can make

Read the gauge

Look at the pressure gauge without removing the casing. If the reading is below the range in the matched user manual, error 108 may be consistent with low system pressure.

Do not rely on a pressure number from a generic boiler article. Use the instructions supplied with your exact boiler and installation.

Look for an obvious leak

Check the visible area around the boiler, accessible pipework and radiators. Do not remove panels, tighten internal parts or dismantle anything.

If you can see water, the gauge keeps falling or you have to refill repeatedly, stop and arrange a qualified inspection.

Match the filling arrangement before using it

The official manual shows a CLAS ONE flexible filling hose and two taps. Only follow that diagram if it matches the arrangement installed in your home and you are confident you understand the controls your installer showed you.

The current manual download contains a general filling note and a separate CLAS ONE diagram with different stop-pressure figures. Because the correct action depends on the installed arrangement, this article deliberately does not give valve-by-valve instructions or choose a target for you. Use the manual left with the boiler or ask a competent heating engineer.

Do not repeatedly reset error 108

Error 108 is tied to water pressure. Repeatedly pressing reset does not fix a leak or an incorrectly filled system.

If the code remains after the correct pressure has been restored using the matched instructions, or if it returns, stop and contact a qualified technician. Do not keep topping up the system.

When to stop and get help

Arrange professional help if:

• the filling loop does not match the official diagram;

• you are unsure which controls to use;

• a valve is stuck or the pressure rises unexpectedly;

• you can see a leak;

• the pressure falls again after filling;

• error 108 stays on the display; or

• the boiler shows another fault or behaves unusually.

Keep the casing closed. Gas, combustion, electrical, flue and internal boiler work is not a homeowner task.

For non-emergency help, use the MPH Boilers repair page and include the exact code, model, gauge reading and any visible leak. You can also review the Ariston boiler range at MPH. An enquiry is not a confirmed appointment; the office confirms coverage and availability.

Gas smell or carbon monoxide concern

Do not treat a gas smell, carbon monoxide alarm or suspected carbon monoxide as a pressure problem.

In Great Britain, move everyone into fresh air for a carbon monoxide concern and call the National Gas Emergency Service on 0800 111 999. If you smell gas, open doors and windows, avoid naked flames and electrical switches, and call the same number. Follow SGN’s current gas-emergency guidance and carbon monoxide advice.

Common questions

Can I keep topping up an Ariston CLAS ONE 24?

No. The official manual says frequent pressure loss can indicate a water leak. Repeated filling is a reason to stop and arrange a qualified check.

Should I reset error 108?

Not as a substitute for correcting the pressure problem. Identify the cause and follow the matched pressure instructions. If the code remains or returns, contact a qualified technician.

Does this guide cover CLAS SYSTEM ONE?

No. It is deliberately limited to the CLAS ONE 24 combi, G.C.N. 47-116-85. The grouped manual contains separate diagrams and instructions for CLAS ONE and CLAS SYSTEM ONE.

Should I open the boiler casing?

No. Ariston’s safety instructions say not to perform operations that involve opening the appliance.

Official sources

• Ariston CLAS ONE UK product page — 24 kW product code 3301043 and official documentation link.

• Ariston CLAS ONE and CLAS SYSTEM ONE user manual — 420011124402, Revision 00, 08/23: applicability p.1; safety pp.2–3; controls p.5; cold pressure and CLAS ONE filling arrangement p.6; code 108 and escalation p.9.

• SGN gas emergency guidance and SGN carbon monoxide guidance, retrieved 3 October 2026.