Chaffoteaux 118 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 118 fault code mean?
Fault code 118 on Chaffoteaux boilers signals that the primary circuit water pressure has dropped too low — typically below 0.5–1.0 bar depending on the model. When the boiler detects this, it shuts down as a protective measure to prevent damage to the heat exchanger and pump. You will usually lose both heating and hot water, and the 118 code will appear on the boiler's digital display. The fix is often as simple as topping up the system pressure via the filling loop, but if pressure keeps falling the underlying cause — a leak, a failed expansion vessel, or a faulty pressure relief valve — must be investigated by a Gas Safe registered engineer. Note: fault code numbering varies across Chaffoteaux's range; the numeric 118 code is associated with modern Ariston-platform Chaffoteaux boilers such as the iC and iC Economiser Plus. Older models (e.g. Britony Combi SE) used LED flash sequences rather than a numeric display — always cross-reference your model's manual if unsure.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Low system water pressure (natural loss over time) Common
Water pressure in a sealed central heating system drops gradually over months, especially after radiator bleeding or seasonal pressure fluctuations. If the pressure gauge reads below 1 bar when the system is cold, this is almost certainly the immediate trigger for code 118. Topping up via the filling loop usually clears the fault straight away.
- Leak in the heating system Common
A slow weep anywhere in the system — pipework joints, radiator valves, a dripping pressure relief valve, or the heat exchanger itself — will cause repeated pressure drops. If you find yourself re-pressurising every few weeks, a leak is the most likely culprit. Look for damp patches under radiators, around the boiler casing, or near pipework joints.
- Faulty or waterlogged expansion vessel Common
The expansion vessel absorbs the rise in water volume as the system heats up. If its internal diaphragm has failed or the air charge has been lost, pressure can swing unpredictably — dropping when cold and spiking dangerously when hot. A failing expansion vessel is one of the most common reasons for repeated 118 faults once simple re-pressurising no longer holds.
- Pressure relief valve weeping Sometimes
The pressure relief valve (PRV) is a safety device that releases water if pressure gets too high. Over time the valve seat can degrade, causing a slow, continuous drip — usually visible as a damp patch or trickle outside the property near the boiler's discharge pipe. This constant loss of water brings the system pressure down gradually.
- Sludge, scale, or a faulty NTC sensor Sometimes
In hard water areas or systems without corrosion inhibitor, scale and magnetite sludge can build up inside the heat exchanger and around the NTC temperature sensor. The insulating effect of scale causes the sensor to misread temperatures, and on some modern Chaffoteaux models this thermal discrepancy can trigger code 118 even when physical pressure is borderline. A noisy pump or cold spots on radiators alongside repeated 118 faults can point to this cause.
- Pressure drop following radiator bleeding Sometimes
Bleeding radiators to remove trapped air is good practice, but it releases water from the system along with the air. If the pressure is not topped up afterwards, the gauge will fall below the acceptable threshold and trigger code 118.
How to fix it
- Check the system pressure gauge DIY safe
Locate the analogue pressure gauge — on most Chaffoteaux iC models it sits behind a panel on the lower left of the boiler casing. The needle should sit between 1.0 and 1.2 bar when the system is cold. Alternatively, press and hold the Eco button for 6 seconds, then tap the + button to scroll to parameter 'A6', which shows a live digital pressure reading. If the gauge reads below 1 bar (and especially below 0.5 bar), low pressure is the confirmed cause of the fault.
- Re-pressurise the system using the filling loop DIY safe
On Chaffoteaux iC and iC Economiser Plus boilers the filling loop is a short copper U-shaped tube that connects between the blue and black isolation valves on the underside of the boiler. If it is stored loose, remove the end caps, fit the loop and finger-tighten the nuts. Open both isolation valves a quarter-turn — you will hear water entering the system. Watch the pressure gauge rise slowly. Close both valves once the gauge reaches 1.0–1.2 bar. Do not exceed 1.5 bar. Remove and store the filling loop, replace the caps. The 118 code should clear and the display should show 'OK'.
- Reset the boiler DIY safe
Press the reset button on the front panel once. Wait at least 60 seconds for the boiler to go through its ignition sequence. If the boiler fires up and runs normally, the fault has been resolved. Do not reset more than two or three times in total — repeated lock-outs mean the underlying problem has not been fixed and further resets will not help.
- Monitor system pressure over the following week DIY safe
Check the pressure gauge daily for 5–7 days. A small drop of 0.1–0.2 bar over several weeks can be normal, but if the gauge falls back below 1 bar within a few days, or you need to top up the system repeatedly, there is an active leak or component fault that needs professional attention. Look for damp spots near radiators, pipework, and under the boiler.
- Inspect visible pipework and radiator valves for leaks DIY safe
Walk around the property and check beneath all radiators, around valve connections, and along any exposed pipework for signs of dampness, water staining, or white limescale deposits (which indicate a slow drip that has dried). Check the boiler's external condensate pipe and the discharge pipe from the pressure relief valve outside. Do not attempt to dismantle any components — just note and photograph anything suspicious to report to your engineer.
- Call a Gas Safe registered engineer if the fault persists or pressure keeps dropping Gas Safe engineer
If code 118 returns within days of re-pressurising, or if you cannot locate an obvious leak, contact a Gas Safe registered engineer. They can pressure-test the system to find hidden leaks, check and re-charge or replace the expansion vessel, inspect the pressure relief valve, and assess whether scale or sludge is affecting the NTC sensor and pump flow. Do not keep topping up the system without resolving the root cause — repeatedly adding fresh water accelerates internal corrosion.
Parts you may need
- Expansion vessel (diaphragm type, combi boiler) · from £85
- Pressure relief valve (3 bar) · from £25
- NTC thermistor / flow sensor · from £20
- Filling loop assembly · from £18
- System inhibitor (e.g. Fernox F1, 1 litre) · from £15
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I re-pressurise a Chaffoteaux boiler myself to fix the 118 fault?
Yes — topping up the system pressure is a straightforward homeowner task on most Chaffoteaux iC models. Attach the filling loop to the two isolation valves under the boiler, open them a quarter-turn until the gauge reaches 1.0–1.2 bar, then close them and remove the loop. If this clears the 118 code and the pressure stays stable for several weeks, no further action is needed. If pressure drops again within days, you need an engineer to find the underlying cause.
Why does my Chaffoteaux boiler keep losing pressure and showing fault 118?
Recurring pressure loss almost always points to one of three things: a slow leak somewhere in the system (pipework, radiator valves, heat exchanger, or the boiler itself), a failing expansion vessel whose diaphragm has perished so it can no longer buffer pressure changes, or a pressure relief valve that is weeping water away continuously. Repeatedly topping up without finding the cause adds fresh oxygenated water to the system, which speeds up internal corrosion — so it is worth having an engineer investigate rather than just re-pressurising indefinitely.
How much does it cost to fix a Chaffoteaux 118 fault in the UK?
If the fix is simply re-pressurising, it costs nothing. If an engineer is needed, a diagnostic visit typically runs £85–£150. Replacing a pressure relief valve usually comes to around £100–£180 including labour. Expansion vessel replacement is the most common paid repair for this fault and typically costs £150–£350 all-in, though it can reach £450 on boilers that are difficult to access. NTC sensor replacement is usually £150–£200. A full power flush to clear sludge or scale ranges from £300–£500 depending on system size. In rare cases where the heat exchanger itself is damaged the cost rises significantly — at that point, for an older boiler, a full replacement may be more economical.
Is fault code 118 the same on all Chaffoteaux boilers?
Not necessarily. Chaffoteaux's fault code numbering has changed across generations. The numeric code 118 for low water pressure applies to modern Ariston-platform boilers such as the iC and iC Economiser Plus ranges. Older models like the Britony Combi SE used LED flash sequences to indicate faults rather than a numbered code on a display. If you are unsure which system your boiler uses, check the model label on the boiler casing and cross-reference with the model-specific manual before acting on any fault code guidance.