Chaffoteaux 111 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 111 fault code mean?
Fault code 111 on most Chaffoteaux boilers indicates that the flow temperature has climbed to or beyond the maximum safe threshold, causing the boiler to lock itself out as a protective measure. The overheating thermostat — mounted on the outlet pipe of the primary heat exchanger — has triggered a shutdown to prevent damage. On Expert Control models, the code appears as '1 11' in the error table (group 1, fault 11), covering overheating and flow-check failures. On Ariston-platform Chaffoteaux models (which share control architecture with Ariston Thermo Group boilers), 111 sits alongside codes 108 and 112 which relate to automatic replenishment and return-pipe sensor faults respectively. On Pigma NOx and similar ranges the same overheating pattern presents under this code family. Important note: older Chaffoteaux models without a screen communicate faults through numbered LED flash sequences rather than a numeric display, so always cross-reference your model's manual to confirm the meaning before acting.
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
- Poor circulation due to a failing pump Common
If the circulating pump is not moving water through the heat exchanger at the correct rate, heat builds up rapidly in the primary circuit. The PCB detects the rising flow temperature and triggers a lockout. A pump that is seized, running slowly, or set to the wrong speed setting is one of the most frequent root causes of an overheating fault on Chaffoteaux boilers.
- Sludge or debris restricting flow Common
Rust particles and other debris from radiators and pipework accumulate over time to form a thick black sludge in the heating circuit. This sludge restricts the flow of water through the boiler and pump, causing localised hot spots in the heat exchanger and triggering the overheating thermostat. Boilers in older systems or systems that have never been chemically cleaned are especially vulnerable.
- Limescale build-up near the flow thermistor Common
In hard-water areas, limescale deposits can form around the NTC thermistor on the flow pipe. Even a modest build-up insulates the sensor from the flowing water, causing it to read a temperature higher than the actual system temperature. The PCB acts on this inflated reading and locks the boiler out unnecessarily.
- Low system pressure Common
If system pressure drops below approximately 0.8–1.0 bar, there is less water in the circuit to absorb and carry away heat. Boiler components can then overheat more quickly than the safety sensors can respond, resulting in a thermal lockout. A slow leak, a weeping radiator valve, or a recently bled system are all common reasons for pressure loss.
- Faulty NTC flow thermistor Sometimes
The NTC thermistor measures the flow temperature and continuously reports it to the PCB. If the sensor itself has failed or drifted out of calibration, it may send false high-temperature readings even when the water is at a perfectly safe temperature. The boiler then locks out without any genuine overheating having occurred.
- Faulty PCB Rare
The printed circuit board interprets all sensor data and controls every major electrical component. In rare cases the PCB misreads or misprocesses thermistor signals, causing spurious overheating faults. PCB failure is worth considering only after all other causes have been ruled out by a qualified engineer.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on the control panel (refer to your model's handbook if unsure). Press and hold it as instructed, then allow the boiler to restart. If the fault code 111 returns immediately or after just one heating cycle, do not reset again — repeated resets on an overheating fault can mask a serious underlying problem and may cause further damage.
- Check the system pressure gauge DIY safe
Find the pressure gauge on the boiler's front panel or pipework. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, top up using the filling loop (a braided flexible hose with a valve, usually beneath the boiler). Open the valve slowly, watch the gauge, and close it once the needle reaches 1.2–1.5 bar. After topping up, attempt one more reset. If the boiler holds pressure and runs normally, monitor it over the next few days for pressure loss.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler locks out again after a single reset and a pressure check, the underlying cause requires professional diagnosis. A qualified engineer will use diagnostic tools to check flow rates, pump performance, thermistor resistance values, and heat exchanger condition. Do not attempt further resets or any internal boiler work yourself.
- Engineer: inspect and test the circulation pump Gas Safe engineer
A Gas Safe engineer will check that the pump is spinning freely, operating at the correct speed, and delivering adequate flow. A pump that is seized, air-locked, or worn will be freed, bled, or replaced as appropriate. Pump replacement on a Chaffoteaux typically requires draining the relevant section of the circuit and re-commissioning the boiler.
- Engineer: power flush or chemical flush the system Gas Safe engineer
If sludge contamination is suspected — indicated by cold radiator spots, noisy pipework, or visibly dirty system water — the engineer can carry out a power flush or targeted chemical flush. They will also dose the system with a corrosion inhibitor and may recommend fitting a magnetic filter to prevent future build-up.
- Engineer: test and replace the NTC flow thermistor if faulty Gas Safe engineer
Using a multimeter, the engineer will measure the thermistor's resistance across a range of temperatures and compare the readings against the manufacturer's specification. A sensor that falls outside tolerance will be replaced. Thermistor replacement is a relatively straightforward job on most Chaffoteaux models but must still be performed by a registered engineer.
- Engineer: assess the PCB if all other components test correctly Gas Safe engineer
If the pump, thermistor, system pressure, and water quality are all satisfactory but the fault persists, the PCB becomes the prime suspect. PCB replacement is the most expensive common repair on these boilers; an honest engineer will confirm the diagnosis before ordering parts. On an older Chaffoteaux (10 years or more), weigh the cost of a new PCB against the cost of a full boiler replacement.
Parts you may need
- NTC flow thermistor · from £25
- Circulation pump (Grundfos or equivalent) · from £85
- Pressure relief valve (PRV) · from £20
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
- Central heating 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 £225–£420, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Chaffoteaux boiler when it shows fault code 111?
No. A single reset is reasonable to see whether the fault clears after a transient event such as a brief pressure drop. However, fault code 111 signals an overheating condition and repeatedly resetting the boiler without finding the root cause puts the heat exchanger and other components under unnecessary stress. If the code returns after one reset, stop and contact a Gas Safe registered engineer.
My Chaffoteaux doesn't have a screen — how do I know if I have fault code 111?
Many older Chaffoteaux models communicate faults through a sequence of LED flashes rather than a numeric display. On these boilers the number of flashes from one or more indicator lights corresponds to a fault code in the manual. Check your model's handbook to decode the flash sequence for your specific boiler. The symptoms of overheating (boiler firing briefly then shutting down, hot pipework near the boiler, pump noise) can help confirm the diagnosis even without a displayed code.
How much does it cost to fix a Chaffoteaux 111 fault in the UK?
For the most common repairs, expect to pay roughly £225–£420 including parts and labour. An NTC thermistor replacement typically costs around £225–£350, a pressure relief valve around £300, and a circulation pump £300–£420 depending on the model. A power flush to clear sludge usually runs £300–£600 depending on system size. PCB replacement is less common but can cost £350–£550 or more; on a boiler over 10 years old that figure makes a full replacement worth considering.
Could sludge in my heating system cause the 111 overheating fault?
Yes, and it is one of the most common causes. Rust and debris from radiators and pipework gradually break down into a thick black sludge that circulates with the heating water. This sludge restricts flow through the pump and heat exchanger, causing the boiler to overheat and lock out. Signs include cold patches at the bottom of radiators, gurgling noises in the pipework, and dirty water when bleeding a radiator. A Gas Safe engineer can confirm the issue and carry out a power flush, followed by dosing the system with inhibitor to slow future corrosion.