Chaffoteaux 109 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 109 fault code mean?
Fault code 109 on a Chaffoteaux boiler indicates that the system pressure has climbed above the safe threshold — typically beyond 3 bar. As a protective measure, the boiler locks out automatically to guard internal components against damage caused by excessive pressure. This is essentially the mirror image of fault code 108 (low pressure). The code is consistent across the main Chaffoteaux ranges including the Pigma, Inoa, Talia Green, and Britony series, though it is always worth confirming the definition against your specific model's manual.
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
- Failed or waterlogged expansion vessel Common
The expansion vessel contains two chambers divided by a rubber diaphragm — one side holds pressurised air or nitrogen, the other connects to the heating circuit. As water heats and expands, it pushes against the diaphragm, which absorbs the pressure rise safely. When the diaphragm splits or the air charge is lost, there is nothing to buffer the expanding water, so system pressure spikes rapidly and can hit 3 bar or above, triggering the 109 lockout. This is by far the most frequent cause of a 109 fault.
- System overfilled after re-pressurising Common
A 109 fault sometimes follows on the heels of a 108 (low-pressure) fault. If the system was topped up via the filling loop and too much water was added, the cold pressure may already be sitting above 2 bar. When the boiler fires up and the water expands, the gauge quickly pushes toward and past 3 bar. This is particularly common when a homeowner has been trying to resolve a low-pressure fault themselves.
- Faulty pressure relief valve (PRV) Sometimes
The PRV is designed to open and discharge water outside the property when pressure reaches its set point (usually 3 bar), preventing damage. If the PRV is stuck shut or has a higher-than-intended opening pressure, it will not release excess pressure when it should, allowing the gauge to climb until the boiler's internal safety circuit triggers the 109 lockout. A tell-tale sign is pressure rising sharply during a heating cycle without any water being discharged from the PRV discharge pipe outside.
- Aged expansion vessel on older Pigma models Sometimes
The expansion vessel on Chaffoteaux Pigma boilers is known to deteriorate after around five to seven years of service. As the vessel ages, the rubber diaphragm becomes brittle and the pre-charge air pressure gradually drops, reducing the vessel's ability to absorb pressure fluctuations. Recurring 109 faults on a Pigma that is several years old strongly point to the expansion vessel needing recharging or full replacement.
How to fix it
- Check the pressure gauge reading DIY safe
Look at the pressure gauge on the front of the boiler. A correctly pressurised cold system should read between 1 and 1.5 bar. If the gauge is showing 2.5 bar or above — especially when the system is cold — there is already too much water in the circuit and the boiler is likely to lock out on 109 once the heating runs.
- Bleed a radiator to reduce pressure if the system was recently overfilled DIY safe
If you recently topped up the system and the cold pressure is above 2 bar, you can carefully bleed a radiator to release a small amount of water and bring the pressure back down to around 1–1.5 bar. Use a radiator bleed key, hold a cloth underneath the bleed valve, and open it just a fraction until water — not air — begins to trickle out, then close it firmly. Check the gauge as you go and stop once the pressure is in the correct range. Do not open the filling loop again after doing this.
- Reset the boiler once DIY safe
Locate the reset button on your Chaffoteaux boiler (consult the user guide if unsure of its position), press it once, and allow the boiler to go through its start-up sequence. If the pressure remains in the normal range and the boiler fires successfully, monitor it over the next few heating cycles. If fault code 109 returns, do not keep resetting — repeated resets without addressing the root cause will not resolve the underlying problem.
- Check the expansion vessel air charge (engineer task) Gas Safe engineer
A Gas Safe engineer can check the air pre-charge in the expansion vessel using a tyre pressure gauge on the Schrader valve (typically located on the vessel itself). The charge should match the system's static fill pressure — usually around 1 bar. If the vessel is flat or waterlogged, the engineer can attempt to recharge it with a pump. If the diaphragm has failed, the vessel will need to be replaced entirely. On Chaffoteaux boilers where the vessel is mounted internally, the boiler casing must be removed and the system partially drained — this work must only be carried out by a qualified engineer.
- Have the pressure relief valve inspected and replaced if necessary Gas Safe engineer
If the expansion vessel is in good condition but pressure still rises sharply during a heating cycle, a Gas Safe engineer should test the PRV. A PRV that is stuck or has a compromised seat may be preventing proper pressure discharge. PRV replacement is a straightforward job for an engineer but involves working on pressurised components and must not be attempted by a homeowner.
- Call a Gas Safe registered engineer Gas Safe engineer
If the 109 fault returns after a single reset and pressure is not simply the result of overfilling, you need a Gas Safe registered engineer to diagnose and repair the fault properly. They will assess the expansion vessel, PRV, and overall system pressure behaviour, carry out any replacements needed, and verify the system is safe before leaving. You can find a Gas Safe registered engineer at gassaferegister.co.uk.
Parts you may need
- Expansion vessel (external, 8–12 litre) · from £65
- Expansion vessel (internal OEM replacement) · from £120
- Expansion vessel diaphragm / membrane · from £25
- Pressure relief valve (3 bar PRV) · from £30
- Schrader valve core (for vessel recharge port) · from £5
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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix a Chaffoteaux 109 fault myself?
Partially. If the fault was caused by overfilling the system — for example after topping up the pressure to fix a previous low-pressure fault — you can safely bleed a radiator to bring the pressure back into the 1–1.5 bar range, then attempt a single reset. However, if the root cause is a failed expansion vessel or a faulty PRV, those jobs require a Gas Safe registered engineer. Attempting to replace or recharge an expansion vessel yourself, particularly an internally mounted one, risks flooding, injury from pressurised water, and potential gas-safety hazards.
Why does my Chaffoteaux boiler keep going to 3 bar when the heating comes on?
A pressure rise of half a bar or so during a heating cycle is normal — water expands as it heats up. But if the gauge is regularly climbing to 2.5 bar or beyond when the heating runs, the expansion vessel is almost certainly the culprit. Either the air pre-charge has dropped (meaning the vessel is no longer absorbing the pressure rise effectively) or the internal diaphragm has failed. Have a Gas Safe engineer check and recharge or replace the vessel. If you also notice water dripping from the discharge pipe outside the property, the pressure relief valve is opening — a clear sign the system pressure is reaching 3 bar.
How much does it cost to replace an expansion vessel on a Chaffoteaux boiler in the UK?
Most expansion vessel replacements on Chaffoteaux boilers cost between £150 and £400 including parts and labour. An externally mounted vessel with good access typically sits at the lower end of that range, while an internally mounted vessel (which requires stripping the boiler casing and draining part of the system) will generally cost more. The vessel part itself typically costs £50 to £120; the rest is engineer time. For unusually complex strip-downs or if a pressure relief valve also needs replacing at the same time, costs can rise to around £500 — though this is less common.
My Chaffoteaux Pigma keeps showing fault 109 — is it worth repairing?
The Pigma's expansion vessel is a known weak point after five to seven years, so a 109 fault on an ageing Pigma is not unusual. If the boiler is under ten years old and otherwise in good condition, an expansion vessel replacement is usually worthwhile and a sensible investment. If your Pigma is pushing twelve or more years old and this is not the first significant repair, it may be worth getting quotes for both repair and a new boiler to make an informed decision — a modern A-rated condensing boiler will also cut your heating bills compared with an older unit.