Chaffoteaux 108 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 108 fault code mean?
Fault code 108 on a Chaffoteaux boiler indicates that the water pressure in the central heating circuit has dropped below the minimum safe operating threshold. The boiler shuts down as a protective measure to prevent damage from running dry. You may also see this displayed as 1P4 or F1P4 on some Chaffoteaux models — these all point to the same underlying issue. Because Chaffoteaux sits within the Ariston Thermo group, this code appears across several Ariston-family boilers under equivalent fault descriptions. Normal operating pressure when the system is cold should sit between 1.0 and 1.5 bar; anything below roughly 0.5–0.8 bar will typically trigger this fault.
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
- System pressure naturally dropped over time or after bleeding radiators Common
The most common trigger. Central heating systems gradually lose a small amount of pressure over months, and bleeding radiators releases water along with air, which can push pressure below the safe minimum. This is a straightforward fix — the system simply needs topping up via the filling loop.
- Leak in the heating circuit Common
A weeping joint, pinhole in a radiator, or a dripping connection inside or outside the boiler can cause pressure to fall repeatedly. After topping the system up, if pressure drops again within days, a leak is the likely culprit. Look for damp patches, staining, or corrosion on radiators and pipework.
- Failing expansion vessel Sometimes
The expansion vessel uses an air-charged diaphragm to absorb pressure fluctuations as water heats and cools. When the diaphragm perishes or the air charge escapes, the system pressure becomes unstable — often dropping when cold and spiking dangerously when hot. A tyre pressure gauge on the vessel's Schrader valve can confirm a low charge.
- Pressure relief valve (PRV) weeping Sometimes
The PRV is designed to open and release water if pressure rises too high. If it develops a partial fault it can weep slowly, allowing water to escape through the overflow pipe and gradually reducing system pressure. Look for a continuously dripping overflow pipe outside the property.
- Faulty water pressure sensor Rare
If the pressure sensor itself develops a fault, it may report low pressure to the PCB even when the actual system pressure is perfectly adequate. This is less common but worth considering when the gauge reads normal yet the fault code persists after repressurising and resetting.
- Blocked or faulty filling loop or filling tap Rare
If the filling loop valve is stuck, seized, or defective, it may be impossible to add water to the system in the normal way, leaving pressure too low to recover. A replacement filling assembly is needed in this case.
How to fix it
- Check the pressure gauge reading DIY safe
Find the boiler's pressure gauge — it is usually a dial or digital display on the front or underside of the boiler. If it reads below 1.0 bar when the system is cold, low pressure is confirmed as the cause of the fault. If the gauge reads 1.0–1.5 bar or higher, do not add more water; move on to resetting and investigating other causes.
- Top up the system using the filling loop DIY safe
Locate the filling loop — typically a short silver braided hose with one or two small lever or screw valves beneath the boiler. Slowly open the valve(s) to allow mains water into the system, watching the pressure gauge closely. Stop when the gauge reaches 1.2–1.5 bar. Never overfill beyond 1.5 bar when cold. Close the valve(s) fully when done and check they are not left partially open.
- Reset the boiler DIY safe
Once the pressure is restored, press and hold the reset button (often marked with a flame or 'R' symbol) for 3–5 seconds, or follow the reset procedure in your boiler's user guide. Allow the boiler a minute or two to restart and attempt ignition. Do not reset more than two or three times in quick succession.
- Monitor pressure over the following days DIY safe
Keep an eye on the pressure gauge over the next few days. If it remains stable at 1.0–1.5 bar when cold, the issue was a one-off pressure drop and no further action may be needed. If it falls again within days, the system has a leak or another underlying fault that requires professional investigation — do not keep topping up indefinitely.
- Check for visible leaks on radiators and pipework DIY safe
Walk around the property and inspect all visible radiators, valves, and exposed pipework for signs of dampness, rust streaks, limescale deposits, or water staining. Also check beneath the boiler casing for drips. If you spot a dripping overflow pipe on an external wall, the pressure relief valve may be weeping. Report your findings to an engineer.
- Call a Gas Safe registered engineer to diagnose persistent pressure loss Gas Safe engineer
If pressure keeps dropping, if you have found a leak you cannot resolve yourself, or if the fault code returns despite a normal-reading gauge, you need a Gas Safe registered engineer. They can pressure-test the circuit to locate hidden leaks, inspect and recharge or replace the expansion vessel, replace the PRV or pressure sensor, and check the filling loop assembly — all tasks that require professional tools and expertise.
Parts you may need
- Water pressure sensor · from £25
- Pressure relief valve (PRV) · from £30
- Expansion vessel (external add-on type) · from £55
- Filling loop assembly · from £20
- Expansion vessel diaphragm / internal replacement · from £80
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 £90–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep topping up the pressure on my Chaffoteaux boiler every few weeks?
Topping up occasionally — say, once or twice a year — is perfectly normal. However, if you find yourself refilling the system every few days or weeks, that is a sign of an underlying problem such as a leak or a failing expansion vessel. Repeatedly adding water without fixing the root cause can accelerate corrosion inside the system and may eventually lead to more costly repairs. Get it checked by a Gas Safe engineer if the pressure keeps dropping.
What does F1P4 or 1P4 mean on a Chaffoteaux boiler — is it the same as 108?
Yes, F1P4 and 1P4 are alternative ways some Chaffoteaux displays show the same low water pressure fault, equivalent to code 108. The underlying meaning is identical — the system pressure has dropped below the safe operating level. The same diagnostic steps and fixes apply regardless of which version of the code appears on your display.
How much does it cost to fix a Chaffoteaux 108 fault in the UK?
If a simple top-up via the filling loop clears the fault permanently, it costs nothing. If an engineer is needed, most people pay between £90 and £350 depending on the cause. Replacing a pressure sensor or PRV typically falls at the lower end of that range, while an expansion vessel replacement (particularly an internal one that is awkward to access) can reach £300–£450. In rare cases where the PCB or heat exchanger is involved, costs can be considerably higher — but those components are not a typical cause of fault 108.
How do I know if my expansion vessel has failed on a Chaffoteaux boiler?
A common clue is that the pressure gauge reads low when the system is cold but rises rapidly — often past 2.5–3 bar — when the boiler is running. You may also notice the pressure relief valve dripping from the external overflow pipe. A Gas Safe engineer can confirm it by pressing the Schrader valve (like a tyre valve) on the expansion vessel: if only water comes out rather than air, the diaphragm has failed and the vessel needs recharging or replacing.