Potterton E118 Fault Code: Causes, Fixes & Repair Costs
What does the Potterton E118 fault code mean?
The E118 code on a Potterton boiler indicates that the central heating system pressure has fallen to a level too low for the boiler to operate safely — specifically below 0.5 bar, at which point the boiler locks out to protect itself and the wider system. In practice, you may notice the fault appearing even before reaching that threshold, as pressure between 0.5 and 1.0 bar is already too low for reliable operation. Most Potterton boilers need to sit between 1.0 and 1.5 bar when cold to function correctly. Until pressure is restored to a safe level, the boiler will not fire, leaving you without heating or hot water.
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
- Gradual pressure loss over time Common
Central heating systems naturally lose a small amount of pressure over months, particularly in older installations. If the boiler hasn't been topped up in a long time, the pressure may have simply drifted down below the acceptable range without any underlying fault. This is the most common reason for an E118 on an otherwise healthy system.
- Water leak from pipework, radiators, or fittings Common
A leak anywhere in the system — at a radiator valve, a pipework joint, or a boiler connection — will cause pressure to drop repeatedly. If you find yourself needing to repressurise more than once every few months, a leak is very likely the culprit. Look for damp patches near radiators, under the boiler, or along pipework runs, and check for pooling water or watermarks on floorboards and ceilings.
- Radiators bled recently without repressurising Common
Bleeding radiators releases trapped air but also reduces the overall water volume in the system. If pressure wasn't topped back up after bleeding, this can easily push the reading below the acceptable threshold and trigger E118.
- Faulty or weeping pressure relief valve (PRV) Sometimes
The PRV is designed to release water if system pressure climbs dangerously high. When it develops a fault — often through wear or mineral build-up — it can weep water continuously or open at the wrong pressure, causing a gradual but persistent pressure drop. You might notice a damp patch near the discharge pipe outside the property or a slow drip from the PRV outlet.
- Failing expansion vessel Sometimes
The expansion vessel absorbs the rise and fall of water pressure as the system heats and cools. If its internal membrane degrades, it can no longer do this effectively, which often leads to the PRV venting water during heat-up cycles and pressure dropping when the boiler cools. This results in recurring E118 faults that return shortly after repressurising.
- Faulty pressure sensor or PCB issue Rare
In less common cases, the pressure sensor itself may be sending an incorrect low-pressure signal to the PCB even when system pressure is actually fine. A faulty PCB can also misinterpret sensor readings and trigger an unnecessary lockout. This tends to be suspected only once genuine pressure issues and leaks have been ruled out.
How to fix it
- Check the pressure gauge on your boiler DIY safe
Locate the pressure gauge — on most Potterton boilers it is either a dial on the front panel or a digital reading on the display. A healthy cold-system pressure sits between 1.0 and 1.5 bar. If the needle or reading is below 1.0 bar, low pressure is confirmed as the likely cause of the E118 fault.
- Repressurise the system using the filling loop DIY safe
The filling loop is usually a flexible braided hose or a built-in valve assembly situated beneath the boiler. Ensure the boiler is switched off and cool before you start. If using an external flexible loop, attach both ends to the correct ports. Slowly open both valves — you should hear water entering the system. Watch the pressure gauge carefully and stop when it reaches approximately 1.2 to 1.5 bar. Close both valves fully. Never overfill beyond 1.5 bar, as this can cause the PRV to vent and worsen the situation.
- Reset the boiler DIY safe
Once pressure is restored to the correct level, press and hold the reset button on the boiler's control panel for 3 to 5 seconds, or follow the reset procedure in your model's user guide. The E118 code should clear and the boiler should attempt to fire. Do not reset more than 2 to 3 times — repeated resets without resolving the underlying cause can cause further issues.
- Monitor pressure over the following week DIY safe
After repressurising, check the gauge daily for 5 to 7 days. A small drop of around 0.1 to 0.2 bar is normal. If pressure falls noticeably within a day or two, or if you find yourself needing to top up the system again within a few weeks, this strongly suggests a leak or a faulty component. Check visually around radiator valves, under the boiler, and along accessible pipework for damp spots or watermarks.
- Call a Gas Safe registered engineer if the fault persists or pressure keeps dropping Gas Safe engineer
If repressurising and resetting does not clear the E118 code, or if pressure continues to fall after topping up, a Gas Safe registered engineer needs to investigate. They will check the pressure relief valve, expansion vessel, pressure sensor, and all pipework connections. If a hidden leak is suspected, specialist leak detection equipment such as tracer gas or thermal imaging cameras can pinpoint the source without unnecessary damage to your property. Do not continue to repressurise repeatedly as a workaround — this can mask a worsening leak.
Parts you may need
- Pressure relief valve (PRV) · from £25
- Expansion vessel · from £55
- Filling loop assembly · from £20
- Pressure sensor / transducer · from £30
- Automatic air vent · from £12
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 £80–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Potterton E118 fault myself?
If the pressure has simply dropped over time or after bleeding radiators, yes — topping up via the filling loop and resetting the boiler is a straightforward job any homeowner can do safely. However, if the pressure keeps dropping after you repressurise, there is an underlying fault (such as a leak or a faulty PRV) that needs a Gas Safe registered engineer to diagnose and repair. Never attempt to work on gas components, the boiler internals, or sealed system components yourself.
Why does my Potterton boiler keep losing pressure and showing E118?
Recurring pressure loss almost always means water is escaping the system somewhere. The most common culprits are a weeping pressure relief valve (often identifiable by a damp patch near the external discharge pipe), a leaking radiator valve or pipework joint, or a failed expansion vessel membrane that causes the PRV to vent water on every heating cycle. Simply topping up the pressure repeatedly is not a fix — it could actually obscure a slow but worsening leak. A Gas Safe engineer should inspect the system if you need to repressurise more than once every couple of months.
What pressure should my Potterton boiler be at?
When the system is cold, the ideal pressure for most Potterton boilers is between 1.0 and 1.5 bar — 1.2 bar is a good target. Pressure will rise slightly (by around 0.3 bar) when the boiler is running and the water is hot; this is completely normal. If the pressure regularly climbs above 2.5 bar when hot, that suggests an expansion vessel issue rather than too much pressure in the system. The E118 lockout triggers when pressure falls below 0.5 bar.
How much does it cost to fix a Potterton E118 fault in the UK?
If you can top up the pressure yourself using the filling loop, there is no cost at all. A Gas Safe engineer callout for investigation, minor adjustments, or bleeding typically costs £80 to £150. Replacing a faulty pressure relief valve or expansion vessel usually falls in the range of £150 to £320 including parts and labour. In cases involving a hidden leak requiring specialist detection equipment, costs can rise further — potentially £400 or more — though this is less common. A faulty PCB is a rarer cause and can cost significantly more to replace, but this should only be considered once all other possibilities have been ruled out.