Potterton E117 Fault Code: Causes, Fixes & Repair Costs
What does the Potterton E117 fault code mean?
The E117 code appears on Potterton boilers when the system water pressure has climbed above approximately 2.9 bar — well beyond the safe operating range. As a protective measure, the boiler shuts itself down and refuses to fire until the pressure is brought back within limits and the fault is cleared. Under normal conditions a cold heating system should sit between 1.0 and 1.5 bar, rising slightly to around 1.5–2.0 bar when fully warmed up. When pressure consistently exceeds that upper threshold and reaches the 2.9 bar trigger point, the boiler interprets this as a risk to internal components such as seals, pipework, and the heat exchanger, and locks out accordingly.
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
- Overfilled heating system Common
The most frequent cause of E117 is simply too much water in the system. This often happens after a homeowner tops up the pressure using the filling loop but adds more water than needed. Even a cold reading of 1.8–2.0 bar can push well past 2.9 bar once the system heats up and the water expands.
- Faulty or weeping filling loop valve Common
If the filling loop valve does not seat properly — due to worn washers, scale build-up, or a damaged seal — mains water can slowly trickle into the closed heating system. Over hours or days this creep of extra water steadily raises the pressure until E117 is triggered. The valve may look closed but still be passing water.
- Waterlogged or failed expansion vessel Common
The expansion vessel is a small pressurised tank designed to absorb the extra volume that water occupies as it heats up. If the internal diaphragm ruptures or the pre-charge pressure drops too low, the vessel can no longer do its job and system pressure spikes every time the boiler fires. This is a very common cause of repeatedly returning high-pressure faults.
- Faulty pressure relief valve (PRV) Sometimes
The PRV is the system's last line of defence — it should open and discharge water outside when pressure gets dangerously high. If the valve sticks closed rather than releasing, pressure has nowhere to go. Conversely, a PRV that has previously opened and then not reseated properly may be masking a bigger underlying pressure issue. Either condition can accompany an E117 lockout.
- Faulty pressure sensor or transducer Rare
The pressure sensor reads the system pressure and reports it to the boiler's control board. Scale deposits or electrical degradation in a damp boiler environment can cause the sensor to report a falsely high reading, triggering E117 even when the actual pressure is normal. This is less common but worth considering if all physical pressure checks appear fine.
How to fix it
- Check the pressure gauge reading DIY safe
Locate the pressure gauge on the boiler's front panel or pipework. If it reads above 2.5 bar with the heating cold, the system genuinely is overpressurised. If the gauge reads normal (1.0–1.5 bar cold) the fault may be sensor-related — note this down for the engineer.
- Switch the boiler off and let the system cool DIY safe
Turn the boiler off at the controls and allow it to cool for at least 30–60 minutes before doing anything else. Never attempt to bleed radiators or adjust anything on a hot pressurised system.
- Bleed one or two radiators to reduce pressure DIY safe
Using a radiator bleed key, carefully open the bleed valve on an upstairs radiator by a quarter turn. Hold a cloth or small container underneath to catch water. Air will escape first, then water will begin to drip. Watch the pressure gauge as you bleed — close the valve once the gauge drops to around 1.2–1.3 bar. Repeat on a second radiator if the pressure is still above 1.5 bar. Do not bleed below 1.0 bar.
- Check that the filling loop valve is fully closed DIY safe
Locate the filling loop — usually a flexible braided hose with one or two handles beneath the boiler. Both handles should be turned so they are perpendicular (at 90°) to the pipe, indicating they are shut. If a handle feels loose, turns easily, or the hose feels warm, the valve may be passing water and will need a Gas Safe engineer to inspect or replace it.
- Reset the boiler DIY safe
Once the pressure is back in the 1.0–1.5 bar range, press and hold the reset button on the boiler's control panel for 3–5 seconds until the fault code clears. Allow the boiler to attempt ignition. If E117 returns within a short period, do not keep resetting — repeated lockouts indicate an underlying fault that needs professional attention.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If bleeding and resetting do not clear E117 permanently, or if the pressure rises again within a day or two, a Gas Safe engineer is needed. They will check the expansion vessel pre-charge pressure and recharge or replace it if necessary, inspect and replace a faulty filling loop valve, test and replace the pressure relief valve or pressure sensor, and carry out a full system health check to establish why pressure keeps climbing.
Parts you may need
- Expansion vessel (8–12 litre, combi boiler) · from £45
- Expansion vessel diaphragm kit · from £18
- Filling loop assembly (replacement) · from £25
- Pressure relief valve (3 bar PRV) · from £20
- Pressure sensor / transducer · from £35
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 £100–£300, depending on the underlying cause.
Frequently asked questions
Can I fix Potterton E117 myself without calling an engineer?
Some initial steps are safe for a homeowner to try: checking the pressure gauge, bleeding a radiator to bring the pressure down to around 1.2 bar, making sure the filling loop is closed, and doing a single reset. If the pressure drops to a normal level and stays there, the boiler may well be fine. However, if the fault returns, or if the pressure keeps climbing on its own, the underlying cause — a failed expansion vessel, a leaking filling loop valve, or a faulty PRV — needs a Gas Safe registered engineer to diagnose and fix properly.
Why does my Potterton boiler keep going over pressure even after I bleed the radiators?
If the pressure creeps back up after you've bled it down, something is continuously adding water to the system. The most likely culprit is a filling loop valve that isn't sealing properly, allowing mains water to slowly trickle in. A failed expansion vessel diaphragm is another common reason — without a functioning vessel to absorb expansion, pressure spikes every time the boiler heats up. Both faults need a Gas Safe engineer to confirm and fix.
Is high boiler pressure dangerous?
Sustained high pressure puts extra stress on seals, pipe joints, and the heat exchanger, which can lead to leaks over time. Modern boilers like Potterton models have a pressure relief valve that should discharge water safely outside the property if pressure becomes extreme, but a PRV that has opened once may not reseal reliably. This is why the boiler locks out at E117 — it is a built-in safety measure. Don't ignore it or repeatedly reset without addressing the root cause.
How much does it cost to fix a Potterton E117 fault?
Most E117 repairs in the UK fall between £100 and £300 including labour. Recharging or replacing an expansion vessel typically costs £100–£200, a filling loop valve replacement around £100–£200, and a pressure relief valve or pressure sensor replacement in the £150–£250 range. If a more significant component such as the PCB or heat exchanger is involved (which is uncommon for a pressure fault), costs can be higher — discuss this with your engineer before authorising any major work, especially on an older boiler.