Overview
E118 on a Potterton Assure (combi or system) boiler means the boiler has detected that the primary heating system water pressure is too low for safe operation. Boilers need a minimum system pressure to circulate water correctly and to avoid pump damage or overheating; when pressure falls below the set threshold the boiler will usually lock out and display this code. Common reasons for low pressure include a leak in the heating system, having recently bled radiators without repressurising, a faulty pressure relief valve (PRV) or expansion vessel problems, or simply that the system hasn’t been topped up for a long time. Severity: this fault is not usually immediately dangerous but it will prevent central heating and hot water from working and, if ignored, can cause pump strain or allow the boiler to overheat during attempts to run. Many homeowners can safely deal with straightforward causes like repressurising the system using the filling loop and checking for obvious leaks. However, if the pressure keeps dropping, the PRV is dripping, you smell gas, or the boiler won’t respond after repressurising and resetting, stop and call a Gas Safe registered engineer — internal gas or sealed-system repairs and component replacements must be carried out by a qualified professional.
Possible Cause: Primary system water pressure too low
Troubleshooting Steps
Safety precautions:
- If you smell gas anywhere, do not attempt any work on the boiler. Turn off the gas supply at the meter if safe to do so, evacuate the property and call the gas emergency number or a Gas Safe engineer immediately.
- Switch off electrical power to the boiler before touching any internal parts or valves. Avoid working on the boiler if the area is wet. Wear eye protection and gloves if you are handling radiator valves or water.
- Do not attempt to modify or bypass safety devices. Any repair to gas valves, PRV, expansion vessel or sealed-system components should be done by a Gas Safe registered engineer.
Initial homeowner checks you can do:
1. Check the pressure gauge on the front of the boiler. Typical cold-system pressure for Potterton boilers is about 1.0–1.5 bar. E118 usually appears when pressure drops well below 1 bar (some manuals state below 0.5 bar triggers the fault).
2. Look around the boiler, pipework, radiators and boiler cupboard for visible signs of leaks, wet spots, or corrosion. Check the PRV discharge pipe (plastic pipe running from the boiler to outside or to a drip tray) for drips or stains.
3. Check whether you, or someone in the household, recently bled radiators. Bleeding radiators can reduce system pressure and requires topping up afterwards.
4. Note whether the filling loop (a silver flexible hose or pair of valves) is fitted and whether its isolation valves are closed.
Specific diagnostic and fix steps (repressurising):
1. Obtain the boiler manual or identify the filling loop location on your model. Many Potterton Assure models have a built-in or removable filling loop beneath the casing or near the boiler valves.
2. Ensure the boiler is switched off (and cooled if recently running).
3. If your filling loop is removable, ensure it is connected correctly and both ends are secure. If it is a permanent fitted loop, ensure its valves are closed before starting.
4. Slowly open the filling loop isolating valve(s) to allow mains water into the system. Open very gradually and watch the pressure gauge. Do not rush or fully open at high flow — introduce water slowly to avoid water hammer or overfilling.
5. Stop when the pressure gauge shows about 1.0–1.5 bar (or as specified in your boiler manual). Do not exceed the recommended pressure (typically keep below about 2.5 bar). Close the filling loop valves fully and, if removable, remove or secure the filling loop as required by your model.
6. Bleed any radiators you recently aired to release trapped air, then recheck the boiler pressure and top up again if necessary to return to the correct range.
7. Turn the boiler back on and reset if required. Monitor operation and note whether the E118 code clears.
If repressurising does not work or pressure falls again:
- If you cannot raise the pressure (filling loop won’t let water in, valves stuck, or pressure won’t rise) or the filling loop is missing/damaged, stop and call a qualified engineer.
- If pressure rises but the code stays or the boiler locks out again, the pressure sensor (switch) or wiring could be faulty, or there may be internal faults — call a Gas Safe engineer.
- If the pressure slowly or quickly drops after topping up, you very likely have a leak, a faulty PRV (dripping discharge pipe), or a failing expansion vessel. Inspect for leaks again and have a registered engineer locate and repair the fault.
When to call a professional:
- Call a Gas Safe registered engineer if you smell gas, see persistent pressure loss, see continuous dripping from the PRV discharge, cannot repressurise, the boiler displays other fault codes, or you are unsure. Any component replacement (PRV, expansion vessel, pressure sensor, valves) or gas-side work must be carried out by a qualified engineer.
Notes and cautions:
- Do not overpressurise the system. Excessive pressure can cause damage and be difficult to reduce.
- Always follow the specific instructions in your boiler’s manual for your model. If in doubt, stop and contact a Gas Safe registered heating engineer for a safe diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.