Overview
E118 on a Potterton Promax Ultra / Gold / Titanium means the boiler has detected that the system water pressure is too low. The boiler needs a minimum pressure in the central heating circuit to operate safely and to allow the pump to circulate water. When pressure falls below the allowed threshold the boiler will lock out or stop working to protect internal components (pump, heat exchanger) and to avoid dry-running. Low pressure commonly happens because water has been lost from the sealed heating circuit. Typical causes are a visible leak (radiator, pipe joint, valve), recent bleeding of radiators without topping the system back up, a leaking pressure relief valve (PRV), or internal component leaks (pump seals, heat exchanger). In some cases a faulty pressure sensor or wiring can give a false low-pressure reading. Severity: this is not usually immediately dangerous but it will stop heating and hot water and, if the underlying cause is a leak, can cause water damage and worsening component damage. If the system simply needs repressurising and holds pressure afterwards it is a straightforward DIY task for a competent homeowner. If pressure keeps dropping, you find leaks, you see water coming from the PRV, or you are unsure about any gas or internal work, you must call a Gas Safe registered engineer.
Possible Cause: System pressure too low
Troubleshooting Steps
Safety precautions:
- Do not remove the boiler casing, work on gas pipes, or attempt internal electrical repairs. Any work involving gas must be done by a Gas Safe registered engineer.
- Turn the boiler off and isolate electrical power at the mains before you touch pipework near the boiler. Allow the boiler to cool if it has been running.
- Keep electrical parts dry. Use towels to catch any drips and protect electrics.
- If you smell gas, evacuate immediately and call the emergency gas number.
Initial checks a homeowner can do:
1. Look at the pressure gauge on the boiler. Most Potterton boilers should be around 1.0–1.5 bar when cold. If it is below 1.0 bar, that is likely the cause of E118.
2. Visually inspect under and around the boiler, radiators, towel rails and visible pipework for signs of leaks or wet patches. Check the condensate/PRV discharge pipe outside or to a drain for recent leakage.
3. Think back: have you recently bled radiators or had any work done that could have released water? Bleeding radiators without topping up will reduce pressure.
4. Check the boiler display/manual for the recommended pressure and any model-specific instructions.
Diagnostic and fix steps you can follow:
1. If you have just bled radiators, or there is no visible leak, repressurising is the usual first step. Before repressurising, bleed radiators if they are air-bound (cold at the top). Bleeding will release air and may reduce pressure further, so you will top up after bleeding.
2. Locate the filling loop. On many boilers this is a flexible silver hose with two small valves or taps at either end, or an integrated filling connection with two isolation screws/valves. If you cannot find a filling loop or are unsure, stop and call an engineer.
3. Ensure both filling loop isolation valves are closed. Attach the filling loop if it is a removable hose and make sure both ends are tightened.
4. Slowly open the cold mains isolation valve(s) on the filling loop one at a time while watching the pressure gauge. Open only enough to allow water into the system. Do not rush.
5. Stop when the gauge reads about 1.0–1.5 bar (many operatives aim for ~1.2 bar cold). Do not over-pressurise. Close both filling loop valves fully and, if removable, remove the flexible hose and stow it (some systems require the hose to be removed to prevent accidental mains water backflow).
6. Wipe up any water, check for leaks again and restore boiler power. Use the boiler’s reset function if the E118 code is still displayed (follow the manual for the reset button and procedure; typically a short press or hold for a few seconds).
7. Turn the heating on and monitor: check that radiators heat correctly and monitor the pressure gauge during the next 24–48 hours. Note any drop in pressure and any visible leakage.
If pressure drops again or you find a leak:
- Look for visible water from radiator valves, pipe joints, under the boiler or from the PRV discharge pipe. If you can isolate a leaking radiator valve or section it off, do so to limit water loss, then call an engineer.
- If the PRV has discharged water, the PRV or system overpressure/thermal events may have caused it to open. A leaking PRV should be checked/replaced by an engineer.
- Persistent pressure loss usually means a leak or failing component (pump seal, heat exchanger) and requires a Gas Safe registered engineer to diagnose and repair.
If the gauge shows normal pressure but E118 remains:
- Try a boiler reset. If the fault persists, there may be a faulty pressure sensor/ switch or an electrical/PCB issue. Do not attempt internal electrical repairs; call a Gas Safe engineer.
What to tell the engineer when you call:
- Exact error code (E118), pressure readings before and after you topped up, whether you bled radiators recently, any visible leaks or discharge from the PRV, and whether the fault recurs or the pressure steadily drops.
Final note:
- Repressurising is a common DIY fix if the system holds pressure afterward, but any work involving gas, boiler internals, persistent leaks or recurring pressure loss must be handled by a Gas Safe registered engineer. Do not attempt to work on gas pipes, the burner, or internal components yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Promax Ultra, Gold/Titanium Combi & System.