Overview
Error 118 on ATAG iC / iS combi boilers means the boiler has detected water pressure that is too low (typically below about 1.0 bar when cold) or it has detected that the pump pressure increase is too low (effectively no pump detection). Low system pressure prevents the boiler from operating safely and will usually cause the boiler to lock out or refuse to fire for central heating and/or hot water. The most common cause is simply loss of water from the heating system (leak, recent bleeding, or a recently drained system). Less commonly it can be caused by a faulty filling loop, a leaking pressure relief valve, a failed expansion vessel, or a circulation/pump fault or wiring/PCB fault that prevents the boiler sensing pump pressure rise. Severity is moderate: the boiler will typically shut down until the pressure is corrected, so you will lose heating/hot water until resolved. Re-pressurising the system is a routine, homeowner-level task if you are comfortable with basic plumbing actions and can follow safe steps (filling loop operation). However, if the code returns after repressurising, you find visible leaks, the filling loop is missing/damaged, the pressure repeatedly drops, or you suspect a pump/electrical fault, you must call a competent Gas Safe registered engineer or ATAG support because further diagnosis and potentially sealed-system repairs or electrical work will be required.
Possible Cause: Pressure less than 1 bar or pump pressure increase too low (no pump detection).
Troubleshooting Steps
Safety precautions:
- Only carry out simple checks and re-pressurising if you are confident and comfortable. If you smell gas, see significant water leaks, or are unsure, stop and call a Gas Safe engineer immediately.
- Turn off timers/room thermostat so the boiler isn’t trying to run while you work. It’s safer to work with the boiler powered but idle; if you choose to isolate electrical supply for other work, switch off at the fused spur or isolator.
- Allow the system to cool if it’s been running before you bleed radiators or open anything; hot water can scald.
- Never force or overtighten the filling loop nuts with tools; hand-tight is advised. Replace the caps on isolation valves when finished.
Initial checks a homeowner can do:
1. Check the pressure reading. Two ways: view the analogue gauge under the boiler (bottom left) or get the digital pressure via the display: press and hold the ECO button for 6 seconds, then press + until the display shows A6 — the screen will alternate with the current cold system pressure. Cold pressure target is about 1.0–1.2 bar.
2. If pressure is below 1.0 bar (or the display shows 118), look under the boiler for the filling loop area (iC models have a filling loop under the boiler between a blue and a black isolation valve; iS models may use an external filling loop). Check for visible leaks around valves, radiators, the boiler base, and the pressure relief discharge pipe.
Specific diagnostic and fix steps (re-pressurising the heating system):
1. Locate the filling loop (a removable copper U-tube with two nuts) or the two isolated taps/caps. If it’s not already connected, fit the loop between the blue and black valve connections and hand-tighten the nuts.
2. Unscrew and remove any protective caps on the valves. Open the valves slowly a quarter turn: open the black tap first (towards you) then slowly open the blue tap until you hear water entering the system. Do not open fully; a quarter-turn is usually sufficient.
3. Watch the digital pressure readout (A6) or the analogue gauge and stop filling when the cold pressure reaches about 1.0–1.2 bar. Do not exceed 1.4 bar. If you cannot see the gauge, listen for the filling noise and check frequently.
4. Close the valves in reverse order (blue first, then black). Tighten the filling loop nuts by hand and replace the protective caps. On some installations the filling loop can remain fitted, but if it’s designed to be removed leave it off unless instructed otherwise.
5. Press the minus button (radiator button) on the boiler panel to clear the error. The boiler may run an aeration/venting program (code 105) for a few minutes to purge air. After this the display should show OK or normal operating screen and the error should clear.
If you overfill:
- If the pressure goes above ~1.4–1.5 bar and the boiler shows code 117 or high pressure, bleed a radiator until the pressure drops back to between 1.0 and 1.2 bar (ensure system is cool before bleeding).
If 118 persists or pressure falls again after topping up:
- Check for visible leaks on pipework, radiators, valves and the boiler itself. A persistent drop indicates a leak, a faulty pressure relief valve, or faulty expansion vessel.
- If you hear no pump running (no vibration/sound) or the boiler indicates 'no pump detection' behaviour, the pump or its wiring/control could be faulty. Do not attempt internal electrical repairs yourself.
- If the filling loop or isolation valves are damaged or missing, or if the boiler repeatedly loses pressure, stop and call a Gas Safe registered heating engineer.
When to call a professional:
- Call a Gas Safe registered engineer or ATAG support if: the pressure will not hold after repressurising; you find a leak; the pump appears not to run or the boiler continues to report 'no pump detection'; the filling loop or valves are damaged; or you are unsure about any step. Do not attempt to replace pump, expansion vessel, pressure relief valve or internal electrical components yourself. If you need ATAG technical help, their homeowner support number in the documentation is 0800 680 0100 (options 1 then 1) but use a Gas Safe registered engineer for on-site repairs.
Final notes:
- Re-pressurising is a common DIY fix for a one-off low-pressure lockout, but repeated issues indicate a fault that needs professional diagnosis. Keep a record of times and circumstances of pressure loss to help the engineer. Always prioritise safety and professional help when in doubt.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag IC & IS Combi Boiler.