Atag Boiler Fault Code 118: Low System Pressure
Fault code 118 on an Atag boiler signals that the central heating system pressure has dropped too low — typically below 0.8–1 bar. The boiler detects insufficient water pressure in the sealed system and shuts itself down as a precaution, cutting off both heating and hot water until the issue is resolved. In some cases, particularly on models such as the iC Econ 27, the code can also point to a problem with the water pressure sensor itself rather than genuine low pressure in the system.
General guidance only — not a substitute for professional advice. 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.
What does the ATAG 118 fault code mean?
Fault code 118 on an Atag boiler signals that the central heating system pressure has dropped too low — typically below 0.8–1 bar. The boiler detects insufficient water pressure in the sealed system and shuts itself down as a precaution, cutting off both heating and hot water until the issue is resolved. In some cases, particularly on models such as the iC Econ 27, the code can also point to a problem with the water pressure sensor itself rather than genuine low pressure in the system.
Common causes
- System pressure gradually lost over time Common
Sealed heating systems naturally lose a small amount of pressure as they age and cycle through heating and cooling. Even the tiniest weep from a joint, radiator valve, or fitting can cause a slow pressure drop that eventually triggers fault 118. This is the most frequent reason homeowners see this code.
- Radiators recently bled Common
Bleeding radiators to remove trapped air also releases a small volume of water from the system. This is enough to drop the pressure below the acceptable threshold, and the boiler will display code 118 until the system is repressurised via the filling loop.
- Faulty or undercharged expansion vessel Sometimes
The expansion vessel contains a pressurised air charge separated from the system water by a rubber diaphragm. If the diaphragm perishes or the air charge depletes, the vessel can no longer buffer pressure changes. Water may then be pushed out through the pressure relief valve, causing repeated pressure loss even after you top the system up.
- Leak in the heating system pipework or components Sometimes
A more substantial leak — at a radiator valve, pipe joint, pump seal, or heat exchanger — will cause pressure to drop quickly and repeatedly. The loss from just 1.5 bar down to 0.5 bar equates to roughly three litres of water escaping the system. In older properties, decommissioned pipework left connected can also be a hidden source of leaks.
- Faulty pressure sensor or wiring fault Rare
On certain Atag models, the pressure transducer or its associated wiring can develop a fault, causing the boiler to report low pressure even when the actual system pressure is within range. Corroded sensor connections are a particular culprit in older installations.
How to fix it
- Check the current system pressure DIY safe
Locate the pressure gauge on the underside of the boiler — typically on the bottom left. The needle should sit between 1.0 and 1.5 bar when the system is cold. Alternatively, on many Atag models you can access a digital pressure reading by pressing and holding the Eco button for around six seconds, then using the + button to scroll to parameter A6. A cold reading below 1.0 bar confirms the fault is pressure-related.
- Repressurise the system using the filling loop DIY safe
Find the filling loop — on iC and iC Economiser Plus models this is a copper U-shaped connector fitted between the blue and black isolation valves beneath the boiler. If it is not already connected, remove the end caps and hand-tighten it into position. Slowly open each valve a quarter turn and allow cold mains water to enter the system. Watch the pressure gauge rise and close both valves once it reaches 1.0–1.2 bar. Do not exceed 1.5 bar. Once finished, remove the filling loop and replace the end caps.
- Reset the boiler DIY safe
After repressurising, press the reset button and allow the boiler to restart. The display should return to normal operation and fault 118 should clear. If the pressure was only slightly low and the reset does not clear the code immediately, allow the boiler to complete one heating cycle and check again. Do not attempt more than two or three resets in quick succession.
- Monitor the pressure over the following days DIY safe
Keep an eye on the pressure gauge over the next week. A healthy sealed system should hold its pressure without needing to be topped up more than once or twice a year. If you find yourself repressurising every few days or weeks, there is an underlying problem that requires investigation — move on to the next step.
- Check for visible leaks around radiators, valves, and pipework DIY safe
Walk around your home and look carefully at all radiator valves, towel rail connections, and any visible pipe joints — particularly in airing cupboards, under floorboards if accessible, and around the boiler itself. Look for water stains, white limescale deposits, or damp patches on walls and ceilings. Also check whether the pressure relief valve discharge pipe (usually a copper pipe exiting the property externally) is dripping or wet. Any sign of moisture here confirms water is being expelled from the system. Note your findings and report them to the engineer.
- Call a Gas Safe registered engineer to investigate the root cause Gas Safe engineer
If the pressure keeps dropping after repressurising, or you have spotted signs of a leak, an engineer is needed. They can pressure-test the system to locate hidden leaks, inspect and recharge or replace the expansion vessel, assess the pressure relief valve, and test the pressure sensor and its wiring. Attempting to work on any of these components yourself is not safe and could void your warranty. Atag boilers typically carry a 10-year manufacturer's warranty — check whether your repair may be covered before booking, and ensure your annual service is up to date, as missed services can invalidate the warranty.
Parts you may need
- Expansion vessel (internal replacement) · from £85
- Pressure relief valve · from £35
- Water pressure sensor / transducer · from £45
- Filling loop assembly · from £18
- Pump seal kit · from £25
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Atag fault code 118 myself without calling an engineer?
If the cause is simply low pressure — which is the most common scenario — yes, you can repressurise the system yourself using the filling loop and reset the boiler, potentially at no cost. However, if the pressure drops again within a short period, that points to a leak, a failing expansion vessel, or another underlying fault that requires a Gas Safe registered engineer to diagnose and repair safely.
How do I check the pressure on my Atag boiler without a traditional gauge?
Many Atag models allow you to read the water pressure digitally from the control panel. Press and hold the Eco button for approximately six seconds, then press the + button to scroll through the parameters until you reach A6 — this displays the live system pressure in bar. A reading below 1.0 bar when the system is cold confirms the pressure is too low.
Why does my Atag boiler keep losing pressure and showing code 118?
Recurring pressure loss almost always means water is escaping from the sealed system somewhere. Common culprits include a small leak at a radiator valve or pipe joint, a pressure relief valve that is weeping, or a failed expansion vessel diaphragm that causes the relief valve to open repeatedly. A Gas Safe engineer can carry out a pressure test to find the source. Note that expansion vessel replacement typically costs £180–£380 including labour, and heat exchanger repairs — if the leak originates there — can cost considerably more.
Will topping up the pressure damage my Atag boiler?
Repressurising to the correct level (1.0–1.2 bar cold) is perfectly safe and is the intended fix for a genuine low-pressure event. The risk comes from overfilling: pressure above 2.5–3 bar when hot can cause the pressure relief valve to open and discharge water. Always stop filling at around 1.0–1.2 bar on a cold system and never exceed 1.5 bar during the filling process.