ATAG 109 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 109 fault code mean?
Error code 109 on an ATAG boiler is a pressure-related fault from the 100-series group. There is a genuine ambiguity between sources about its precise direction: the most likely interpretation, supported by multiple technical sources, is that it signals the system pressure has exceeded the maximum permitted level (sometimes described as 'Pressure > Pmax'). However, at least one Gas Safe engineer source associates this code with low pressure — specifically pressure falling below 1 bar or insufficient pump pressure increase. The key thing to do first is check your boiler's pressure gauge directly. If the needle is above 2.5–3 bar, you likely have a high-pressure condition. If it reads below 1 bar, this is a low-pressure situation. Both can trigger pressure-series codes on ATAG boilers, and both need addressing promptly to avoid further damage.
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
- Failed or waterlogged expansion vessel Common
The expansion vessel contains a pressurised diaphragm that absorbs the increase in water volume as the system heats up. If the diaphragm splits or the vessel loses its nitrogen charge, the system has nowhere to accommodate expanding water and pressure spikes rapidly — often causing the PRV to release water and triggering high-pressure codes. This is one of the most frequent causes of a Pmax-type fault.
- System leak causing low pressure Common
A slow or hidden leak anywhere in the pipework, radiators, or boiler connections causes the system to gradually lose water volume, dropping pressure below 1 bar. If pressure keeps falling back after you top it up, a leak is almost certainly present somewhere in the circuit.
- Overfilled system — too much water added via the filling loop Common
If the system has been topped up too aggressively via the filling loop — for example after a previous low-pressure fault — it can push pressure well above 1.5 bar, particularly once the boiler heats up and the water expands. This is an easy mistake to make and a common cause of high-pressure codes.
- Pressure relief valve (PRV) fault Sometimes
The PRV is a safety device that opens to dump water if pressure becomes dangerously high. A PRV that is stuck partially open will allow continuous weeping of water, causing pressure to fall. Conversely, a PRV that fails to open at all cannot protect the system against excess pressure. Signs include water dripping from the overflow pipe outside the property.
- Faulty pressure sensor or transducer Sometimes
The sensor that monitors system pressure and feeds readings to the PCB can give inaccurate readings if it has failed or become coated with debris. This can cause the boiler to report a pressure fault even when the actual system pressure is within normal range.
- Blockage or closed isolation valve Rare
A partially closed isolation valve or a blockage in the pipework can create a localised pressure spike in part of the circuit, causing the system to register an abnormally high pressure reading even if the overall system looks fine.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge on the front or underside of the boiler. Normal operating pressure for most ATAG boilers is between 1 and 1.5 bar, usually shown in green. A reading above 2.5–3 bar indicates high pressure; a reading below 1 bar indicates low pressure. This single observation will guide everything that follows.
- If pressure is low (below 1 bar): top up via the filling loop DIY safe
Locate the filling loop — usually a braided flexible hose with one or two valves connecting the cold mains supply to the heating circuit, often beneath the boiler. Slowly open the valve(s) and watch the gauge rise. Stop when the needle reaches around 1.2–1.5 bar. Close the valve(s) firmly. Consult your ATAG boiler manual for the exact filling procedure for your model, as designs vary.
- If pressure is high (above 2.5 bar): bleed a radiator to relieve pressure DIY safe
Use a radiator bleed key to open the bleed valve on the highest radiator in the house by a quarter turn. Hold a cloth beneath the valve. Air — and a small amount of water — will escape. Watch the pressure gauge and close the bleed valve once the reading drops to around 1.2–1.5 bar. Do not over-bleed; stop as soon as pressure is in the normal range.
- Reset the boiler and observe DIY safe
Once pressure is in the correct range, press the reset button on the boiler (refer to your ATAG manual for its location) and allow the boiler to restart. Monitor the pressure gauge over the next 20–30 minutes as the boiler heats up. If the fault returns, do not reset more than two or three times in total, as repeated resetting without fixing the underlying cause can mask a developing problem.
- Check for visible leaks around the boiler and pipework DIY safe
Inspect pipework joints, radiator valves, and any visible sections of the heating circuit for signs of moisture, staining, or dripping. Check outside your property for water weeping from the pressure relief valve outlet pipe — this indicates the PRV may be releasing excess pressure or may itself be faulty. If you find a leak, do not attempt to open sealed components; note where it is and report it to an engineer.
- If pressure drops repeatedly or the fault returns, call a Gas Safe registered engineer Gas Safe engineer
Recurring pressure loss points to a system leak or a failed component such as the expansion vessel, PRV, or pressure sensor. A Gas Safe engineer will pressure-test the system to locate leaks, check and re-pressurise the expansion vessel, replace a faulty PRV or pressure transducer, and carry out any internal boiler work safely. Do not attempt to open the boiler casing, replace components, or adjust internal parts yourself.
Parts you may need
- Expansion vessel (replacement) · from £60
- Pressure relief valve (PRV) · from £25
- Pressure sensor / transducer · from £35
- Filling loop assembly · from £18
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 £90–£350, depending on the underlying cause.
Frequently asked questions
Is ATAG fault code 109 a high-pressure or low-pressure fault?
Sources differ, which is genuinely confusing. The weight of evidence — including a Gas Safe engineer's interpretation and the literal description 'Pressure > Pmax' — points to high pressure as the primary meaning. However, at least one other source links 109 to low pressure. The safest approach is to check your pressure gauge immediately: above 2.5 bar confirms a high-pressure event; below 1 bar confirms low pressure. Both conditions sit within the ATAG 100-series pressure fault group and both need fixing.
Can I fix ATAG fault 109 myself?
Partly. If the fix is simply topping up a low system pressure via the filling loop, or bleeding a radiator to relieve slightly elevated pressure, those are homeowner-safe tasks. However, if the fault keeps returning, or if internal components such as the expansion vessel, pressure relief valve, or pressure sensor need replacement, you must use a Gas Safe registered engineer. Opening the boiler casing or working on sealed heating components without Gas Safe registration is unsafe and may void your warranty.
Could my expansion vessel be the cause — and how much does it cost to replace?
Yes — a failed expansion vessel diaphragm is one of the most common causes of recurring high-pressure faults. When the diaphragm splits, the vessel can no longer absorb the expansion of heated water, causing pressure to spike every time the boiler fires. Replacing an expansion vessel typically costs £120–£300 including parts and labour for a straightforward job, though this can rise if access to the boiler is difficult or if it is a larger vessel. Always check whether your ATAG boiler is still within its warranty period (ATAG offers a standard 10-year warranty) before paying for repairs.
Water is dripping from a pipe outside my house — is that related to fault code 109?
Very likely, yes. That external pipe is almost certainly the outlet for the pressure relief valve (PRV). When system pressure exceeds its maximum safe threshold, the PRV opens automatically to release water and protect the system — this is the correct and intended behaviour. It strongly suggests a high-pressure fault, consistent with the Pmax interpretation of code 109. Common root causes are a waterlogged expansion vessel or an overfilled system. A Gas Safe engineer needs to identify and fix the underlying cause; the PRV itself may also need replacing if it no longer seals properly after opening.