Overview
Error code 108 on an ATAG boiler means the central heating system pressure has dropped below the minimum safe level (below about 0.5 bar). The boiler locks out or refuses to run as a safety measure because low water pressure can cause the boiler to overheat or damage internal components. A related code, 1P4, indicates a slightly higher low-pressure range (about 0.5–0.8 bar) but the practical remedy is the same: re-pressurise the system to the correct cold pressure. This is commonly a simple, low-severity fault that homeowners can often fix themselves by topping up the system with the filling loop until the cold pressure is around 1.0–1.1 bar. However, if the pressure regularly drops, if you find visible leaks, if topping up does not clear the fault, or if other error codes appear, the underlying cause may be a leak, a faulty expansion vessel, a pressure relief valve problem or a sensor/pump fault — these need a qualified heating engineer. If in doubt, contact ATAG support on 0800 680 0100 or email info@atagheating.co.uk.
Possible Cause: Pressure needs filling to < 0.5 bar
Troubleshooting Steps
Safety precautions:
- Before you start, turn off the boiler via the controller and isolate any room thermostat or timed controls for heating. Do NOT attempt major repairs yourself — only re-pressurise when you have basic confidence. Keep children and pets away. If you spot a significant leak or water on the floor, switch the boiler off at the isolator and at the mains and turn off the water supply to the heating system if you can safely do so, then call a professional.
Initial checks a homeowner can do:
- Look at the boiler display: the pressure reading is normally shown in the top-right corner. If it reads well below 1.0 bar (and particularly below 0.5 bar) the 108 code is expected.
- Inspect around radiators, pipework, valves and under the boiler for any visible signs of water, corrosion or damp. Check the pressure relief valve (PRV) discharge pipe for leaks or recent water.
- Note whether the fault is one-off or recurring. If pressure drops again within days, do not keep repeatedly topping up without booking an engineer.
How to re-pressurise the system (typical ATAG iC / iC Economiser procedure):
1) Locate the filling loop: on many ATAG iC models it is a removable copper ‘U’ shaped tube that connects between a small blue and a black isolation valve under the boiler. Some iS models have an external filling loop. If you cannot find it, stop and call a qualified plumber/engineer.
2) Remove the plastic caps from the two isolation valves (if fitted). Fit the filling loop between the valves and hand-tighten the nuts (do not overtighten).
3) With the boiler powered on so you can watch the display, open the black tap a quarter turn, then slowly open the blue tap a quarter turn. You should hear water entering the system and see the pressure rise on the display.
4) Watch the pressure and stop filling when the cold pressure reaches about 1.0–1.1 bar (do not exceed 1.4 bar). Close the blue tap first, then the black tap.
5) Remove the filling loop if it is a removable type and replace any caps. Many manufacturers recommend leaving the loop disconnected when not in use to prevent accidental overfilling.
6) Return the boiler to normal operation. The 108 error should clear. If you see an “air purge active” message, allow the automatic purge program to complete (it can last several minutes).
If you overfill the system:
- If pressure goes too high and you receive code 109 or the gauge is above the recommended range, bleed a radiator until the pressure falls back to around 1.0–1.2 bar.
If the error persists or pressure will not hold:
- If topping up does not clear the 108 code, or the pressure drops again quickly, do not keep refilling. Possible causes include a leak in the system, a faulty expansion vessel, a leaking pressure relief valve, or a pressure sensor fault. These require a qualified heating engineer to diagnose and repair.
- If you find water leaking from a valve, radiator or pipe, turn the boiler off, isolate the water if possible and call an engineer.
When to call a professional:
- Call a Gas Safe registered heating engineer (or contact ATAG support) if you cannot locate or operate the filling loop, if there are visible leaks, if the boiler will not accept pressure, if the fault recurs repeatedly, or if other error codes appear. A trained engineer will check for leaks, test the expansion vessel and PRV, inspect sensors and pump operation and carry out safe repairs.
Contact information:
- If you need manufacturer support, ATAG can be contacted on 0800 680 0100 or info@atagheating.co.uk.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.