ATAG boilers
ATAG makes gas boilers for UK homes. Its model names all start with a small i. The iC is the combi, the iS is the system boiler and the iR is the regular boiler. The iC Economiser is a combi with heat recovery built in. It takes extra heat from the flue gases.
ATAG is known for long warranties. Its boilers come with 12 years as standard, and you can extend that to 18. The heat exchanger is the heart of the boiler. ATAG calls it the iCon, and it’s guaranteed for life.
Two ranges, two sets of codes
This is the most important thing to know about ATAG codes. They changed in 2019. ATAG publishes two lists: one for boilers made from 2015 to 2018, and one for 2019 and newer. The same number can mean different things on each.
Take code 118. On a 2015 to 2018 boiler, it means the pressure is too low. On a 2019 or newer boiler, it means a sensor check has failed. That’s a job for an engineer. On the newer boilers, low pressure shows as 108 or 1P4 instead.
The screen also shows a symbol next to the code:
- A spanner means a blocking code. It’s temporary and may clear by itself.
- A bell means the boiler has locked out. It needs a reset, or a visit from an engineer.
Not sure which list fits your boiler? Check its user guide, or ask ATAG.
ATAG pressure is lower than most
Most makers ask for 1 to 1.5 bar. ATAG asks for a bit less. It says the pressure should be about 1.0 to 1.1 bar when the water is cold. When the heating is hot, it can rise to around 1.8 to 2.4 bar. ATAG says that’s normal, because hot water expands.
When you top up, ATAG says not to go over 1.4 bar. If you overfill it, bleed a radiator to bring it down. Turn off any timers and thermostats before you start.
On iC models, the filling loop is a copper U-shaped tube with a nut at each end. It fits under the boiler. Some iS boilers use a braided hose on the pipes instead. Open the taps a quarter of a turn, and close them at the right pressure. Our guide shows how to top up low boiler pressure step by step.
Heating comes on at night?
ATAG has a tip for this one. Check the lowest temperature set on your room thermostat. If it’s set to 18°C, the heating will come on whenever the house cools below that. ATAG suggests a low setting, such as 8°C to 12°C.
ATAG gas boiler ranges
These are the gas boiler ranges on the ATAG website as of 21/10/2026.
- iC Economiser Combi
- iC Combination Combi
- iS System System
- iR Regular Regular
Common ATAG fault codes
Codes can differ between models and ages of boiler. Always check the user manual for your exact model.
| Code | What it means | What you can do |
|---|---|---|
108 | The pressure is under 0.5 bar (2019 and newer boilers). | Top up to about 1.0 to 1.1 bar with the heating cold. Don't go over 1.4 bar. |
1P4 | The pressure is between 0.5 and 0.8 bar (2019 and newer boilers). | Top up to about 1.0 to 1.1 bar with the heating cold. Don't go over 1.4 bar. |
118 | On 2019 and newer boilers, a sensor check has failed. On 2015 to 2018 boilers, the pressure is too low. | On an older boiler, check the gauge and top up. On a newer one, call ATAG or a Gas Safe engineer. |
109 | The pressure is too high (2019 and newer boilers). | Let the heating cool and check the gauge. If it's still high, call a Gas Safe engineer. |
501 | No flame was detected (2019 and newer boilers). | Check your other gas appliances and any prepayment meter. Reset once. If it comes back, call an engineer. |
304 | The boiler has been reset too many times (2019 and newer boilers). | Stop resetting it. Call ATAG or a Gas Safe engineer. |
ATAG manuals
ATAG puts its user guides and installation manuals on its own website. You can find ATAG manuals here. You'll need your model name. The GC number helps too. Our guide shows you how to find your boiler's model and GC number.
Stay safe. You can reset a boiler, top up its pressure and thaw a frozen condensate pipe yourself. Anything inside the case is a job for a Gas Safe registered engineer.
If you smell gas, leave the building and call 0800 111 999.