On this page
- The flame icon
- The radiator and tap icons
- The frost or snowflake icon
- The pressure gauge or bar reading
- The filling loop or tap-and-key icon
- Eco, comfort or modulation icons
- When an icon appears alongside an error code
- Why your specific model’s manual still matters
- Book with Gasmaster
- Frequently asked questions
Quick answer
Most modern boiler displays use a set of small icons alongside any numeric readout to show status at a glance: a flame symbol for active heating or hot water demand, a snowflake or frost symbol for frost protection active, a tap or droplet for hot water mode, a radiator symbol for central heating mode, and a pressure gauge icon or bar reading for system pressure. Exact icons vary by manufacturer, so your boiler’s own manual remains the definitive guide for your specific model.
Key takeaways
- Display icons are a quick-glance status system distinct from the numeric fault codes covered in our separate guide on error codes.
- A flame icon generally shows the burner is actively firing, useful for quickly confirming the boiler is genuinely working rather than just powered on.
- A frost or snowflake icon indicates frost protection has activated, explained in more depth in our dedicated frost protection guide.
- A pressure gauge symbol, or a simple bar figure, shows current system pressure, and most boilers use colour coding (often green for normal, red for low or high) to make this readable at a glance.
- Icons vary meaningfully between manufacturers and even between generations of the same brand’s range, so treat this guide as a general orientation rather than a definitive key for your exact model.
Every boiler display uses a slightly different set of icons, but most modern condensing combi and system boilers share a broadly similar visual language, and knowing roughly what you are looking at helps you understand your boiler’s status without needing to reach for the manual every time.
The flame icon
A flame symbol appearing on the display generally indicates the burner is actively firing, whether for central heating or hot water demand. This is a genuinely useful quick check: if you are troubleshooting a “no heat” complaint and the flame icon is not appearing when you would expect it to, that points towards an ignition problem rather than, for example, a circulation issue where the burner is firing fine but heat is not reaching the radiators.
The radiator and tap icons
Many boilers show a small radiator symbol when in central heating mode and a tap or water droplet symbol when in hot water mode, sometimes both simultaneously on a combi boiler briefly switching between the two. This helps you confirm which function the boiler currently believes it is serving, useful when diagnosing why, for example, hot water seems fine but heating does not, or vice versa.
The frost or snowflake icon
A snowflake or frost symbol indicates the boiler’s frost protection function has activated, firing the heating briefly to protect the boiler and its immediate pipework from freezing, overriding your normal programmed schedule. Seeing this appear during genuinely cold weather is expected and normal; seeing it appear when the weather is not particularly cold can be worth mentioning to an engineer, since it may indicate a sensor reading incorrectly.
The pressure gauge or bar reading
Most boilers show system pressure either as a simple analogue-style gauge graphic or a numeric bar reading, often colour-coded, commonly green for the normal operating range and red for too low or too high. If this consistently shows in the red low zone, your system likely needs topping up via the filling loop, covered in our guide on how to repressurise a combi boiler.
The filling loop or tap-and-key icon
Some boilers display a specific icon, often resembling a tap with a key or lever, when the system detects low pressure and is prompting you towards the filling loop procedure specifically, distinct from the general pressure gauge display. This is the boiler’s way of proactively flagging that a top-up is needed rather than you having to notice the pressure gauge reading low yourself.
Eco, comfort or modulation icons
Many current boilers display a small eco or leaf-style icon when running in a lower-output, more efficient modulation mode, and some show a comfort or boost icon when running at a higher output to meet demand quickly. These reflect the boiler’s internal modulation, automatically adjusting its output to match demand, rather than something you generally need to interact with directly.
When an icon appears alongside an error code
If you see an icon alongside an actual alphanumeric fault code rather than during normal operation, treat the numeric code as the primary diagnostic information; our guide on common boiler error codes covers what those codes generally mean by category, and any code involving ignition, flame, fan or flue faults needs a Gas Safe engineer rather than DIY troubleshooting.
Why your specific model’s manual still matters
Icon sets vary meaningfully between manufacturers, Worcester Bosch, Vaillant, Ideal, Baxi, Glow-worm and Viessmann each use their own visual language, and even generations within the same brand’s range can differ. This guide gives you a general orientation to what most boilers show, but your specific boiler’s handbook, or the quick-reference card often printed inside the front casing, remains the definitive source for your exact model.
Book with Gasmaster
Confused by something your boiler’s display is showing, or want a proper diagnosis rather than guesswork? Gasmaster Plumbing & Heating covers Stoke-on-Trent, Newcastle-under-Lyme and Cheadle for boiler servicing and fault diagnosis. Get in touch through our boiler service booking page.
Frequently asked questions
What does a flashing icon usually mean, compared with a solid one?
This varies by manufacturer, but a flashing icon often indicates an active or transitional state (such as frost protection currently firing), while a solid icon often indicates a steady status; check your specific model’s manual for the exact convention it uses.
Why does my boiler show both the radiator and tap icon at once?
On a combi boiler, this can happen briefly during normal operation as the boiler prioritises hot water demand over heating, or vice versa, and is not usually a fault in itself.
Is a red pressure icon always urgent?
A red low-pressure reading generally means the system needs topping up soon, which is straightforward to do yourself in most cases, though a pressure that drops repeatedly within days of topping up points to a leak worth having checked rather than continuing to top up indefinitely.
Do all boilers use the same icons?
No, icon sets vary between manufacturers and even between model generations within the same brand, so treat this guide as general orientation and your boiler’s own manual as the definitive reference.
Can I find a full icon key online for my specific boiler?
Manufacturer websites typically publish user guides and quick-reference materials for current models; your boiler’s original handbook, or the quick-reference card sometimes printed inside the boiler casing, is usually the most reliable source.
Sources
- Manufacturer published user guides (Worcester Bosch, Vaillant, Ideal, Baxi, Glow-worm, Viessmann)
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
