Vaillant Radiator Symbol Flashing? What It Means and When to Worry
A flashing radiator symbol on a Vaillant ecoTEC is usually normal: the burner is firing for the heating, or the boiler is pausing between cycles. How to tell which, and the signs that point to a real fault.
On most Vaillant ecoTEC boilers a flashing radiator symbol is not a fault. On current ecoTEC plus and ecoTEC pro models it means the burner is lit and heating the radiators; a steady symbol means heating mode is on but there is no demand. On older ecoTEC plus models with the earlier display, a flashing radiator symbol means the boiler is in its anti-cycling pause, a short wait between burner cycles that stops it switching on and off too often. It becomes a concern when the radiators stay cold while the symbol flashes, when the pressure bars are flashing too, when an F code appears, or when the boiler seems to be in anti-cycling most of the time.
Key Takeaways
- On current ecoTEC plus and ecoTEC pro displays, a flashing radiator symbol means the burner is on for the heating. Steady means heating is enabled with no demand.
- On older ecoTEC plus displays, a flashing radiator symbol means the anti-cycling timer is running. It clears on its own.
- The status code tells you exactly what the boiler is doing. S.04 or S.004 is burner on for heating; S.08 or S.008 is anti-cycling.
- Flashing pressure bars are a separate warning: Vaillant says to top up when the pressure is below 0.8 bar, if your system has a filling loop you have been shown how to use.
- Cold radiators with a flashing symbol, constant anti-cycling or an F code are worth getting looked at before the weather turns.
On most Vaillant ecoTEC boilers, a flashing radiator symbol is not a fault. On current ecoTEC plus and ecoTEC pro models it means the burner is lit and heating the radiators, while a steady symbol means heating mode is switched on but nothing is asking for heat. On older ecoTEC plus models with the earlier display, a flashing radiator symbol means the boiler is in its anti-cycling pause: a short, deliberate wait between burner cycles. Either way the boiler is working as designed. It only becomes a concern when the radiators stay cold while the symbol flashes, when the pressure bars flash as well, when an F code appears, or when the boiler seems to be pausing most of the time.
What the flashing radiator symbol means
Vaillant uses the radiator symbol for heating mode and the tap symbol for hot water. Both can be steady or flashing, and the meaning depends on the display generation. The table below is taken from Vaillant's own operating instructions.
| Boiler and display | Radiator symbol steady | Radiator symbol flashing |
|---|---|---|
| ecoTEC plus, current display (instructions dated 2023) | Heating mode on, burner off, no heat demand | Burner on, heat demand present |
| ecoTEC pro (VUW 6/5-3) | Heating mode heat requirement | Burner on in heating mode |
| ecoTEC plus, earlier display | Heating mode active | Anti-cycling mode active |
The tap symbol follows the same pattern for hot water. On the current ecoTEC plus it flashes when the burner is on for hot water, and on the earlier display a flashing tap means the hot water cylinder is calling for heat. None of these are warnings. A genuine fault replaces the normal display with an F code, such as F.22 for low water pressure or F.28 for a failed ignition.
Which display do you have?
The model name is printed on the front of the boiler, usually ecoTEC plus or ecoTEC pro followed by a number. If you're still not sure which display generation you have, the manual that came with the boiler will say, and the data plate carries the article number. Don't take any panels off to find it.
As a rough guide, the current ecoTEC plus has a menu system with a MAIN MENU and a five-bar pressure symbol along the top of the screen. The ecoTEC pro has a simpler display with its own separate symbol for the burner anti-cycling time. The earlier ecoTEC plus has a small LCD with fixed radiator, tap and flame symbols, and shows status codes when you press the i button.
Check the status code to be certain
The quickest way to know what the boiler is doing is to read its status code. On the current ecoTEC plus, go to MENU, then INFORMATION, then Status code. On older models, the status code appears when you press the information button. Vaillant's instructions list the codes that matter here:
- S.04 or S.004: heating mode, burner on. The boiler is heating the radiators.
- S.07 or S.007: heating mode, pump overrun. The burner has stopped and the pump is running on to move the last of the heat.
- S.08 or S.008: heating mode, anti-cycling time. The boiler is waiting before it relights. The ecoTEC pro shows the minutes remaining.
- S.31 or S.031: heating mode switched off and no hot water demand, for example in summer mode.
- S.34 or S.034: frost protection is active.
Write the code down along with the time. If you end up calling an engineer, a note like "S.08 for most of the evening, radiators lukewarm" tells them far more than "the radiator light keeps flashing".
Anti-cycling explained
When the radiators are already warm, the boiler can reach its target flow temperature within a minute or two of lighting. Without a pause it would switch off, cool slightly and relight again, over and over. That is called short cycling, and it wastes gas and wears the ignition parts. Vaillant's ecoTEC pro instructions describe the anti-cycling time as a way to avoid frequent switching on and off, which increases the product's working life.
So a spell of anti-cycling after the heating has been running is expected. The time is set at the installer level, which Vaillant locks behind a code, so it isn't something to change from the user menu. You'll often see it in mild weather or when only a few radiators are calling for heat, because the boiler has so little heat to deliver.
When a flashing radiator symbol points to a problem
The symbol itself is normal. What matters is what the rest of the system is doing while it flashes.
- Radiators stay cold while the burner is on. If the boiler says it is heating but the radiators never warm up, the heat is not reaching them. Air in the system, a pump that is not circulating, sludge, a stuck zone valve or closed radiator valves are the usual suspects. Our guide to radiators not heating up covers the room-by-room checks.
- The boiler seems to be in anti-cycling all the time. Constant anti-cycling with lukewarm radiators suggests the boiler heats its own water very quickly but can't pass that heat on. Poor circulation, a partly blocked heat exchanger, a bypass set wrongly or a boiler much larger than the house needs can all do this. It isn't dangerous, but it is inefficient and worth an engineer's time.
- The pressure bars are flashing too. This is a separate warning that the filling pressure is outside the permitted range. Vaillant recommends at least 1 bar and says to top up below 0.8 bar. Low pressure can stop the heating working properly and may lead to fault F.22.
- An F code appears. F.83 means the burner lit but the flow and return sensors saw little or no temperature change, which usually points to a circulation problem. See our Vaillant F83 guide and F28 ignition guide for what each one means.
- The boiler keeps switching off completely. That is a different problem from anti-cycling. Our guide to a boiler that keeps turning off explains the common causes.
Safe checks you can do yourself
- Read the status code and note it with the time, as above.
- Check the pressure. On the current ecoTEC plus the value is shown at the bottom right of the basic display, or under MAIN MENU, INFORMATION, Water pressure. If it is below 0.8 bar and you have a filling loop you've been shown how to use, top up following the instructions for your model. Our guide to repressurising a boiler explains the general process and when to stop.
- Check the controls. Make sure the room thermostat is set above the current room temperature and the timer is on. If the heating works but only just, look at the heating flow temperature. Our guide to what temperature to set your boiler at explains the heating dial.
- Feel the radiators. Cold at the top usually means air, and bleeding them will help, but it will also drop the pressure, so check the gauge afterwards. Our guide to bleeding a radiator covers it step by step.
- Leave it for a few minutes. If the status code is S.08, the pause should end on its own. The burner should relight if the heating still needs heat.
Don't remove the boiler casing, adjust installer settings or keep resetting the boiler to clear a code. Vaillant's instructions are clear that work inside the boiler is for a competent person, and anything involving the gas side needs a Gas Safe registered engineer.
Radiators cold while the symbol flashes?
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When to call an engineer
Call a Gas Safe registered engineer if the radiators stay cold while the boiler reports that the burner is on, if the boiler sits in anti-cycling for most of the time the heating is on, if the pressure keeps dropping after you top it up, or if any F code comes back after a reset. It is much easier to sort a circulation or pressure problem in early autumn than on the first cold night of the year, when every heating engineer in London is busy.
When you book, give the boiler model, the status or fault code, the pressure reading and what the radiators are doing. Say whether the problem affects heating, hot water or both. That lets the engineer arrive with a sensible idea of what to check first. You can check anyone who works on your boiler through the Gas Safe Register, as the HSE explains.
We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Our boiler repair, no heating and boiler pressure services cover homes and flats across London, and our boiler error codes guide links to every Vaillant code we cover.
If you smell gas or a carbon monoxide alarm goes off, stop and treat it as an emergency: leave the property and call the National Gas Emergency Service on 0800 111 999.
Frequently Asked Questions
Why is the radiator symbol flashing on my Vaillant boiler?
On current ecoTEC plus and ecoTEC pro models it means the burner is lit and heating the radiators, which is normal. On older ecoTEC plus models with the earlier display, it means the boiler is in its anti-cycling pause between burner cycles, which is also normal and clears on its own. Check the status code to be sure which one you are seeing.
Is a flashing radiator symbol on a Vaillant boiler a fault?
Not on its own. Vaillant's operating instructions list it as a normal operating state, not a fault. A fault shows as an F code, such as F.22 or F.28, in place of the normal display. It is worth investigating if the radiators stay cold while the symbol flashes, the pressure bars are flashing too, or the boiler seems to spend most of its time in anti-cycling.
How long does Vaillant anti-cycling last?
The anti-cycling time is a setting the installer can adjust, so it varies from one installation to another. On ecoTEC pro boilers the status display shows the remaining anti-cycling time in minutes. It should end on its own and the burner should then relight if the heating still needs heat.
Can I turn off anti-cycling on my Vaillant boiler?
Not from the user menu. The anti-cycling time is set at the installer level, which Vaillant protects with a code because it should only be changed by someone with the expertise to do it. If the boiler seems to be in anti-cycling most of the time, the fix is usually finding why it heats up so quickly, not switching the timer off.
What does it mean when the pressure bars flash on a Vaillant boiler?
The five-bar pressure symbol flashes when the filling pressure is outside the permitted range. Vaillant recommends at least 1 bar and says to top up when it falls below 0.8 bar. If you have a filling loop and have been shown how to use it, follow your model's instructions. If the pressure keeps dropping, or you can't top it up, the system needs checking for a leak or a faulty part.
Sources and further reading
Source links checked 28 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.