What the flashing radiator symbol means
The radiator icon on a Vaillant display is the heating mode symbol. In every Vaillant operating manual we checked, a flashing heating symbol means the burner is lit and heating your radiators, so in most cases it is the boiler telling you it is doing its job.
The meaning of the steady (not flashing) symbol is where models differ, and it is the reason people get confused:
| Boiler and Vaillant document | Heating symbol steady | Heating symbol flashing |
|---|---|---|
| ecoTEC pro 24 and 28, operating instructions 0020244996 | There is a heat requirement | Burner on in heating mode |
| ecoTEC plus VU, VUW, VUI ../5-5, operating instructions 0020245033 | There is a heat requirement | Burner on in heating mode |
| New-generation ecoTEC plus CS/1-5, operating instructions 0020308119 | Burner off, no heat demand | Burner on, heat demand present |
So a steady radiator means the heating is calling but the burner is not lit yet on an ecoTEC pro or ../5-5, and no demand at all on a CS/1-5. On other models, such as the ecoTEC sustain or exclusive, check the operating instructions, as we have only confirmed these three.
When it is normal and when to look closer
A radiator symbol that flashes while the heating is running, and stops when the thermostat or timer is satisfied, is normal. On a cold day it can flash for long periods because the burner simply stays on.
It is worth looking more closely if:
- an F code appears on the display, because a fault code replaces the normal display on these models;
- the symbol flashes but the radiators stay cold or only lukewarm after 20 or 30 minutes;
- the pressure bars or pressure reading are flashing alongside it;
- the symbol is steady (on an ecoTEC pro or ../5-5) for a long time and the burner never seems to light;
The other symbols you may see next to it
Here is what the other common symbols mean, according to Vaillant's operating instructions:
- Hot water symbol (tap): on the ecoTEC pro and ../5-5, steady means hot water is being drawn before the burner lights and flashing means the burner is on for hot water. On the CS/1-5 it follows the same pattern as the radiator: steady means burner off, flashing means burner on. If hot water is the problem, see our Vaillant boiler no hot water guide.
- Burner symbol: on the ecoTEC pro and ../5-5, the burner is operating correctly.
- Pressure display: steady while the pressure is within the permitted range and flashing when it is outside it. The CS/1-5 shows the pressure in five bars.
- Spanner: maintenance is required. Vaillant says the boiler is not in fault mode and keeps running. On the CS/1-5, more detail appears as an I code.
- Anti-cycling symbol: on the ecoTEC pro and ../5-5, this shows the burner is being held off for a short time so it does not switch on and off too often. The CS/1-5 has no anti-cycling symbol; the nearest is a "Product is busy with a task" symbol.
- F followed by a number: a fault. It appears instead of the normal display.
The ecoTEC pro and ../5-5 also have status codes showing what the boiler is doing. The relevant ones are S.00 (no heat demand), S.04 (heating, burner on), S.08 (anti-cycling, with the time remaining), S.31 (summer mode) and S.34 (frost protection). S.31 and S.34 are not anti-cycling codes; S.08 is the one that explains a heating symbol waiting to light.
Flashing, but the radiators are cold
If the burner is on but the heat is not arriving, work through these:
Low system pressure
On the ecoTEC pro and ../5-5, Vaillant says the boiler switches off below 0.5 bar and shows F22, but pressure can be low enough to affect circulation before that point. A flashing pressure display is the giveaway.
Air trapped in the radiators
Radiators that are warm at the bottom and cold at the top usually have air in them. Bleeding fixes this, though it lowers the pressure, so you may need to top up afterwards.
Temperature set too low
If the heating flow temperature on the boiler is turned right down, the burner can run while the radiators only reach lukewarm. Check the heating temperature setting on the boiler and the setting on your room thermostat or app.
A pump or circulation fault
If water is not being pushed round the system, the boiler heats the water inside itself but the radiators stay cold. This often ends with a fault code such as F75. A pump fault is inside the sealed boiler and is a job for a Gas Safe registered engineer.
Checking the pressure and bleeding radiators
Vaillant's guidance on pressure varies slightly by generation:
| Model | What Vaillant recommends |
|---|---|
| ecoTEC pro and ecoTEC plus ../5-5 | 1.0 to 2.0 bar with the system cold; top up below 0.8 bar; boiler switches off below 0.5 bar |
| New-generation ecoTEC plus CS/1-5 | At least 1 bar; top up below 0.8 bar; maximum 2.5 bar; add 0.1 bar per metre of system height to the minimum |
On the CS/1-5 you can read the exact figure through the main menu under Information, then Water pressure. Vaillant's instructions expect your installer to show you where the filling device is. Open the valves slowly, watch the reading rise, then close them fully. Our Vaillant boiler pressure guide walks through it step by step.
To bleed a radiator, let the heating cool, open the bleed valve with a radiator key and close it once water appears. Then recheck the pressure.
Resetting, fault codes and when to call an engineer
A flashing radiator on its own is not a fault, so only reset when an F code is showing. The method differs by generation:
- ecoTEC pro and ../5-5: press the fault clearance button for one second. If the fault is still there after three attempts, Vaillant says to consult a competent person. On these models F.28 appears after three failed ignition attempts.
- New-generation CS/1-5: press and hold the reset button for more than three seconds, up to three times. On these boilers the ignition fault is shown as F.028, after five failed ignition attempts.
Codes often seen with heating complaints are F22 (low pressure), F28 (ignition failure), F29 (flame loss) and F75 (pump or water shortage). Our Vaillant fault codes hub explains each one.
When to call a Gas Safe registered engineer
Call an engineer if a fault code returns after two or three resets, the pressure keeps dropping, the radiators stay cold after bleeding and topping up, or you suspect the pump. Freeing or replacing a pump means working inside the sealed boiler, so it is never a DIY job.
Do not remove the boiler casing. If you smell gas, turn the supply off at the meter if it is safe to do so, open windows and call the National Gas Emergency Service on 0800 111 999.
Keeping the heating side healthy
Most "flashing but no heat" problems come down to water, air and sludge. Vaillant says an annual inspection by a competent person is a prerequisite for safe, reliable operation. The new-generation CS/1-5 manual also says the second year of its two-year guarantee depends on an annual service one year after installation. If your boiler has a longer guarantee, check its paperwork for the servicing conditions.
| Job | Typical UK cost |
|---|---|
| Annual boiler service | Typically £80 to £150 |
| Magnetic filter, fitted | Typically £120 to £260 |
| Chemical flush | Typically £150 to £350 |
| Powerflush | Typically £400 to £800 |
A magnetic filter catches iron sludge before it reaches the boiler, and a flush clears sludge already in the radiators. Ask your engineer about either if several radiators are cold at the bottom.