Vaillant F.23: the temperature spread says water is barely moving
Vaillant F.23 reports too great a temperature spread, which means water circulation is too low. See what the boiler measured and what an owner can check.
What this means
Vaillant defines F.23 as the temperature spread being too great, or water circulation too low. The boiler measures flow and return temperature and compares them, and when the difference grows beyond what the appliance allows, it concludes that not enough water is passing through the heat exchanger to carry the heat away.
Equipment and model context
Vaillant ecoTEC gas boilers covered by the Vaillant United Kingdom fault code list
- ecoTEC plus combi
- ecoTEC plus system
- ecoTEC exclusive
The definition used here is Vaillant's own fault code list for ecoTEC appliances sold in the United Kingdom. Vaillant uses different fault numbering on other ranges and in other markets, and a Glow-worm appliance, though part of the same group, has its own published list. Check the code against the label inside your own casing flap.
What you may notice
- The boiler fires, runs briefly, and then shuts down with F.23 displayed, repeating over successive attempts.
- Pipework near the boiler becomes hot within a minute while radiators further away stay cool.
- Kettling, banging, or gurgling noises come from the boiler during the short period it runs.
Likely causes
- A pump that is not moving water at the rate the appliance needs, whether seized, set too low, or failing, leaves flow through the heat exchanger below design.
- Air trapped in the primary circuit occupies space that water should fill, so the pump circulates less than its setting suggests.
- Debris or corrosion products can restrict the heat exchanger or a valve, which the boiler experiences as low flow regardless of what the pump is doing.
Diagnostic paths
Radiators are cold while the boiler pipework is scorching
The flow pipe leaving the boiler heats rapidly within a minute of firing, yet radiators around the house stay cool or warm only at the top.
Note which radiators stay cold and how quickly the boiler pipework heats. Heat going into water that is not travelling is the essence of this fault.
The boiler is making noise as it fires
Banging, kettling, or a sound like a boiling kettle comes from inside the casing during the short run before the fault appears.
Stop using the heating and report the noise. Localised boiling in the heat exchanger is what happens when heat input outruns the water carrying it away.
It began after work on the heating system
A radiator was changed, a valve was fitted, or the system was drained and refilled shortly before the fault started appearing.
Say what was done. Air introduced during refilling and a valve left partly closed both produce this reading without any boiler component failing.
There is no flow meter in most domestic boilers, so circulation is inferred. Heat input is known, flow and return temperatures are measured, and the difference between them stands in for how much water is passing.
- Heat inputKnown from the firing rate
- Flow sensorReads water leaving the exchanger
- Return sensorReads water coming back
- The spreadWidens as circulation falls
| Condition | Why the spread widens | Where it is corrected |
|---|---|---|
| Pump underperforming | Less water carries the same heat | At the pump or its setting |
| Air in the circuit | Water is displaced by gas | Venting the system |
| Restricted circuit | Flow is throttled downstream | Valves, strainers, or the exchanger |
| Closed zone valves | The circuit has nowhere to go | Controls and valve actuators |
A boiler protecting a heat exchanger it cannot see into
The reason this fault exists has nothing to do with comfort. A heat exchanger with too little water passing through it develops local hot spots, and water in those spots can boil against the metal. That produces the noise people describe as kettling, deposits scale onto the surfaces, and over time it damages the exchanger. Shutting down on a wide temperature spread is the appliance refusing to continue doing that to itself.
This also explains why the fault repeats rather than clearing. Nothing about firing again changes how much water the pump is moving, so the same spread develops on the next attempt, often faster than the last as the system gets no chance to distribute heat.
The fault tends to be in the house, not the boiler
It is worth saying plainly because it changes what a homeowner should expect. The boiler is reporting a property of the water circuit, and that circuit is pipework, radiators, valves, and a pump distributed through the building. Sludge accumulated over years, a zone valve that has stopped opening, a pump set to its lowest speed after a repair, and air never vented after a refill are all conditions outside the appliance.
A Gas Safe engineer will measure the temperature difference and work outward from there. What helps them is a description of which radiators heat and which do not, because that maps the circuit in a way no measurement at the boiler can. The radiators cold at the bottom page covers what different cold patterns indicate, and the boiler losing pressure reference covers the related symptom that often accompanies air in a system.
What to give the technician
Tell the engineer the ecoTEC model and serial number, the F.23 code, which radiators heat and which do not, the system pressure reading, any noise from the boiler, and any recent heating work.
Safe homeowner checks
- Feel each radiator after a heating attempt and note which get warm, which stay cold, and whether the cold ones are cold at the top or the bottom.
- Check the pressure gauge and record the reading, since a system that has lost water often has air in its place.
- Confirm that radiator valves and any thermostatic heads have not been closed off, particularly on circuits recently worked on.
Is a reset appropriate?
Resetting allows another firing attempt against unchanged circulation, and each attempt puts the exchanger through the same conditions. Two attempts establish that the fault repeats; more than that is not diagnosis.
When to call a technician
- Setting or replacing a pump, venting a sealed primary circuit, power flushing, and testing flow and return sensors are Gas Safe registered tasks on this appliance.
- Kettling or banging from the boiler is a reason to stop using the heating rather than to keep trying, because boiling against the exchanger causes cumulative damage.
Questions people ask about this
Is Vaillant F.23 a pump problem?
A pump not moving enough water is one route to it, and so are air in the circuit, a restricted heat exchanger, and a valve that has not opened. The code reports the consequence rather than naming the component.
Can I bleed the radiators to fix F.23?
Bleeding removes air from radiators and is reasonable owner maintenance, but air can also sit in the boiler and the primary pipework where only an engineer can reach it. Bleeding may help and may not resolve the fault.
Why does the boiler get hot so quickly with this fault?
The same heat is going into far less water, so the temperature of that water rises much faster than it should. The speed at which the flow pipe heats is itself a sign of how little is circulating.
Evidence recordHow this page was checked
official manufacturer support article, operation manual · checked 2026-09-15
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Vaillant UK and Vaillant technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article, operation manual
- Scope of the definition
- Vaillant ecoTEC gas boilers covered by the Vaillant United Kingdom fault code list
- Last checked
- 2026-09-15