A Vaillant F23 fault code means the boiler has shut itself down because water isn’t moving round the system properly. The boiler compares the temperature of the water leaving it (the flow) with the water coming back (the return). When the gap gets too big, it assumes heat isn’t being carried away and locks out. Low water pressure has its own code, F22, so don’t mix the two up.
A few checks are fine to try yourself. The pump, the sensors and anything behind the casing need a Gas Safe registered engineer.
What Does the Vaillant F23 Fault Code Mean?
On most Vaillant boilers, F23 (F.23 on some displays) is a water shortage or circulation fault. Codes can vary between models, so check the manual for yours.
The code tells you water isn’t circulating well. It doesn’t tell you why. The cause could be a pump that’s stuck or running slowly, air in the system, sludge or a blockage, or a temperature sensor giving a false reading.
What You’ll Notice
The display shows F23 and the boiler stops firing. Sometimes the heating dies completely, and sometimes it runs for a while before locking out again. Radiators may be cold or only warm in patches. If the pump hums but nothing moves, that can be the reason, and our guide to a central heating pump not working shows how to tell.
Vaillant F23 vs F22: What’s the Difference?
F22 is the low water pressure fault, so topping up the system is the right response there. F23 is the temperature spread fault and points to circulation. Pressure can still matter, because a badly under-filled system won’t circulate well, so check the gauge. If the pressure is fine and F23 stays on the display, topping up won’t help.
Common Causes of the Vaillant F23 Fault Code
| Cause | What It Can Look Like | Who Deals With It |
|---|---|---|
| Pump stuck, worn or running too slowly | Cold or slow-to-heat radiators, a pump that is silent, humming or noisy | Engineer |
| Air in the system | Gurgling, radiators cold at the top | You can bleed radiators. Engineer if it keeps coming back |
| Sludge or a blockage | Radiators cold at the bottom, poor circulation overall | Engineer |
| Closed or restricted valve | One or more radiators not heating | You can check radiator valves. Engineer for system valves |
| Faulty flow or return sensor, or its wiring | F23 keeps returning and no circulation problem turns up | Engineer |
| Expansion vessel problem (on some models) | Pressure that swings up and down as the system heats and cools | Engineer |
| Low system pressure | Gauge well below normal | You can top up as the manual describes. Engineer if it keeps dropping |
Can a Faulty Central Heating Pump Cause F23?
Yes, and an engineer will usually look at it early. The pump moves water round the system. If it’s seized, worn or running slowly, the flow temperature climbs while the return stays cool, which is the gap F23 reacts to. On a combi boiler the pump sits inside the casing, so only an engineer can check it. Our guide to the signs of a failing central heating pump covers what to look for.
How to Fix a Vaillant F23 Fault: What You Can Check Yourself
Stay on the outside of the boiler and take these in order.
Start with the pressure gauge. With the system cold, it should sit where your manual says, commonly around 1 to 1.5 bar. If it’s well below that, top it up with the filling loop as the manual describes. If it keeps dropping, there’s a leak somewhere and you’ll need an engineer.
Next, once the system has cooled, bleed any radiators that are cold at the top, then check the pressure again, because bleeding lowers it. Make sure every radiator valve is open.
Then reset the boiler once, following the manual. If F23 comes straight back, stop there. Resetting clears the message and nothing else, so repeating it won’t fix a circulation fault. If you can hear grinding or rattling from the boiler, our guide to a noisy central heating pump goes through what each sound can mean.
What Not to Do
- Don’t open the boiler casing or touch the pump, sensors or wiring.
- Don’t keep topping up the pressure hoping F23 will clear. It’s a circulation fault.
- Don’t reset the boiler over and over.
If you ever smell gas, turn off the supply if it’s safe to do so, open the windows and call the National Gas Emergency Service on 0800 111 999.
When to Call an Engineer
- F23 returns after a reset and bleeding the radiators
- The boiler gets hot but the radiators stay cold
- The pump is silent, noisy or humming without moving water
- The pressure keeps dropping
- Anything that means opening the casing
What an Engineer Will Check
An engineer will check that the pump is running and moving water at the right speed, test the flow and return sensors and their wiring, and look for air, sludge and restricted valves. They’ll then decide whether the pump needs repairing or replacing. F23 only shows the symptom, so finding the cause is most of the job.
How Much Does It Cost to Fix a Vaillant F23 Fault?
The cost depends on what’s behind the circulation problem and whether any parts need replacing. BeFix charges £95 an hour from 7AM to 5PM, £125 from 5PM to 9PM and £160 from 9PM to 7AM, all exclusive of VAT, with no call-out fee. Parts are charged separately from labour.
If the circulation pump needs replacing, the final cost depends on the boiler model, access and the right replacement part. Our guide to central heating pump replacement cost explains what affects the price.
How to Avoid F23 Coming Back
An annual boiler service gives an engineer the chance to check circulation before something fails. Bleed radiators when you notice cold spots, and if sludge is a problem, ask whether the system needs cleaning. A magnetic filter and the right inhibitor in the system water help keep sludge from building up. In summer, run the heating for a few minutes each month so the pump doesn’t stick.
Book a BeFix Engineer for a Vaillant F23 Fault
BeFix provides boiler fault diagnosis and boiler repair in London, including Vaillant F23 circulation faults. Our Gas Safe registered engineers can inspect the pump, temperature sensors and heating circuit to find the underlying problem.
We offer same-day appointments, subject to availability, and engineers typically arrive within 1 to 3 hours. All repair work comes with a 12-month workmanship warranty. Photos of the display and the pressure gauge, plus the boiler model and serial number, help us prepare.
You can also book a boiler repair online or email info@befix.co.uk with photos. We’re open Mon to Fri 7AM to 10PM and Sat and Sun 9AM to 8PM. BeFix is at 227B Nether Street, London N3 1NT.
FAQs
What does the Vaillant F23 fault code mean?
The temperature difference between the flow and return water has become too large, which points to poor circulation, and the boiler shuts down to protect itself. The code doesn’t say whether the pump, air, sludge or a sensor is to blame.
Is F23 a low pressure fault?
No, low water pressure is F22. Low pressure can still affect circulation, so check the gauge, but topping up won’t clear an F23 caused by the pump, air or sludge.
What does F23 mean on a boiler?
On a Vaillant boiler, F23 means the temperature difference between the flow and return water has become too large. The boiler has shut down because water isn’t circulating properly. Other makes use different codes, so check your manual if your boiler isn’t a Vaillant.
Can I fix an F23 fault myself?
You can check the pressure gauge, bleed cold radiators, make sure the radiator valves are open and reset the boiler once. The pump, the sensors and anything inside the casing are engineer jobs.
How do I reset a Vaillant F23 fault code?
Follow the reset steps in your boiler’s manual, and only do it once. If F23 comes straight back, the cause is still there, so stop resetting and call a Gas Safe registered engineer.
Why does F23 keep coming back?
A reset only clears the message. If F23 returns, the pump, trapped air, sludge or a sensor is still causing the problem and the boiler needs proper diagnosis.
Can a faulty pump cause F23?
Yes. A pump that is stuck, worn or running too slowly can stop water moving fast enough, and that can trigger F23.
Does F23 mean I need a new boiler?
Not by itself. F23 is a symptom, and many of the causes can be repaired. What’s needed depends on the diagnosis.
How much does it cost to fix F23 with BeFix?
It depends on the cause. BeFix charges £95, £125 or £160 an hour depending on the time of day, exclusive of VAT, with no call-out fee, and parts are charged separately. If the pump needs replacing, the cost depends on the boiler model, access and the right replacement part.
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