Vaillant F23 Fault Code Explained
Hertfordshire, Bedfordshire & Cambridgeshire
An F23 fault on a Vaillant boiler means the difference between the flow and return temperatures has become too large.
In plain terms, water is going out hot and coming back far too cold, which means it is not circulating as it should.
What F23 actually means
In a healthy system the water leaving the boiler and the water returning to it are reasonably close in temperature, because it is moving steadily around the circuit. A large gap means flow has slowed or stopped somewhere.
What you can safely check yourself
Check the pressure gauge. It should read 1 to 1.5 bar when the heating is cold. If it is low, repressurise using the filling loop, and bleed your radiators if any are cold at the top.
If the pressure is correct and the fault stays, stop there.
When to stop, and why
If pressure and bleeding do not clear it, stop. Continuing to run a boiler with poor circulation puts the heat exchanger under stress.
What causes it beyond the simple checks
A failing pump, a pump set too low, sludge in the system, partially closed valves, or a faulty thermistor. This is one of the codes where the underlying cause is often sludge, and treating only the symptom means it returns.
How Vantage can help
We are Gas Safe registered and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day.
Call us on 01462 506374. See also boiler repair and Vaillant boiler installation.
Frequently asked questions
Is F23 the same as F83?
They are related. Both point to circulation, measured slightly differently.
Will a power flush fix it?
If sludge is restricting flow, it addresses the cause.
Can I run the heating meanwhile?
No, the boiler will not run while the fault is displayed.
Related fault codes
Other Vaillant codes: Vaillant F22 · Vaillant F75 · Vaillant F28 · Vaillant F29 · Vaillant F27 · Vaillant F20 · Vaillant F83 · Vaillant F32 · Vaillant F62 · Vaillant F61 · Vaillant F72
The same problem on other makes (circulation and flow faults): Baxi E125
See also our guide to radiator not heating up, the full boiler advice section, and boiler repair.
In Vaillant’s own words
Vaillant describes F23 as the temperature spread between flow and return being too great.
In plain terms, the water leaving the boiler is far hotter than the water coming back. That normally means the water is not moving quickly enough, so the pump, trapped air or sludge are the usual suspects.
When to call an engineer
Still stuck? We repair this fault regularly across Hertfordshire, Bedfordshire and Cambridgeshire. Call 01462 506374 to get an engineer booked, or to talk it through first if you would rather. Genuine 24 hour cover, 365 days a year.
Need help today?
Call and speak to a person, any hour, any day of the year, or get a fixed price quote online.
Get a fixed price quote Call 01462 506 374Trusted & Accredited Professionals
What our customers say
Speak to a local engineer
Vantage Plumbing and Heating covers Hertfordshire, Bedfordshire and the surrounding areas. Call and talk to an engineer, or send us the details and we will come back to you with a price.
Gas Safe registered, number 956889. Emergency line answered 24/7.