Boiler fault codes
Vaillant F24 Fault Code: The Boiler Heated Up Too Quickly
Published
Vaillant display
F24
What it means
The temperature inside the boiler rose faster than it should after the burner lit, which means water is not flowing through the heat exchanger properly. The boiler shuts down before it can overheat.
Safe to check yourself
- Check the pressure gauge reads between 1 and 1.5 bar with the heating cold (sealed systems)
- On an open-vented system, check the small feed tank in the loft has water in it
- Bleed radiators that are cold at the top
- Press reset once
Call an engineer if
- F24 comes back after one reset
- The boiler bangs, rumbles or hums before locking out
- You topped up the pressure and it has dropped again
- You smell gas: open windows, turn the meter off if it is safe to, keep off switches, get out and call 0800 111 999
F24 on a Vaillant boiler means the temperature is increasing too quickly. As soon as the burner lights, the water inside heats up faster than normal because it is not being pumped round, so the boiler shuts down. Air, low pressure or a failing pump are the usual reasons.
What causes F24 on a Vaillant?
A boiler expects the water temperature to climb at a steady rate once it fires. If water is barely moving through the heat exchanger, the same small amount keeps getting hotter and the temperature shoots up. The boiler reads that through its sensors and stops with F24 before anything boils.
Vaillant's installer manuals for ecoTEC plus models up to 2021 list a blocked or weak pump, air in the boiler, system pressure too low, and a non-return valve that is blocked or fitted the wrong way, and the 2021 edition adds the pump's plug and cable and the boiler's internal automatic air vent. The 2023 manual for the newest models keeps air, low pressure and the pump, and adds the flow and return sensors and their wiring, the diverter valve and the internal expansion vessel. Vaillant's homeowner page also names the expansion vessel.
Does it matter what kind of system I have?
Yes. Vaillant's homeowner advice for F24 splits by system type. For a combi or system boiler (sealed system, pressure gauge on the front), the checks below apply and a Vaillant engineer is the next step if they do not work. For an open-vented boiler, the kind with a feed tank in the loft and usually a hot water cylinder, Vaillant says to contact the installer, because circulation depends on how the boiler was plumbed into the rest of the system.
Open-vented systems have no pressure gauge to check. What you can check is the small tank in the loft that keeps the heating topped up. If it is empty, its float valve has probably stuck, and the heating will be running short of water.
What can I do safely tonight?
On a sealed system, read the gauge with the heating cold. Below 1 bar, top up through the filling loop to between 1 and 1.5 bar; our Vaillant boiler pressure guide covers how. Then bleed any radiator that is cold at the top and warm at the bottom, and check the pressure again afterwards, because bleeding lowers it.
Press reset once. If the boiler fires and runs normally for a full heating cycle, keep an eye on it. If it trips F24 again within minutes of lighting, stop there.
A boiler that bangs or rumbles like a kettle before showing F24 is boiling water in the heat exchanger. Switch it off at the controls and do not keep resetting it. The pump and air vent are inside the casing, and the non-return valve sits in the boiler or its pipework; none of them are householder checks.
What the engineer does, and typical costs
The engineer starts with pressure and air, running Vaillant's check programme P.0 to purge the boiler. Then they check whether the pump is running and at what setting, whether the internal air vent is working, the diverter valve and the temperature sensors, and on older models whether a non-return valve is blocked or back to front, which is worth mentioning if the boiler or pipework has recently been changed.
Purging air or freeing a stuck air vent is a short job. A new air vent is an inexpensive part. A new pump costs more: a complete genuine pump for a recent ecoTEC plus is listed at roughly £250 to £300 as a part alone. If the system water is full of sludge, the engineer may suggest a flush so the new pump lasts.
Our visit starts with the £60 call-out, and the repair price is agreed with you on site before any work starts. Book through Vaillant boiler repair.
Questions people ask
Is Vaillant F24 serious?
Often not: air or low pressure causes plenty of F24 lockouts, and both are easy to sort. It becomes serious if you keep resetting a boiler whose pump has failed, because the heat exchanger is being heated with too little water in it.
What is the difference between F24 and F23?
F24 watches how fast the temperature rises; F23 watches the gap between flow and return. Both mean poor circulation. If you have had both, a weak pump or a blockage is the likeliest cause.
Could F24 lead to other codes?
Yes. If the water does overheat, the boiler's overheat protection can trip and show F20. Deal with the circulation problem and both should stop.