Vaillant F.24 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.24 fault code mean?
The F.24 code appears on Vaillant boilers when the PCB detects that water temperature is climbing too fast, almost always because water isn't moving around the system properly. Without adequate circulation the heat exchanger can't shed heat at a safe rate, so the boiler locks out to protect itself. Until the underlying circulation problem is fixed you'll have no heating and no hot water. The F.24 fault is common across a wide range of Vaillant models — ecoTEC Plus, ecoTEC Pro, TurboMAX, and ecoMAX — as well as some Glow-worm boilers built on the same platform.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Low system pressure Common
When system pressure drops below roughly 0.8 bar there isn't enough water in the circuit to circulate properly. The cold-fill target for most Vaillant boilers is 1.0–1.5 bar, and anything below that is the single most common trigger for F.24. Check the pressure gauge on the boiler front panel first.
- Faulty or seized circulation pump Common
The pump is responsible for pushing water around the circuit. If internal components have seized — often due to a long period of inactivity or a build-up of debris — water flow drops dramatically and F.24 is triggered. A pump that feels excessively hot or is making grinding or humming noises is a strong indicator.
- Airlocks in the system Common
Trapped air pockets in the pump, pipework, or radiators can partially or completely block water movement. Airlocks often appear after draining work, a pressure loss event, or at the start of the heating season.
- Closed or partially closed isolation valves Sometimes
The isolation valves on the boiler's flow and return pipes may have been left closed or only partly open after a service or repair. This is a simple oversight but it completely stops circulation and immediately triggers F.24.
- Sludge, limescale, or debris blockage Sometimes
Years of corrosion produce iron-oxide sludge; hard-water areas add limescale on top of that. Either can restrict flow through the heat exchanger, pump, or pipework to the point where the boiler overheats. Systems without a magnetic filter or chemical inhibitor are most at risk.
- Pump speed set too low Sometimes
Vaillant boilers have adjustable pump speed settings. If the speed was set incorrectly during installation or after a service — typically too low for the size of the property or heating circuit — flow rate can fall short of what the boiler needs, causing F.24.
- Wiring fault between pump and PCB Rare
Loose or corroded wiring connecting the pump to the PCB can cause the board to behave as though the pump has failed, even when the pump itself is perfectly functional. This produces a false F.24 signal.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, low pressure is very likely the cause of your F.24.
- Re-pressurise via the filling loop if pressure is low DIY safe
Locate the filling loop — usually a silver braided hose or a pair of taps beneath the boiler. Slowly open the valve(s) and watch the gauge rise. Stop when it reaches 1.2–1.5 bar, then close the valve(s) firmly. Never leave the filling loop open permanently.
- Reset the boiler DIY safe
Press and hold the reset button (usually marked with a flame or reset symbol) for about 3 seconds. Allow the boiler a minute or two to restart. If F.24 clears and the boiler runs normally, monitor the pressure over the following days — if it keeps dropping there may be a leak that needs a Gas Safe engineer.
- Check that isolation valves are fully open DIY safe
The isolation valves on the boiler's flow and return pipes should have their slot running in line with the pipe (open position). If any are across the pipe (closed), turn them so they are fully open, then attempt a reset.
- Bleed radiators to remove trapped air DIY safe
Turn the heating off and let radiators cool completely. Starting on the ground floor and working upward, use a radiator bleed key to open each bleed valve a quarter turn. Allow the hissing air to escape until water begins to trickle out, then close the valve. Re-check boiler pressure afterwards and top up if needed, then reset.
- Do not repeatedly reset without resolving the fault DIY safe
Resetting more than two or three times without fixing the root cause allows the heat exchanger to overheat repeatedly, which can cause cracks and lead to a very costly repair. If F.24 returns after one or two resets, stop and call an engineer.
- Call a Gas Safe registered engineer to inspect the pump, pipework, and controls Gas Safe engineer
If the homeowner checks above haven't cleared the fault — or if it keeps returning — a Gas Safe engineer needs to inspect and test the pump (checking for seizure, speed setting, and airlock), examine isolation valves and pipework for blockages, check wiring connections between the pump and PCB, and carry out a power flush if sludge is suspected. Never attempt to dismantle the pump, adjust internal boiler components, or work on the PCB yourself.
Parts you may need
- Circulation pump (Vaillant-compatible) · from £120
- Pump head seal / pump repair kit · from £25
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
- Central heating inhibitor (e.g. Fernox F1, 1 litre) · from £15
- Filling loop assembly (replacement) · from £20
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£450, depending on the underlying cause.
Frequently asked questions
Can I fix Vaillant F.24 myself?
Partly. Topping up the system pressure via the filling loop, bleeding radiators to remove airlocks, and checking that isolation valves are open are all safe DIY actions. These fix the majority of F.24 faults at no cost. However, anything involving the pump, internal boiler components, wiring, or a power flush must be done by a Gas Safe registered engineer — both for safety and to avoid invalidating your Vaillant warranty.
How much does it cost to fix a Vaillant F.24 fault in the UK?
If low pressure is the cause you can re-pressurise for free yourself, or pay around £80–£120 for an engineer call-out if you'd prefer professional help. A pump replacement typically costs £300–£450 including labour. A power flush to clear sludge usually runs £400–£700 depending on system size. For most people the total repair bill falls between £100 and £450. In rare cases where the heat exchanger or PCB has been damaged by repeated overheating, costs can exceed £500–£800, which is why it's important not to keep resetting without resolving the fault.
Why does my Vaillant boiler keep showing F.24 after I reset it?
A fault that returns within minutes or hours of resetting almost always means the underlying circulation problem hasn't been fixed. Common culprits are a failing pump, an airlock that bleeding radiators didn't fully clear, a partially closed valve, or a growing sludge blockage. Each time the boiler locks out it's protecting the heat exchanger from damage — so rather than resetting again, it's better to call a Gas Safe engineer to diagnose the root cause properly.
Does the F.24 fault appear on Glow-worm boilers too?
Yes. Because Vaillant owns Glow-worm, a number of Glow-worm boilers share the same underlying platform and control logic. These models display F.24 with the same meaning — insufficient water circulation causing a rapid temperature rise. The diagnostic steps and likely causes are identical to those for Vaillant-branded boilers.