Vaillant F.23 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.23 fault code mean?
The F.23 fault code appears on Vaillant boilers when the temperature gap between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water coming back) exceeds approximately 35°C. Under normal operation this difference should be modest, because water circulates steadily around the system. When the gap grows too large it tells the boiler that something is preventing proper circulation — water is being heated but not moving around the system efficiently enough. As a safety measure, the boiler triggers a lockout and shuts down to avoid overheating the heat exchanger or other internal components. The boiler cannot restart until the underlying problem has been resolved and, if necessary, a manual reset is performed.
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
- Faulty or seized circulation pump Common
The pump is responsible for pushing water around the heating circuit. If it has seized, is running below its rated speed, or has failed partially, water moves too slowly and the return temperature drops well below the flow temperature. This is the single most common cause of F.23 on Vaillant boilers.
- Central heating sludge or system blockage Common
Over time, corrosion by-products and limescale accumulate in pipework, radiators, and the pump itself. A partially blocked circuit restricts water flow in exactly the same way as a weak pump, creating the same excessive temperature differential.
- Airlock in the system Common
Trapped air pockets can form anywhere in the circuit — in radiators, the pump head, or towel rails. An airlock interrupts water flow locally or entirely, preventing the return water from picking up enough heat energy to keep the differential within limits.
- Low system pressure Common
If the system pressure has dropped below around 1.0 bar, there simply is not enough water in the circuit to circulate properly. The boiler heats a reduced volume very quickly, spiking the flow temperature while the return stays cool.
- Pump speed set too low Sometimes
Vaillant pumps typically have adjustable speed settings. If the pump speed was inadvertently set to a lower position during a service or installation, water moves through the circuit more slowly than the boiler expects, widening the temperature gap.
- Faulty NTC thermistor (flow or return temperature sensor) Sometimes
The boiler uses NTC thermistors to measure both flow and return temperatures. If one sensor is drifting or has failed, it can report a false temperature, making the boiler believe the differential is dangerously high even when circulation is perfectly normal.
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 is below 1.0 bar, use the filling loop (usually a braided hose beneath the boiler) to top up the pressure slowly until the gauge sits around 1.2 bar. Consult your boiler manual if you are unsure where the filling loop is located.
- Bleed your radiators to release any trapped air DIY safe
Starting on the ground floor and working upward, use a radiator bleed key to open each bleed valve slightly until water trickles out steadily (have a cloth ready). This removes airlocks that may be restricting water flow. Once finished, re-check the system pressure and top up again if needed, as bleeding can cause a small pressure drop.
- Reset the boiler DIY safe
Once you have addressed pressure and bled the radiators, reset the boiler by pressing and holding the reset button (typically marked with a flame symbol or a circular arrow) for around 3 seconds, or as described in your manual. Allow the boiler to complete its ignition sequence. Do not reset more than two or three times in quick succession — repeated failed resets can mask a worsening fault.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in the property (cooker, gas fire) are working normally. If there is no gas supply, contact your gas supplier rather than attempting any boiler work. If gas is present elsewhere but the fault persists, move on to the next step.
- Have a Gas Safe engineer inspect and test the circulation pump Gas Safe engineer
If the basic checks above have not cleared the fault, the most likely culprit is the pump. A registered engineer will test pump operation, check the speed setting, and determine whether the pump needs adjustment or full replacement. Pump replacement on a Vaillant boiler is not DIY work.
- Ask the engineer to power-flush or chemically clean the system if sludge is suspected Gas Safe engineer
If the pump is functioning correctly but flow is still restricted, a power flush carried out by a qualified engineer will clear sludge and debris from the pipework and radiators. A corrosion inhibitor should be added afterwards to slow future build-up.
- Have the NTC thermistors tested and replaced if faulty Gas Safe engineer
A Gas Safe engineer can use diagnostic equipment to read live temperature sensor values and compare them against actual pipe temperatures. A drifting or failed thermistor must be replaced with a genuine Vaillant part to restore accurate temperature monitoring.
- Call a Gas Safe registered engineer if the fault has not cleared Gas Safe engineer
If none of the homeowner checks have resolved the F.23 lockout, do not continue resetting the boiler. Contact a Gas Safe registered engineer to carry out a full diagnosis. You can verify an engineer's registration at GasSafeRegister.co.uk before booking.
Parts you may need
- Vaillant circulation pump (e.g. Grundfos UPS or equivalent OEM) · from £85
- Flow NTC thermistor (temperature sensor) · from £18
- Return NTC thermistor (temperature sensor) · from £18
- Magnetic system filter (e.g. Adey MagnaClean) · from £75
- Central heating inhibitor (1 litre) · from £12
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix a Vaillant F.23 fault myself?
There are a few safe checks you can do yourself: topping up the system pressure via the filling loop, bleeding radiators to remove airlocks, and performing a boiler reset. These resolve a meaningful proportion of F.23 faults at no cost. However, if those steps do not clear the lockout, the cause is most likely a failing pump, a sludged system, or a faulty thermistor — all of which need a Gas Safe registered engineer. Attempting to replace a pump or thermistor without Gas Safe registration is illegal and potentially dangerous.
How much does it cost to fix a Vaillant F.23 fault in the UK?
For most households, the repair falls somewhere between £100 and £350 once you include the engineer's callout, labour, and any parts. A simple pump speed adjustment or inhibitor top-up sits at the lower end, while a full pump replacement typically costs £200–£350 all-in. A power flush, if sludge is the cause, usually runs around £250–£400 depending on system size and your location. In rare cases where a PCB fault is mimicking a sensor fault, costs can rise higher — worth mentioning to your engineer if standard repairs have not worked.
Why does my Vaillant boiler keep showing F.23 after I reset it?
If F.23 returns shortly after each reset, the underlying circulation problem has not been resolved. The boiler is simply detecting the same fault each time it tries to run. Repeated resets without fixing the root cause can accelerate wear on components. You should have a Gas Safe engineer diagnose the system properly rather than continuing to reset.
Could low boiler pressure cause an F.23 fault?
Yes. If system pressure drops below about 1.0 bar there is insufficient water in the circuit, so the boiler heats the small volume present very rapidly and the return temperature lags far behind the flow temperature. Check the pressure gauge — if it is below 1.0 bar, top up to around 1.2 bar using the filling loop. If pressure keeps dropping, there is likely a leak somewhere in the system that a Gas Safe engineer should trace and fix.