Vokera AL21 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL21 fault code mean?
The AL21 code on a Vokera boiler indicates that the temperature of water returning from the central heating system to the boiler is dangerously high. Under normal operation, water should lose a good portion of its heat as it circulates around your radiators; if it arrives back at the boiler still very hot, it suggests heat is not being shed around the system as it should be. Vokera's safety logic detects this condition and triggers an automatic lockout — shutting the boiler down before any damage can occur to the heat exchanger or other internal components. AL21 belongs to Vokera's overheat and temperature-fault group, which also includes codes AL10, AL20, AL26, AL28, and AL34. Note that code formats can vary slightly across the Vokera range: older models may display a simpler 'A' prefix, Mynute series boilers typically use 'AL' codes like this one, and newer Evolve boilers use 'E'-prefixed codes. Always confirm the code against your specific model's documentation.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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
If the pump has stopped working, slowed down, or become partially seized — often due to internal sludge build-up — hot water cannot circulate properly around the system. The return water never cools sufficiently before coming back to the boiler, triggering the overheat lockout. A failed pump is one of the most frequent causes of this code.
- Sludge or limescale restricting circulation Common
Over time, corrosion byproducts create a black sludge that settles in pipework, radiators, and the boiler itself. In hard-water areas, limescale from mineral deposits also narrows internal passages. Either problem reduces flow and causes heat to build up in the return circuit, mimicking or directly causing an overheat condition.
- Faulty return thermistor or temperature sensor Common
The thermistor responsible for measuring the return water temperature may have drifted out of calibration or failed entirely. If it is sending an incorrect reading to the PCB — reporting higher temperatures than actually exist — the boiler will lock out even when the physical water temperature is perfectly normal.
- Trapped air in the system Sometimes
Air locks in radiators or pipework reduce the volume of water circulating and create hot spots. This can cause uneven temperature distribution and, in some cases, cause the return temperature to register as abnormally high as flow is restricted.
- Faulty pressure relief valve (PRV) Sometimes
A PRV that is weeping or has partially opened can cause system pressure to drop unexpectedly, disrupting circulation and contributing to overheating episodes. While less commonly the primary cause, a failing PRV can make other underlying issues worse.
- Blocked or closed zone valves or radiator valves Sometimes
If a significant portion of the system's radiators or zones are shut off — by closed TRVs, a stuck zone valve, or a commissioning error — the boiler has nowhere to dissipate heat, and return temperatures climb rapidly.
How to fix it
- Switch the boiler off and allow it to cool down fully DIY safe
Do not attempt to reset or inspect the boiler while it is still hot. Turn it off at the programmer or thermostat and give it at least 20–30 minutes to cool. Attempting to reset a boiler that is still overheating will simply trigger the lockout again immediately.
- Bleed your radiators to release trapped air DIY safe
Starting with the radiators farthest from the boiler and working back towards it, use a radiator bleed key to open each bleed valve slightly until water (not air) comes out, then close it again. Air locks reduce circulation efficiency and can contribute to high return temperatures.
- Check the system pressure on the boiler's pressure gauge DIY safe
Most Vokera boilers operate correctly at 1.0–1.5 bar when cold. If the gauge reads below 1.0 bar, use the filling loop (the braided hose or valve under the boiler) to top up pressure to around 1.2 bar. Do not overfill beyond 2.0 bar. After topping up, re-bleed any radiators that release air.
- Attempt a single reset DIY safe
On button-equipped models, press and hold the reset button (marked 'R') for 3–5 seconds until the fault indicator stops flashing, then release and allow the boiler to restart. On dial-operated models, turn the selector to the reset or off/reset position, hold briefly, then return to the desired mode. Only attempt one or two resets — repeated resets without identifying the underlying fault can mask a genuine safety issue.
- Check that all radiator valves and zone valves are open DIY safe
Walk around your home and confirm that thermostatic radiator valves (TRVs) on radiators are open (not turned fully closed) and that any motorised zone valves are not stuck shut. A large portion of the system being closed off prevents heat from dissipating, which directly causes return-water overheating.
- If the fault returns, call a Gas Safe registered engineer Gas Safe engineer
A persistent AL21 after a reset, bled radiators, and correct pressure points to a component fault — most likely the pump, thermistor, sludge blockage, or PRV. An engineer will measure actual flow and return temperatures, test the thermistor resistance values against specification, assess pump output, and inspect the PRV. They may recommend a power flush to clear sludge and limescale if circulation is heavily restricted. Do not continue operating the boiler if it keeps locking out on this code.
Parts you may need
- Return thermistor / NTC temperature sensor · from £25
- Central heating circulation pump (compatible replacement) · from £85
- Pressure relief valve (PRV) · from £30
- Magnetic system filter (e.g. Magnaclean) · from £65
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 £220–£400, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler when AL21 appears?
You can try one or two resets after allowing the boiler to cool and checking pressure and radiator bleeding, but you should not keep resetting it repeatedly. AL21 is a safety lockout — the boiler has shut itself down to prevent damage. If it keeps coming back, there is an underlying issue that resetting will not fix, and continued resets could mask a worsening problem. At that point, you need a Gas Safe engineer to diagnose the root cause.
How much does it cost to fix a Vokera AL21 fault in the UK?
The most common repairs fall in the range of £220–£400 including parts and labour. Replacing a faulty thermistor typically costs around £225–£350, while a new pump tends to be £300–£400 depending on the model. A PRV replacement runs to roughly £300 all-in. A power flush to clear sludge and limescale is usually £300–£500 depending on the size of your system. Bear in mind that some engineers charge a call-out or diagnostic fee of £85–£150, which may or may not be included in the final repair cost. In rare cases where the PCB or heat exchanger is involved, costs can be significantly higher — at which point it is worth comparing repair costs against the price of a new boiler, especially if yours is over 10 years old.
What is the difference between AL21 and other Vokera overheat codes like AL20 or AL26?
All of these codes sit within Vokera's overheat and temperature-fault group, but they relate to different sensors or conditions. AL20 typically indicates a general overheat condition on the flow side, while AL21 specifically flags high temperature in the return water circuit. AL26 and AL28 relate to other thermal safety devices or sensor readings within the same protection group. The underlying investigation and remedies are similar — checking circulation, the pump, and thermistors — but an engineer will identify which specific sensor or component triggered the lockout based on the code displayed.
Could sludge in my heating system be causing the AL21 code?
Yes, this is a very common culprit. Black iron oxide sludge builds up inside older heating systems from internal corrosion of pipework and radiators. It tends to settle in the pump, heat exchanger, and lower sections of radiators, restricting flow and causing the boiler to overheat. If your radiators have cold spots at the bottom, your pump sounds noisy or laboured, or the system has never been flushed, sludge is a likely contributor. A Gas Safe engineer can assess whether a power flush is warranted and may also recommend fitting a magnetic system filter to prevent future build-up.