Vokera AL20 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL20 fault code mean?
The AL20 code appears on Vokèra boilers when the temperature of water returning from the central heating system to the boiler is detected as dangerously high. This triggers a safety lockout, shutting the boiler down to protect the heat exchanger, burner, and other internal components from heat damage. AL20 sits within a family of Vokèra overheat and temperature-related codes — alongside AL10, AL21, AL26, AL28, and AL34 — all of which relate to dry fire, overheat, or thermistor conditions. The exact code displayed can vary slightly by model: Mynute models commonly show AL-prefixed codes, while newer Evolve boilers may use E-format codes, but the underlying safety logic is consistent across the range.
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
The pump is responsible for keeping heated water moving around the system. If it slows, seizes, or fails entirely, hot water stagnates inside the boiler rather than circulating away, causing return temperatures to spike rapidly. This is the single most common trigger for AL20.
- Sludge or limescale blocking circulation Common
Over time, rust particles and debris accumulate in the system as magnetite sludge. This can clog the heat exchanger, restrict flow through pipework, and reduce the pump's effectiveness — all of which push temperatures upward. Systems that have never been power-flushed or had inhibitor added are especially vulnerable.
- Faulty return thermistor Sometimes
The return thermistor monitors water temperature as it comes back into the boiler. If the sensor itself has drifted out of calibration or developed a fault, it may report falsely elevated temperatures and trigger a lockout even when actual water temperatures are acceptable.
- Airlock in the system Sometimes
Trapped air pockets can interrupt water flow, creating hotspots and uneven circulation. This is more likely after recent work on the system, a period of low pressure, or if radiators have not been bled for some time.
- Blocked or scaled heat exchanger Sometimes
A heavily scaled or partially blocked heat exchanger reduces the transfer of heat away from the burner. Water inside the exchanger can overheat quickly, with the elevated temperature showing up on the return sensor as the hot water eventually moves through the circuit.
How to fix it
- Allow the boiler to cool down completely DIY safe
The AL20 lockout is a safety mechanism. Do not attempt to reset the boiler while it is still hot. Give it at least 15–20 minutes to cool before doing anything else.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when cold. If it is below 1 bar, top it up using the filling loop until it reaches the correct level, then close the filling loop valves securely.
- Bleed your radiators DIY safe
Trapped air can disrupt circulation and contribute to overheating. Use a radiator bleed key to release any air from each radiator, starting on the ground floor and working upward. Re-check the boiler pressure afterwards, as bleeding can cause it to drop slightly.
- Attempt a single reset DIY safe
Once the boiler has cooled and you have bled the radiators, press and hold the reset button (labelled R on most Vokèra models) for 3–5 seconds until the fault code stops flashing, then release it. On some models, move the mode selector to OFF/RESET and then back to your preferred heating mode. If the AL20 returns after one reset, do not keep resetting — this points to an underlying fault that needs professional attention.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
A qualified engineer will carry out a full diagnosis, testing the circulation pump for speed and output, checking both the flow and return thermistors for accuracy, inspecting the heat exchanger for limescale or blockage, and assessing whether a power flush is needed to clear sludge from the system. Only a Gas Safe registered engineer is legally permitted to work on gas appliances in the UK. You can verify any engineer's credentials at the Gas Safe Register website.
Parts you may need
- Return thermistor (temperature sensor) · from £25
- Circulation pump · from £80
- Pressure relief valve (PRV) · from £20
- Magnetic system filter (e.g. Fernox TF1 or Magnaclean) · from £55
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 £225–£450, depending on the underlying cause.
Frequently asked questions
Can I keep resetting the boiler to clear the AL20 code?
A single reset after the boiler has cooled is reasonable if this is the first time you have seen AL20. However, if the code comes back after a reset, repeated resetting will not fix the underlying problem and may mask a worsening issue. At that point you should stop resetting and call a Gas Safe registered engineer to investigate properly.
How much does it cost to fix a Vokèra AL20 fault?
For the most common repairs — thermistor replacement or pump replacement — most people pay somewhere between £225 and £450 including parts and labour. A power flush to clear system sludge typically adds £300–£500 on top of any component work. If the heat exchanger turns out to be the root cause, replacement costs can exceed £800–£1,000, though this is less common and usually only applies to older boilers where the whole appliance may be worth replacing instead.
What is the difference between AL20 and the other Vokèra AL codes like AL10, AL21, or AL26?
All of these codes fall within the same family of overheat, dry fire, and thermistor-related faults on Vokèra boilers. AL20 specifically points to high temperature detected in the return water circuit. AL10 typically relates to a flow overheat condition, while AL21, AL26, AL28, and AL34 can indicate variations such as dry fire detection or sensor faults at different points in the system. An engineer will use the specific code shown — along with live temperature readings and component tests — to pinpoint exactly which part of the circuit has triggered the lockout.
Could sludge in my system really cause an AL20 lockout?
Yes, and it is more common than many homeowners realise. Magnetite sludge — a black, gritty mixture of rust and debris — builds up in systems that have not had inhibitor added or been cleaned. It can clog the heat exchanger passages, coat pump impellers, and restrict flow through radiator circuits. All of these effects reduce how efficiently heat is carried away from the boiler, pushing return temperatures up. A power flush combined with a magnetic system filter and fresh inhibitor is the standard solution, and it will also improve your boiler's efficiency and reduce wear on the pump.