Andrews A20 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews A20 fault code mean?
On Andrews commercial gas-fired water heaters, fault codes are displayed as a letter followed by a number. The letter 'A' indicates a lockout condition — meaning the burner has shut itself down and will not restart until the fault is cleared and the unit is manually reset. The number '20' identifies the specific fault within the Honeywell integrated burner controller used across Andrews ranges such as the MAXXflo, HIflo EVO, ClassicFlo, and COMBIflo. An A20 code therefore signals that the burner attempted to light but either failed to establish a flame or lost it during operation, triggering a safety lockout. Because Andrews units are commercial appliances — found in hotels, apartment blocks, schools, and large commercial premises — this fault must be treated seriously and, in almost all cases, requires a Gas Safe registered engineer with commercial gas competency to diagnose and repair.
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
- Ignition failure or faulty spark electrode Common
The most frequent cause of an A20 lockout. If the spark electrode is worn, misaligned, or fouled with carbon deposits, it cannot reliably ignite the burner. The Honeywell controller allows a set number of ignition attempts before locking out, and A20 is the result when none of those attempts succeed. Electrode replacement or realignment is required.
- Dirty or degraded flame sensor (ionisation probe) Common
The ionisation probe monitors whether a flame is present after ignition. Over time, particularly in commercial appliances running for long daily periods, the probe tip becomes coated with combustion residue, reducing its ability to detect the flame. If the controller cannot confirm a flame is present, it locks out as a safety precaution — even if the burner did actually light briefly.
- Gas supply interruption Common
If the gas supply to the building or the unit is interrupted — for example due to a closed isolation valve, a pressure drop from the network, or a meter issue — the burner cannot light and A20 will be triggered. Checking that other gas appliances on the same supply are working normally is a simple first step that can rule this out quickly.
- Flue or air supply blockage Sometimes
Andrews commercial water heaters rely on adequate combustion air and a clear flue path. A blocked or partially obstructed flue outlet, a failed or restricted air intake, or a bird nesting in the terminal can all disrupt the combustion process and cause the burner to fail to sustain a flame, resulting in lockout.
- Fan fault Sometimes
The combustion fan must run at the correct speed before and during burner operation. If the fan motor is failing, the bearing is worn, or wiring to the fan is damaged, the controller will not receive the correct air-proving signal and will prevent ignition — or will lock out shortly after the burner lights. Fan faults are more common on older or heavily used commercial units.
- Gas valve fault or PCB communication issue Rare
The gas valve opens on a signal from the controller PCB. If a relay on the PCB has failed, wiring is damaged, or the gas valve solenoid itself is faulty, gas may not reach the burner at all during an ignition attempt. PCB faults can also cause spurious lockouts where the hardware is physically sound but the controller misreads sensor data.
How to fix it
- Verify that the gas supply to the building is active DIY safe
Check whether other gas appliances in the same building — such as a gas hob or another boiler — are working normally. If nothing gas-powered is working, the issue is likely with the meter or incoming supply. Contact your gas supplier rather than attempting to reset the Andrews unit.
- Inspect the unit display and note the exact code shown DIY safe
Andrews units alternate the letter and the burner module number on the display. Make a note of the full code as shown — for example whether it reads 'A' then '20' alternately, or shows a different number. This information will be useful for the engineer and for the Andrews technical support team if you call them.
- Attempt a single reset using the unit's reset procedure DIY safe
Refer to the Andrews operating instructions for your specific model to locate the reset button or reset sequence. Press reset once and observe whether the burner lights and the unit returns to normal operation. If the A20 code returns immediately or within a short period, do not continue resetting — repeated lockouts on a commercial appliance indicate a genuine fault that resetting alone will not resolve.
- Do not open the casing or attempt internal inspection Gas Safe engineer
Andrews Water Heaters are commercial gas appliances. All internal servicing — including checking the spark electrode, flame sensor, gas valve, fan, or PCB — must be carried out by a Gas Safe registered engineer who holds the appropriate commercial gas qualifications (such as CIGA or COCN1/CODNCO1 as relevant). Unauthorised intervention can void any remaining warranty and may create a serious safety hazard.
- Contact Andrews Water Heaters technical support or a Gas Safe registered commercial gas engineer Gas Safe engineer
Andrews (part of the Baxi Heating group) operate a dedicated commercial technical support and service team. Alternatively, engage a Gas Safe registered engineer who can demonstrate commercial gas competency for water heater servicing. The engineer will typically inspect and clean the ionisation probe and spark electrode, test the gas valve operation, check the fan speed and air-proving circuit, and run a full combustion analysis before returning the unit to service.
Parts you may need
- Spark electrode and lead assembly · from £45
- Ionisation probe (flame sensor) · from £35
- Combustion fan motor · from £220
- Gas valve (Honeywell or equivalent) · from £275
- Honeywell burner controller / PCB · from £380
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 £200–£550, depending on the underlying cause.
Frequently asked questions
Is the Andrews A20 fault the same on all Andrews water heater models?
The 'A' prefix meaning — a burner lockout — is consistent across Andrews commercial water heater ranges including the MAXXflo, HIflo EVO, ClassicFlo, and COMBIflo. However, the precise fault numbered '20' can vary slightly depending on which generation of Honeywell integrated controller is fitted to your particular model. Always cross-reference with the fault code table in your specific model's installation and maintenance manual, or call the Andrews technical helpline with your model number to hand.
Can I reset the Andrews unit myself, and how many times is safe?
Yes, a single reset attempt is reasonable and is the first practical step. Locate the reset button as described in your Andrews operating guide and press it once. If the unit fires up and runs normally, monitor it over the next hour or so. If the A20 code returns, stop resetting — a unit that repeatedly locks out is signalling a genuine fault. Continuing to reset a faulty commercial water heater risks damaging components further and, more importantly, may mask a safety issue that requires proper diagnosis.
Why are Andrews repairs potentially more expensive than a standard domestic boiler repair?
Andrews units are commercial appliances designed for high-demand environments. Replacement parts — particularly Honeywell controller boards, gas valves, and fan assemblies specific to commercial water heaters — are more specialised and often more expensive than equivalent domestic boiler parts. Engineers must also hold appropriate commercial gas qualifications, and call-out rates for commercial heating contractors are generally higher than residential rates. For very old Andrews units, parts availability can be limited, which can increase both cost and lead time. Most A20 faults caused by a probe or electrode will sit at the lower end of the repair range; fan, gas valve, or PCB replacements will be at the higher end or beyond.
Should I call Andrews directly or find my own Gas Safe engineer?
Either route is valid. Andrews Water Heaters (part of the Baxi Heating group) have their own technical support and service network, which is often a sensible first call for commercial units as their engineers are factory-trained on the specific product range. If you use an independent Gas Safe engineer, ensure they hold commercial gas qualifications relevant to water heater servicing — not all domestic heating engineers are qualified or experienced on commercial Andrews units. Ask the engineer to confirm their commercial gas competency before booking.