Andrews A11 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews A11 fault code mean?
The A11 code appears on Andrews MAXXflo and Maxiflo commercial gas-fired water heaters when the flow temperature on burner module 1 has climbed above 95°C, causing that module to lock out as a safety measure. On these multi-burner units, the display alternates a letter with a number: the letter 'A' signals a lockout-type fault, and '11' identifies both the fault type (overtemperature) and the affected module (module 1). The unit has deliberately shut itself down to prevent a dangerous situation — the root cause must be identified and resolved before the system is returned to service. A single reset attempt is permissible, but if the unit locks out again immediately, a qualified engineer must investigate before any further resets are tried.
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
- Limescale build-up on the heat exchanger Common
In hard water areas, scale accumulates on heat exchanger surfaces and progressively reduces the rate at which heat transfers into the water. The burner continues to fire but the heat cannot dissipate efficiently, so local water temperature rises rapidly until the 95°C safety limit is breached. This is the single most common trigger for A11 on commercial water heaters in the UK.
- Circulation pump failure or reduced flow Common
If the pump slows, seizes, or fails entirely, water stops moving through the heat exchanger at the required rate. Heat builds up in the stationary or sluggish water, pushing the flow temperature above the safety threshold. Sludge and scale deposits can also restrict the pipework enough to mimic a pump failure.
- Faulty high-limit or safety thermostat Sometimes
The high-limit thermostat is designed to cut the burner before temperatures reach a dangerous level. If the thermostat has drifted out of calibration or developed an internal fault, it may fail to regulate correctly — either tripping too readily (causing repeated lockouts at normal temperatures) or failing to sense the true water temperature, allowing the flow to climb beyond the 95°C threshold before the safety circuit eventually responds.
- System blockage from sludge or debris Sometimes
Internal corrosion and scale can produce sludge that partially blocks pipework, manifolds, or the heat exchanger itself. Restricted flow has the same overheating effect as a failed pump and can be harder to identify without a thorough system inspection.
- Gas valve fault causing uncontrolled burner output Rare
A gas valve that sticks open or fails to modulate correctly can allow excessive gas flow to the burner, generating more heat than the water circuit can absorb and driving the flow temperature above the lockout threshold.
How to fix it
- Check the display and note the full fault code DIY safe
Confirm the display is showing A11 (alternating 'A' and '11'). Make a note of the code before touching anything — this confirms which module has locked out and that the fault is an overtemperature event rather than an ignition or gas-pressure issue.
- Attempt a single reset DIY safe
Press the reset button on the control panel once. Stand back and observe whether the unit fires and runs normally, or whether it locks out again within a short time. If it restarts and runs stably, monitor it closely over the next hour. If it locks out again, do not reset a second time — the underlying fault must be diagnosed first.
- Check the cold water supply and any isolation valves DIY safe
Ensure the cold water inlet isolation valve is fully open and that water pressure to the unit is within the normal operating range. A restricted water supply can reduce flow through the heat exchanger and contribute to overheating.
- Arrange a Gas Safe registered engineer to inspect the heat exchanger for scale Gas Safe engineer
On commercial storage water heaters in hard water regions, limescale is the most probable cause of A11. An engineer can carry out a chemical descale of the heat exchanger or recommend a power flush to restore efficient heat transfer. This work must not be attempted by unqualified personnel.
- Have the engineer inspect and test the circulation pump Gas Safe engineer
The engineer should check pump operation, flow rate, and whether sludge or scale is restricting circulation. A seized or underperforming pump will need replacement; a blocked circuit will need flushing before a new pump is fitted.
- Have the engineer test the high-limit thermostat and temperature sensors Gas Safe engineer
The thermostat and any associated NTC temperature sensors should be tested for correct calibration and continuity. Faulty components should be replaced with manufacturer-approved parts to ensure the safety circuit functions correctly.
- Have the engineer inspect the gas valve and burner output Gas Safe engineer
If limescale, pump, and thermostat checks do not reveal the cause, the gas valve should be tested for correct modulation. An uncontrolled gas valve can cause the burner to fire at excessive output. Gas valve work requires Gas Safe registration and proper gas-tightness testing after any intervention.
- Consider installing a scale inhibitor or water softener Gas Safe engineer
If limescale was identified as the cause, an engineer can fit an inline scale inhibitor on the cold water feed to the unit. This is particularly worthwhile in hard water areas and can significantly reduce the likelihood of repeat A11 lockouts.
- Contact Andrews Water Heaters before arranging third-party repair if under warranty DIY safe
Andrews provides a two-year parts-and-labour warranty on units commissioned by Andrews Water Heaters by Baxi. If the unit is still within this period, contact Andrews directly first, as third-party intervention may affect warranty cover.
Parts you may need
- High-limit safety thermostat · from £45
- NTC flow temperature sensor · from £35
- Circulation pump · from £180
- Gas valve (module 1) · from £220
- Slaveboard / module control board · from £290
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–£600, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the A11 code to get hot water?
You should limit yourself to one reset attempt. If the unit locks out again straight away, the safety system is detecting a genuine problem — repeated resets without resolving the underlying fault can stress components and, in the case of an overtemperature condition, could eventually cause damage to the heat exchanger or associated pipework. On a commercial water heater serving a hotel, care home, or similar premises, you also have a duty-of-care obligation to ensure the appliance is safe before returning it to use.
How much will it cost to repair an Andrews A11 fault?
Most A11 repairs fall in the range of £200–£600 once you include the engineer's time, any parts, and — where applicable — descaling or flushing. A thermostat or sensor replacement sits at the lower end, a pump replacement in the middle, and a gas valve or control board towards the upper end. Heat exchanger replacement is rare but can push costs above £600; at that point it is worth getting a quote for a new unit alongside the repair quote. If the heater is still within its two-year Andrews warranty, parts and labour should be covered at no cost.
What does the 'A' prefix mean on an Andrews fault code — and what would A21 or A31 mean?
On Andrews MAXXflo and Maxiflo units, the letter in the fault code indicates the fault category. 'A' denotes a lockout fault — the module has shut down due to a safety-critical condition such as overtemperature or ignition failure. The first digit after the letter identifies the burner module number, and the second digit indicates the fault type. So A11 is an overtemperature lockout on module 1; A21 would be an overtemperature lockout on module 2; A31 on module 3, and so on. If you see a different second digit (for example A12 or A13), this points to a different fault category on that same module — consult your Andrews service manual or the Andrews technical helpline for codes not listed here.
Does an Andrews water heater repair have to be done by a Gas Safe engineer?
Yes. Andrews themselves state that all installation and service work must be carried out by competent, qualified personnel, and working on the gas components of any gas appliance in the UK is a legal requirement that only Gas Safe registered engineers can fulfil. In a commercial setting, the duty holder (employer or building owner) also has obligations under the Gas Safety (Installation and Use) Regulations 1998 to ensure gas appliances are maintained by competent persons. Always ask to see an engineer's Gas Safe ID card before work begins.