Andrews A15 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews A15 fault code mean?
On Andrews commercial water heaters and boilers (including the MAXXflo and CWH range), fault codes beginning with the letter 'A' indicate a lockout condition — the appliance has shut itself down as a safety precaution and will not restart without investigation and a manual reset. Code A15 specifically points to the air pressure switch failing to confirm that the fan is running correctly within the expected timeframe (typically around 20 seconds after fan start). The air pressure switch is a critical safety device: it monitors whether the fan is generating sufficient draught to safely expel combustion gases through the flue. If the switch does not detect the expected pressure differential — for any reason — the appliance locks out rather than risk firing without proper ventilation. Note that Andrews appliances cover several product families (MAXXflo, CWH, ClassicFlo, ECOflo); while A15 most commonly relates to the air pressure switch circuit on MAXXflo and CWH series units, always cross-reference with your specific model's technical manual or contact Andrews' technical helpline (part of Baxi Heating UK) to confirm the code definition for your exact appliance.
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
- Fan not running or running below required speed Common
The fan must spin up to operating speed before the air pressure switch can detect a sufficient pressure differential across the sensing ports. If the fan motor is seized, partially blocked by debris, or simply worn out and running too slowly, the switch will never receive the signal it needs and the lockout is triggered. This is the single most frequent root cause of an A15 on Andrews commercial appliances.
- Blocked or kinked pressure sensing hoses Common
Flexible rubber or plastic hoses carry the pressure signal from the fan assembly to the air pressure switch. If any of these tubes are kinked, cracked, disconnected at a joint, or blocked with condensate, the switch reads zero pressure regardless of whether the fan is actually running. Hose degradation is particularly common on older units.
- Faulty air pressure switch Common
The switch itself contains a small flexible diaphragm that deflects when the fan creates a pressure difference. Over time the diaphragm can harden, split, or stick, causing the switch to fail to close its electrical contacts even when fan conditions are correct. Switches can also suffer from internal moisture ingress, especially on appliances installed in damp plant rooms or commercial kitchens.
- Blocked flue or air intake Sometimes
Any obstruction in the flue outlet or combustion air intake — bird nests, debris, ice in winter, or a collapsed flue liner — reduces the draught and may prevent the fan from generating the pressure differential the switch is looking for. On commercial installations the flue runs can be long, and partial blockages may only show up intermittently.
- Wiring or connection fault to the air pressure switch Sometimes
Corroded terminals, chafed cables, or a loose plug at the switch or PCB can interrupt the electrical signal path. The PCB may then interpret the open circuit as a switch-not-closed condition and trigger the A15 lockout even if the switch and fan are both mechanically sound.
- Moisture or condensation inside the switch or hoses Sometimes
Commercial plant rooms can experience significant condensation. Water finding its way into the sensing tubes or the switch body can block the pressure path or short out the switch contacts, triggering an intermittent or persistent A15 fault. This tends to be more common on older appliances where rubber hoses have become porous.
- PCB fault Rare
In rare cases the control board itself fails to correctly process the signal from the air pressure switch, generating a false A15 lockout even though all components downstream are working normally. This is a diagnosis of exclusion — a qualified engineer should confirm all other causes are clear before suspecting the PCB.
How to fix it
- Check the appliance display and note the exact code shown DIY safe
Confirm the display is showing A15 and not a similar code such as A14 or A16, as the recommended actions differ. Take a photo of the display if possible before attempting a reset, since the code may clear temporarily after reset.
- Attempt a single manual reset via the appliance control panel DIY safe
Andrews lockout faults require a manual reset before the appliance will attempt to fire again. Locate the reset button on the control panel (refer to your user guide for its exact position — it is typically labelled 'Reset' or indicated by a flame symbol with a line through it) and press and hold it as directed. Allow the appliance a full start-up sequence before judging success. If it locks out again with A15, do not reset more than two or three times in total — repeated unsuccessful resets will not fix the underlying fault and can mask important diagnostic information.
- Visually inspect the flue terminal and air intake for obvious blockages DIY safe
If you can safely access the external flue terminal without going onto a roof or using a ladder at height, check that it is clear of bird nests, leaves, ice, or other debris. Similarly check the combustion air intake grille if it is accessible at ground level. Do not attempt to dismantle any flue components — this inspection is visual only.
- Have a Gas Safe engineer inspect and test the fan assembly Gas Safe engineer
An engineer will check that the fan starts correctly, reaches the expected speed, and is not obstructed by debris or seized bearings. They will also check the fan's electrical supply and any tachometer or speed-sensing wiring. The fan sits within the sealed combustion section of the appliance and must not be accessed by an unqualified person.
- Have the engineer check and clear the pressure sensing hoses Gas Safe engineer
The engineer will trace each sensing tube from the fan housing to the switch, checking for kinks, splits, loose push-fit connections, or condensate pooling inside the tube. Blocked tubes are often cleared with a brief burst of compressed air; cracked or hardened tubes need replacement.
- Have the engineer test the air pressure switch with a manometer Gas Safe engineer
Using a manometer (a pressure measuring instrument), the engineer can confirm whether the switch opens and closes at the correct pressure threshold. If the switch is sluggish, does not operate at all, or operates at the wrong pressure, it should be replaced. Andrews commercial switches are not widely stocked at general plumbers' merchants, so the engineer may need to order the correct part from Andrews/Baxi Heating's parts supply chain.
- Have the engineer inspect wiring and terminal connections Gas Safe engineer
The engineer will check cables and plug connectors at both the switch and the PCB for corrosion, chafing, or loose terminals and repair or replace as required. A simple wiring fix is often the quickest and lowest-cost resolution.
- If no other cause is found, have the engineer assess the PCB Gas Safe engineer
If the fan, hoses, switch, and wiring all test satisfactory, the control board may be generating a false lockout signal. PCB replacement on a commercial Andrews unit is a significant job that should only be carried out after all other causes are eliminated.
- Call a Gas Safe registered engineer experienced with commercial water heaters Gas Safe engineer
Because Andrews appliances are commercial grade, it is worth specifically requesting an engineer with experience on commercial hot water systems or Andrews/Baxi commercial products. You can contact Andrews Water Heaters (Baxi Heating UK) directly for their approved service network. Do not leave the appliance attempting to restart indefinitely — if it is supplying a commercial premises such as a hotel or leisure centre, arrange cover promptly.
Parts you may need
- Air pressure switch (Andrews MAXXflo/CWH compatible) · from £75
- Pressure sensing hose/tube kit · from £25
- Fan motor assembly (model-specific) · from £180
- Fan assembly wiring loom/connector · from £30
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 £175–£380, depending on the underlying cause.
Frequently asked questions
Can I reset an Andrews A15 fault myself?
Yes, you can attempt a manual reset using the button on the appliance's control panel — this is a standard homeowner or facilities action. However, if the appliance locks out again with A15 after one or two resets, the underlying fault has not cleared and you should stop resetting and call a Gas Safe registered engineer. Repeated resets without a fix will not resolve the problem and the appliance will simply lock out again each time it tries to fire.
How much does it typically cost to fix an Andrews A15 fault in the UK?
For the most common repairs — replacing the air pressure switch, clearing or replacing the sensing hoses, or fixing a wiring connection — most businesses and landlords pay in the region of £175–£380 including parts and labour. If the fan motor also needs replacing, costs rise toward the top of that range or slightly above. In the rare situation where the PCB is at fault, a commercial PCB replacement can cost considerably more (potentially £400–£700 or beyond depending on the model), but this outcome is uncommon. Emergency call-out surcharges of £100–£150 may also apply outside normal working hours.
Why does Andrews use the letter 'A' in its fault codes?
On Andrews MAXXflo and CWH series appliances the letter prefix in a fault code indicates the fault category. The 'A' prefix specifically denotes a lockout fault — meaning the appliance has performed a controlled safety shutdown and requires a manual reset and investigation before it will operate again. The number that follows (in this case 15) identifies which safety circuit or component triggered the lockout. Other prefix letters may indicate different fault categories; always check your specific model's technical manual for the full code list.
Do I need a specialist engineer for an Andrews commercial water heater, or will any Gas Safe engineer do?
Any Gas Safe registered engineer is legally permitted to work on gas appliances in the UK, but Andrews commercial water heaters have different controls, components, and fault logic compared with typical domestic boilers. It is strongly advisable to use an engineer with experience on commercial hot water systems. Andrews is part of Baxi Heating UK, and contacting their commercial technical helpline (details in your appliance documentation) is a good starting point — they can recommend approved service partners who are familiar with the MAXXflo, CWH, ClassicFlo, and ECOflo product ranges and have access to genuine Andrews parts.