Alpha 0A16 Fault Code: Causes, Fixes & Repair Costs
What does the Alpha 0A16 fault code mean?
Fault code 0A16 on Alpha boilers indicates that the flow temperature is not rising quickly enough after the burner fires. The boiler's control board monitors how fast the water temperature climbs at the flow (outlet) side of the heat exchanger. When that rise is slower than expected within the allowed timeframe, the board treats it as an anomaly and triggers a lockout as a precaution. The '0A' prefix means this is a resettable fault — you can attempt a reset once the boiler has completed its post-ventilation cycle — while '16' identifies the specific flow-sensor temperature-rise fault. This code is most commonly seen on the Alpha Pro Tec Plus range (50, 70, 90 and 115 kW system boilers).
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
- Poor water circulation or trapped air Common
If air has become trapped in the heat exchanger or the wider system, water cannot circulate freely enough for heat to transfer efficiently. The flow temperature then climbs far too slowly, triggering the fault. Gurgling noises from the boiler or radiators that are cold at the top are telltale signs of an airlock.
- Seized or underperforming circulation pump Common
A pump that has seized, is running below its rated speed, or is failing intermittently will not push water around the system fast enough. Without adequate flow, heat builds up in the heat exchanger instead of being carried away to the radiators, and the flow thermistor records a sluggish temperature rise.
- Blocked or scaled heat exchanger Common
Limescale deposits — especially in hard-water areas — can coat the inside of the heat exchanger and create flow restrictions. Sludge from corroded pipework can also accumulate over time. Both reduce the volume of water passing through and make the boiler work harder for a slower temperature rise.
- Faulty or drifting flow sensor (NTC thermistor) Sometimes
The NTC thermistor that monitors flow temperature can fail outright or gradually drift out of calibration, sending misleading readings to the PCB. A localised hotspot near the sensor — caused by a kinked pipe, limescale build-up directly around the sensor pocket, or a poorly seated sensor — can also make the PCB think the temperature is rising correctly when it is not, eventually triggering 0A16 when the mismatch becomes too great.
- System restriction or flue blockage causing overheating Sometimes
A partially closed isolation valve, a blocked zone valve, or a restriction in the flue run can all reduce flow or combustion efficiency enough to prevent a normal temperature rise pattern. The boiler may fire but fail to distribute or exhaust heat as designed.
How to fix it
- Wait for the post-ventilation cycle to finish, then attempt a single reset DIY safe
Alpha boilers run a short fan post-purge after a lockout. Once the display has settled back to showing the fault code, press the reset button once. If the boiler fires and runs normally, monitor it over the next few hours. Do not reset more than two or three times in total — repeated resets without identifying the underlying fault can mask a more serious problem.
- Check your boiler pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top it up using the filling loop (refer to your Alpha user guide for the location). Low pressure can contribute to poor circulation and slow heat transfer.
- Bleed the radiators to release trapped air DIY safe
Turn the heating off and let the system cool. Starting from the ground floor, use a radiator bleed key to open each bleed valve a quarter-turn until water (not air) comes out steadily, then close it. Work your way upstairs. Re-check the boiler pressure afterwards and top up if it has dropped below 1 bar.
- Check that the gas supply to the boiler is on DIY safe
Make sure the gas isolation valve on the supply pipe to the boiler is in the open (inline) position. Check that other gas appliances in the property — hob, fire — are working normally. If you suspect a gas supply issue, contact your gas supplier rather than the boiler engineer.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
An engineer will test the pump's speed, current draw, and head pressure to confirm it is circulating water at the correct rate for the Pro Tec Plus output. A seized or worn pump will be replaced; in some cases, de-airing the pump housing resolves the fault without a full replacement.
- Have a Gas Safe engineer test the flow sensor (NTC thermistor) Gas Safe engineer
The engineer will measure the thermistor's resistance at a known water temperature and compare it against the Alpha specification. A reading outside tolerance means the sensor is replaced — it is an inexpensive part. The engineer will also check the sensor pocket for limescale build-up or pipe kinks nearby that could cause a localised hotspot.
- Have a Gas Safe engineer assess the heat exchanger for scale or sludge Gas Safe engineer
If circulation and sensor tests pass, the heat exchanger is likely partially blocked. The engineer may carry out a chemical flush or recommend a full powerflush of the system. A magnetic system filter and inhibitor dose should be fitted afterwards to prevent recurrence.
- Have a Gas Safe engineer check the flue and system for restrictions Gas Safe engineer
The engineer will inspect the flue terminal for obstructions (bird nests, debris) and verify that all isolation valves and zone valves in the system are fully open. Any restriction that limits flow or combustion air can contribute to this fault code.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If 0A16 returns after a reset, or if the steps above have not identified a clear cause, contact a Gas Safe registered engineer. Continuing to reset a locking-out boiler without a proper diagnosis risks damage to the heat exchanger and other components.
Parts you may need
- NTC flow thermistor (temperature sensor) · from £20
- Central heating circulation pump (compatible replacement) · from £80
- System inhibitor fluid (1 litre) · from £15
- Magnetic system filter · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I reset my Alpha boiler myself when fault code 0A16 appears?
Yes — one reset attempt is a reasonable first step. Wait until the boiler has finished its post-ventilation fan cycle, then press the reset button once. If the boiler runs normally and does not lock out again, bleed your radiators and monitor it over the next day or two. However, if 0A16 returns within a short period or locks out repeatedly, stop resetting and call a Gas Safe engineer. Repeated resets without a fix can cause further heat exchanger damage.
How much does it cost to fix Alpha fault code 0A16 in the UK?
Most owners with this fault end up paying somewhere between £120 and £380 all in, depending on the cause. A flow sensor (NTC thermistor) swap is one of the cheaper fixes — the part is typically £15–£30 and labour makes the total around £120–£180. A circulation pump replacement usually runs £200–£350 including parts and labour. A powerflush to clear sludge or scale can cost £300–£600 depending on system size. Heat exchanger replacement is a much rarer outcome but can exceed £700 once parts and labour are added — at that point, comparing quotes for a new boiler is often worthwhile, particularly if the Pro Tec Plus is already more than ten years old.
Why does my Alpha Pro Tec Plus keep showing 0A16 after I reset it?
A recurring 0A16 almost always points to an underlying issue that a reset cannot clear — typically a failing circulation pump, a heavily scaled heat exchanger, or a drifting flow thermistor. Each time the boiler fires without enough water movement, heat builds up unevenly and the control board locks out again. A Gas Safe engineer needs to run flow, pump, and sensor tests to pinpoint the root cause. Continuing to reset without a fix risks overheating and permanent heat exchanger damage.
Could a powerflush fix Alpha fault code 0A16?
It can, if sludge or limescale build-up is restricting water flow through the heat exchanger or the wider system. A powerflush uses high-velocity water and chemical cleaners to flush out deposits, restoring proper circulation. However, a powerflush is only the right solution if restricted flow is confirmed as the cause — it will not help a failing pump or a faulty thermistor. Your engineer should diagnose the root cause first before recommending a flush. After any flush, fitting a magnetic filter and dosing with inhibitor fluid will help prevent the problem returning.