Alpha E25 Fault Code: Causes, Fixes & Repair Costs
What does the Alpha E25 fault code mean?
The E25 fault code on Alpha boilers is a protective shutdown triggered when the boiler attempts to fire but cannot confirm that water is flowing through the system. This is often called a 'dry fire' fault — the boiler's safety logic detects conditions consistent with running without adequate water, which could cause serious damage to internal components. The fault is most commonly traced to low system pressure or a faulty flow sensor, but blocked heat exchangers, trapped air, and in some cases a PCB fault (particularly noted on the Alpha E-Tec 28) can also trigger the same code. On some models, a temperature sensor or wiring fault may produce an E25 even when pressure is normal, so it is worth considering sensor issues if pressure checks out fine.
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
- Low System Pressure Common
If the system pressure drops below the minimum threshold — typically around 0.5–0.8 bar on Alpha boilers — the boiler cannot confirm adequate water is present and will lock out with E25. This is the most frequent cause and is easily checked on the pressure gauge on the front of the boiler. A reading below 1 bar warrants repressurisation.
- Faulty Flow Sensor Common
The flow sensor monitors water movement through the boiler. If this component fails or develops a fault, it may report no flow even when water pressure and circulation are perfectly normal. The boiler interprets this as a dry fire risk and locks out. Sensor failures can also be caused by limescale build-up around the sensor tip in hard water areas.
- Airlock in the Heating System Sometimes
Pockets of trapped air in the system can interrupt water circulation past the flow sensor, causing the boiler to register a dry fire condition. Airlocks commonly develop after the system has been drained, after radiator work, or when system pressure has been very low for a period.
- Blocked or Scaled Heat Exchanger Sometimes
A heat exchanger heavily scaled with limescale or partially blocked by sludge restricts water flow to the point where the flow sensor cannot register adequate circulation. This mimics the dry fire condition that E25 is designed to detect. More common in older boilers or systems without magnetic filters or inhibitor.
- PCB or Wiring Fault Rare
In some Alpha models, particularly the E-Tec 28, the E25 code can be generated by a PCB fault or damaged wiring to the flow sensor or temperature sensor rather than an actual water flow problem. If all hydraulic checks come back normal, a PCB or wiring issue should be investigated by an engineer.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, low pressure is likely contributing to the fault. If the gauge shows zero or is barely registering, proceed to repressurise before anything else.
- Repressurise the system using the filling loop DIY safe
Locate the filling loop — usually a braided silver hose or built-in valve beneath the boiler. Slowly open the valve(s) to allow mains water in, watching the pressure gauge rise. Stop when the needle reaches 1 to 1.5 bar, then close the valve(s) securely. Never overfill beyond 2 bar. If you are unsure where the filling loop is, consult your boiler manual or look for a label near the boiler.
- Bleed the radiators to clear any airlocks DIY safe
Starting with the radiators furthest from the boiler, use a radiator bleed key to open the bleed valve slightly until any hissing air escapes and a steady trickle of water appears, then close it. Work your way back towards the boiler. After bleeding, check the system pressure again and top up if needed, as bleeding lowers pressure slightly.
- Reset the boiler DIY safe
Once you have addressed pressure and any airlocks, attempt a reset. On most Alpha models, turn the selector dial to the RESET position, hold it there for approximately 30 seconds, then return it to the desired operating position (e.g. central heating or hot water). The boiler should attempt to fire. If E25 returns immediately or after a short time, do not keep resetting repeatedly — move on to the next step.
- Arrange for a Gas Safe engineer to inspect the flow sensor and wiring Gas Safe engineer
If the fault persists after repressurising and resetting, a Gas Safe registered engineer should test the flow sensor for correct operation and check wiring connections for corrosion, damage, or disconnection. A faulty flow sensor is straightforward to replace and typically resolves the fault if no hydraulic cause is found.
- Have the heat exchanger inspected and cleaned or replaced if necessary Gas Safe engineer
An engineer can carry out a power flush or chemical clean to clear sludge and scale from the heat exchanger if restricted flow is suspected. In severe cases where the heat exchanger is beyond cleaning, replacement will be required. An inhibitor should be added after any flush to protect the system going forward.
- Have the PCB inspected if all other checks are normal Gas Safe engineer
If the flow sensor, wiring, pressure, and heat exchanger all check out, the engineer should assess the PCB for faults. This is particularly relevant on the Alpha E-Tec 28. PCB repair or replacement is a more involved job but will be required if the board is generating a false E25 signal.
- Contact a Gas Safe registered engineer if the fault has not been resolved Gas Safe engineer
If the steps above have not cleared the E25 fault, call a Gas Safe registered engineer to carry out a full diagnostic. Do not continue to reset and run the boiler if the fault keeps returning, as a dry fire condition can damage the heat exchanger over time.
Parts you may need
- Flow Sensor / Flow Switch · from £35
- NTC Temperature Sensor · from £25
- Circulation Pump · from £85
- Printed Circuit Board (PCB) · from £220
- Heat Exchanger · from £280
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 fix the Alpha E25 fault myself without calling an engineer?
There are a few checks you can safely carry out yourself — checking and topping up the system pressure via the filling loop, bleeding radiators to release trapped air, and resetting the boiler. These steps resolve a proportion of E25 faults, particularly when low pressure or a minor airlock is the cause. However, anything beyond these checks — testing or replacing the flow sensor, inspecting the PCB, or flushing the heat exchanger — must be carried out by a Gas Safe registered engineer. It is illegal for an unqualified person to remove the boiler casing and work on internal components.
How much does it cost to fix an Alpha E25 fault in the UK?
Most people with an E25 fault pay somewhere between £120 and £380 all in, covering the callout, labour, parts, and VAT. A flow sensor replacement sits in the middle of that range at roughly £150–£250. If the heat exchanger needs replacing the cost rises considerably — typically £450–£700 — and PCB replacement usually falls in the £300–£500 bracket. If your boiler is several years old and facing a repair bill above £400, it is worth comparing that against the cost of a new boiler installation.
Why does my Alpha boiler keep showing E25 even after I've topped up the pressure?
If the pressure gauge reads a healthy 1–1.5 bar but E25 returns, the issue is unlikely to be low pressure. The next most probable causes are a faulty flow sensor that cannot correctly detect water movement, trapped air in the system causing intermittent circulation problems, or a scaling issue within the heat exchanger. On the Alpha E-Tec 28 in particular, a PCB fault can also generate a persistent E25. An engineer will be able to diagnose which component is at fault using a multimeter and flow tests.
Is the Alpha E25 fault dangerous?
The E25 code is actually a safety feature — the boiler has locked out to prevent itself from firing without adequate water, which could otherwise cause serious damage or overheating. Because the boiler shuts down, the immediate risk to safety is low. However, you should not repeatedly reset the boiler hoping the fault clears on its own, as this can stress internal components. Get the underlying cause diagnosed and fixed promptly, especially heading into winter.