Alpha E25 Fault Code: Dry Fire / No Water Detected
The E25 code is Alpha's way of telling you that the boiler attempted to fire but could not confirm adequate water flow through the system. Often called a 'dry fire' warning, it is a protective lockout designed to prevent the heat exchanger from overheating and being damaged when water isn't circulating properly. The most common triggers are low system pressure and a faulty flow sensor, though in some E-Tec models the fault can occasionally point toward a PCB issue.
General guidance only — not a substitute for professional advice. 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.
What does the Alpha E25 fault code mean?
The E25 code is Alpha's way of telling you that the boiler attempted to fire but could not confirm adequate water flow through the system. Often called a 'dry fire' warning, it is a protective lockout designed to prevent the heat exchanger from overheating and being damaged when water isn't circulating properly. The most common triggers are low system pressure and a faulty flow sensor, though in some E-Tec models the fault can occasionally point toward a PCB issue.
Common causes
- Low system pressure Common
If the system pressure has dropped below the boiler's minimum threshold (typically around 0.8–1.0 bar on Alpha boilers), the flow sensor cannot confirm adequate water is present and the E25 lockout is triggered. Pressure can drop gradually through micro-leaks or after bleeding radiators.
- Faulty or stuck flow sensor Common
The flow sensor (sometimes called a flow switch or water flow detector) monitors water movement through the primary circuit. If this component fails, becomes stuck, or its wiring becomes loose or corroded, it will report no flow even when pressure and circulation are perfectly normal.
- Airlocks in the heating circuit Sometimes
Pockets of trapped air in the pipework or heat exchanger can prevent water from circulating freely past the flow sensor, mimicking a dry fire condition. This is especially common after system drains, radiator additions, or periods of inactivity.
- Scaled or partially blocked heat exchanger Sometimes
Limescale and sludge build-up inside the heat exchanger can restrict water flow to the point where the boiler cannot detect adequate circulation. This tends to be more common in hard-water areas and in systems without inhibitor protection.
- Faulty PCB Rare
On some Alpha E-Tec models, a failing printed circuit board can generate a spurious E25 code even when water flow and pressure are both normal. This is typically only suspected once all other causes have been ruled out by an engineer.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, this is almost certainly contributing to the fault.
- Top up pressure using the filling loop DIY safe
Locate the filling loop beneath or near the boiler — it is usually a braided hose with one or two valves. Slowly open the valve(s) until the gauge reads around 1.2–1.3 bar, then close it firmly. Never exceed 2.0 bar. If you are unsure how to do this, your boiler manual shows the exact procedure for your model.
- Bleed any radiators that feel cold at the top DIY safe
Using a radiator bleed key, open the bleed valve on any radiators that have cold spots at the top (a sign of trapped air). Hold a cloth underneath and close the valve as soon as water — rather than air — begins to escape. Re-check the boiler pressure afterwards, as bleeding can lower it slightly, and top up again if needed.
- Reset the boiler DIY safe
On most Alpha E-Tec models, turn the selector switch to the RESET position, hold it there for around 30 seconds, then return it to your desired heating mode. Give the boiler a minute to attempt restart. If it fires up and runs normally, monitor it over the next few days. Attempt a reset no more than two or three times — repeated lockouts mean an underlying fault needs attention.
- Check your gas supply DIY safe
Make sure other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier rather than an engineer, as there may be a supply interruption to your property.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the boiler locks out again after resetting, or if pressure was not the cause, a Gas Safe engineer needs to inspect the flow sensor and its wiring, test the pump, check for scale or sludge in the heat exchanger, and — if all else is ruled out — assess the PCB. None of these tasks should be attempted by a homeowner. Always ask to see the engineer's Gas Safe ID card before work begins.
Parts you may need
- Water flow sensor / flow switch · from £25
- Central heating pump · from £65
- Inhibitor fluid (e.g. Fernox F1 or Sentinel X100) · from £18
- Filling loop assembly · from £20
- PCB (printed circuit board) · from £220
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
Is the Alpha E25 fault dangerous?
The E25 lockout is a safety feature, not a dangerous failure in itself — the boiler has shut itself down to prevent damage. However, you should not keep resetting it repeatedly and leaving it unattended, as the underlying cause (whether that is low pressure, a blocked heat exchanger, or a failing sensor) does need to be properly identified and fixed. If at any point you smell gas, leave the property, avoid operating electrical switches, and call the National Gas Emergency Service on 0800 111 999.
Can I fix an Alpha E25 fault myself?
There are a few safe checks you can carry out: topping up system pressure via the filling loop, bleeding radiators to remove trapped air, and resetting the boiler up to two or three times. Beyond these steps, the fault requires a Gas Safe registered engineer. It is a legal requirement in the UK that gas appliance work is carried out by a Gas Safe registered professional — attempting to open the boiler and work on components yourself is both illegal and potentially dangerous.
How much does it cost to fix an Alpha E25 fault in the UK?
For the most common causes — a faulty flow sensor or low pressure — you can typically expect to pay somewhere between £120 and £380 including parts and labour. A straightforward sensor swap or system flush with inhibitor sits toward the lower end of that range. If the fault turns out to be a failed PCB, costs can rise to around £350–£500 once parts and labour are included, and a heat exchanger replacement is more expensive still — that is worth discussing with your engineer before authorising any major work, as on an older boiler it may be more economical to replace the boiler entirely.
Why does my Alpha boiler keep showing E25 after I reset it?
A fault that clears on reset but keeps coming back is a strong signal that a component is failing rather than that a one-off event (like air entering the system) caused the lockout. The most likely culprits on an Alpha E-Tec are a flow sensor that is intermittently sticking or failing, a pump that is beginning to lose performance, or sludge build-up that is partially blocking the heat exchanger. Repeated lockouts cause additional wear, so it is better to book an engineer promptly rather than keep resetting and hoping the problem resolves itself.