Glow-worm F25 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F25 fault code mean?
The F25 code on a Glow-Worm boiler signals that the appliance has detected a problem with water circulation through the heating system. When water cannot flow freely around the circuit, heat builds up rapidly inside the boiler and the flue gas temperature climbs beyond safe limits. The boiler's safety controls recognise this and trigger a protective lockout before any damage can occur. In practice the two meanings — poor circulation and elevated flue gas temperature — are two sides of the same coin: restricted or inadequate water movement is what causes both. The boiler will remain locked out until the root cause is addressed and the fault is cleared by a reset.
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.
Common causes
- Low system pressure Common
If system pressure has dropped below around 1.0 bar, the circulating pump cannot push water efficiently around the heating circuit. This is one of the most frequent triggers for F25, particularly in older systems or those that have recently been bled. Check the pressure gauge on the boiler front panel — a healthy reading sits between 1.0 and 1.5 bar when the system is cold.
- Trapped air causing an airlock Common
Air pockets in radiators or pipework block the flow of water and force the pump to work against resistance it cannot overcome. Airlocks often appear after radiators have been bled, following maintenance work, or when a system has stood idle for a long period. Gurgling noises from radiators or cold patches at the top of panels are tell-tale signs.
- Sludge or scale buildup causing a blockage Common
Over time, magnetite sludge, limescale, and debris accumulate inside the heat exchanger and pipework. This progressively narrows the flow path, reducing circulation to the point where the boiler overheats. Systems without an inline magnetic filter are particularly vulnerable. A power flush by a Gas Safe engineer is the usual remedy.
- Faulty or seized circulation pump Sometimes
If the pump impeller has jammed — sometimes through sludge fouling or a prolonged idle period — water simply cannot move around the circuit. A pump that hums but produces no flow, or runs intermittently, is a strong indicator. Pump replacement is a job for a Gas Safe engineer.
- Inaccurate flow or return thermistors Sometimes
Glow-Worm boilers compare temperature readings from sensors on the flow and return pipework. If either sensor is reading incorrectly due to limescale coating, wiring damage, or component failure, the boiler may calculate an exaggerated temperature difference and trigger F25 even when circulation is physically fine. A Gas Safe engineer can test sensor resistance with a multimeter.
- System leak causing pressure loss Sometimes
A slow leak anywhere in the heating circuit — at radiator valves, compression joints, or within the boiler itself — gradually reduces system pressure, eventually causing circulation problems. Look for damp patches around pipework, drips beneath radiators, or a pressure gauge that repeatedly drops back to zero after topping up.
- Faulty pressure sensor Rare
The boiler's internal pressure sensor feeds readings to the PCB. If the sensor has degraded through limescale or electrical wear, it may report falsely low pressure and trigger a lockout even when the actual pressure is perfectly normal. This is less common than a genuine pressure drop but should be considered when topping up makes no difference.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. It should sit between 1.0 and 1.5 bar when the heating is cold. If it reads below 1.0 bar, the low pressure itself may be causing the F25 lockout, and topping up should be your first step.
- Top up system pressure using the filling loop DIY safe
Locate the filling loop — usually a braided flexible hose with one or two valves underneath the boiler. Slowly open the valve(s) and watch the gauge climb. Stop when it reaches approximately 1.2–1.3 bar, then close the valve(s) securely. Never overfill beyond 1.5 bar (cold).
- Bleed any radiators that feel cold at the top DIY safe
Use a radiator bleed key to open the bleed valve on any radiator that has cold patches near the top — a sign of trapped air. Hold a cloth beneath the valve and open it slowly until water (not air) trickles out steadily, then close it. Work through each radiator from the ground floor upwards. After bleeding, recheck boiler pressure and top up again if it has dropped.
- Reset the boiler DIY safe
Once pressure is correct and radiators have been bled, press and hold the reset button (marked with a circular arrow or labelled RESET) for around three seconds until the boiler attempts to restart. If it lights up and runs normally, monitor it over the next hour. If F25 returns promptly, do not keep resetting — move on to professional diagnosis.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in your home — such as a gas hob or fire — are working normally. If they are not, contact your gas supplier, as an interruption to the mains gas supply could be contributing to boiler shut-down.
- Inspect visible pipework and radiator valves for leaks DIY safe
Walk around your home and look for damp patches on walls or ceilings near pipework, drips at radiator valve connections, or any signs of corrosion on pipe joints. If you spot an active leak, turn off the boiler and call a Gas Safe engineer — do not attempt to repair pipework joints yourself.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
If pressure is correct and radiators are clear of air but F25 keeps returning, the circulation pump itself may be seized or failing. An engineer can test whether the pump is running at the correct speed, check the impeller for debris, and replace the unit if necessary.
- Have a Gas Safe engineer test the flow and return thermistors Gas Safe engineer
An engineer will use a multimeter to check the resistance values of both temperature sensors against the manufacturer's specification for the current water temperature. A sensor reading outside tolerance will be replaced — a relatively straightforward repair that clears phantom overheating faults.
- Commission a power flush if sludge is suspected Gas Safe engineer
If the system has never been flushed, is many years old, or if the engineer finds restricted flow across the heat exchanger, a power flush — using specialist equipment to force chemical cleaner and high-velocity water through the circuit — will remove accumulated magnetite and scale. Fitting a magnetic system filter afterwards helps prevent the problem recurring.
- Call a Gas Safe registered engineer Gas Safe engineer
If F25 persists after all DIY checks, or if you are unsure of the cause at any point, contact a Gas Safe registered engineer. Continuing to reset a boiler with an unresolved circulation fault risks damaging the heat exchanger and will not solve the underlying problem. You can verify an engineer's registration at gassaferegister.co.uk.
Parts you may need
- Circulation pump (Grundfos or equivalent) · from £235
- Flow thermistor (NTC sensor) · from £25
- Return thermistor (NTC sensor) · from £25
- Pressure sensor / transducer · from £35
- Magnetic system filter (e.g. Magnaclean) · from £65
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 £100–£400, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F25 fault myself?
Some causes are safely within DIY reach: topping up system pressure via the filling loop, bleeding trapped air from radiators, and resetting the boiler are all things you can do without specialist knowledge. However, if F25 returns after those checks, the problem is likely the pump, a sensor, or sludge — all of which require a Gas Safe registered engineer. Attempting repairs on internal boiler components yourself is unsafe and will invalidate any remaining warranty.
Why does my boiler keep showing F25 after I top up the pressure?
If the pressure drops back repeatedly within days or weeks of topping up, there is almost certainly a leak somewhere in the system. This could be at a radiator valve, a compression joint, the pressure relief valve discharge, or inside the boiler itself. A Gas Safe engineer will carry out a pressure test to locate the leak and repair it. Continuing to top up without finding the source masks the problem and risks more serious damage over time. Note that in rare cases a faulty pressure sensor can give misleading low-pressure readings even when the actual pressure is fine — an engineer can distinguish between the two.
How much does it cost to fix the F25 fault on a Glow-Worm boiler?
For most people the repair falls between £100 and £400, covering engineer call-out plus parts such as a pressure sensor (around £35 for the part), thermistors (around £25 each), or a circulating pump (roughly £235–£285 for the part alone before labour). If a full system power flush is needed — typically required when heavy sludge is found — expect to pay £300–£600 for that work. A new boiler, which may be the better long-term choice on an older or repeatedly failing unit, generally runs £1,500–£3,000 installed.
How do I prevent the F25 fault from coming back?
Annual boiler servicing is the single most effective measure — an engineer will check pump operation, sensor readings, and system pressure during a service. Fitting a magnetic inline filter (such as a Magnaclean) captures iron oxide sludge before it can block the heat exchanger or pump. Adding central heating inhibitor to the system water reduces corrosion and scale. If your system has never been power-flushed and is over ten years old, scheduling one proactively is worthwhile.