Glow-worm F89 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F89 fault code mean?
The F89 code appears on Glow Worm boilers when the appliance detects a problem with the circulation pump or the system's ability to move water around the heating circuit. On startup, the boiler's control board checks that the pump is running and that water is circulating correctly — if it cannot confirm this within an expected timeframe, it triggers F89 and locks out as a precaution. This protects the heat exchanger and other internal components from overheating due to stagnant water. The fault sits alongside related pump and pressure codes F73 and F25 on Glow Worm's diagnostic system. Because Glow Worm is part of the Vaillant Group, engineers familiar with Vaillant pump diagnostics will generally recognise this fault behaviour.
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
- Seized or failing circulation pump Common
The most frequent cause. Over time, the pump impeller can seize — particularly if the boiler has been idle for a long period (e.g. after summer). A seized pump cannot move water, so the boiler locks out immediately. You may also hear an unusual vibrating or humming noise from the boiler before the fault appears.
- Sludge or magnetite blocking the pump or pressure sensor Common
Black iron oxide sludge (magnetite) builds up in systems that have not been chemically cleansed or power flushed. This debris can clog the pump impeller, restrict water flow, or block the differential pressure sensor so the board cannot confirm circulation is happening. Glow Worm boilers are noted to be particularly sensitive to sludge contamination.
- Faulty or blocked pressure sensor Sometimes
Even if the pump itself is mechanically sound, a pressure sensor clogged with sludge or debris can send false readings to the PCB — making the boiler believe there is no circulation when there actually is. Replacing or cleaning the sensor resolves this without a full pump replacement.
- Damaged wiring or loose electrical connections Sometimes
The wiring harness connecting the pump and sensors to the PCB can suffer from vibration-related wear, especially in older units. A loose connector or damaged wire can interrupt the signal from the pump, triggering F89 even though the pump itself is functional.
- PCB fault or sensor sending false data Rare
In some cases the control board itself misinterprets data from otherwise healthy components, generating a persistent or recurring F89 without a clear mechanical cause. This is typically only diagnosed after other components have been ruled out.
How to fix it
- Check your boiler's system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, top up via the filling loop (consult your boiler manual for the filling loop location). Low pressure can sometimes contribute to pump-related faults. Do not overfill — stop at around 1.2–1.5 bar.
- Attempt a boiler reset DIY safe
Once you have confirmed the pressure is correct, press the reset button (usually marked with a flame symbol or a reset icon) and hold for 3–5 seconds. If the boiler fires up and runs normally, monitor it over the next day or two. Attempt a reset no more than 2–3 times in total — repeated resets without a fix risk causing further damage.
- Listen and look for obvious signs of pump trouble DIY safe
With the boiler cover closed, listen for any unusual humming, rattling, or complete silence from the pump area. A pump that is completely silent when it should be running, or one that sounds strained, is a strong indicator of a mechanical fault. Note these observations to pass on to your engineer.
- Do not attempt to open the boiler casing or touch internal components Gas Safe engineer
Working inside the boiler casing — including accessing the pump, wiring, sensors, or PCB — must only be carried out by a Gas Safe registered engineer. Doing so yourself is unsafe and invalidates your warranty and home insurance cover.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will remove the pump, test it electrically and mechanically, inspect the pressure sensor, and check the wiring harness with a multimeter. Depending on findings, they may replace the pump, clean or replace the pressure sensor, flush the system, or investigate the PCB. Always confirm your engineer is on the Gas Safe Register at gassaferegister.co.uk before booking.
Parts you may need
- Circulation pump (Grundfos or equivalent OEM replacement) · from £120
- Differential pressure sensor · from £45
- Pump wiring harness · from £30
- Magnetic system filter (e.g. Magnaclean) · from £65
- 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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow Worm F89 fault myself?
The only homeowner-safe steps are checking system pressure, topping up via the filling loop if it is below 1 bar, and attempting a reset up to two or three times. Everything beyond that — removing the pump, testing sensors, or inspecting wiring — requires a Gas Safe registered engineer. Attempting internal boiler repairs yourself is unsafe and may void your warranty.
How much does it cost to fix a Glow Worm F89 fault?
Most people pay between £150 and £400 in total. If the pump simply needs freeing or a sensor needs replacing, costs sit at the lower end. A full pump replacement including parts and labour typically falls in the £200–£380 range. If a power flush is also needed to clear system sludge, this adds £300–£500 on top. In rare cases where the PCB is at fault, that component alone can cost £250–£400 for parts, so the total bill can exceed £500 — though this is uncommon.
Why does my Glow Worm keep showing F89 after a reset?
A fault that clears on reset but returns within hours or days suggests the underlying cause has not been addressed. This is most commonly a pump that is on the verge of full failure, a sludge-blocked pressure sensor giving intermittent false readings, or a wiring connection that is loose rather than fully broken. Each reset puts additional strain on the pump — book an engineer promptly rather than continuing to reset.
Is F89 related to F73 or F25 on Glow Worm boilers?
Yes, all three codes point to the pump and water circulation system. F73 typically relates to a pressure sensor fault (low or no differential pressure detected), while F25 is associated with an overheat or circulation-related temperature fault. F89 is specifically a pump operation fault where the board cannot confirm the pump is running correctly. An engineer may encounter more than one of these codes when diagnosing a sludge or pump problem, as the root cause can trigger multiple alerts.