Glow-worm F7 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F7 fault code mean?
The F7 fault code on Glow-Worm boilers signals that the boiler cannot confirm a change in water pressure after the pump starts up. When the boiler fires, it briefly runs the circulation pump and expects to detect a corresponding rise in system pressure — proof that water is moving safely around the system. If the pressure sensor fails to register that change, the boiler locks out and displays F7 as a safety precaution. On newer Glow-Worm models this same underlying fault appears as F75, which is simply a more precise variant of the same detection failure. Both codes point to the same root cause: the boiler cannot verify proper water circulation. The fault may stem from the pressure sensor itself, the circulation pump, or low system water pressure — each of which prevents the boiler from completing its start-up self-check.
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
- Faulty or worn pressure sensor Common
The pressure sensor is the component most commonly responsible for this fault. After several years of service — particularly in areas with variable water quality — the sensor can develop electrical faults or simply wear out. When it fails, it cannot detect the pressure spike generated when the pump runs, causing a false lockout even if the rest of the system is working normally.
- Blocked pressure sensor port Common
Sludge, limescale, or debris from corroding pipework can accumulate inside the small port connecting the sensor to the system. This is especially prevalent in hard water areas such as the South East of England. The blockage prevents the sensor from registering any pressure movement, mimicking a genuine pressure detection failure and triggering the F7 or F75 lockout.
- Low system water pressure Common
If the system pressure has dropped — due to a minor leak, recent radiator bleeding without re-pressurising, or a failing expansion vessel — the baseline pressure may be too low for the sensor to detect meaningful change when the pump activates. Checking the pressure gauge on the boiler front panel is a quick first step: it should read between 1.0 and 1.5 bar at rest.
- Malfunctioning circulation pump Sometimes
If the pump itself has seized, suffered bearing wear, or developed an electrical fault, it cannot generate sufficient water movement. With no real change in flow, the pressure sensor — even if working perfectly — has nothing to detect, and the boiler shuts down. Pump failures are more likely in older boilers or larger properties where the pump works harder.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of your Glow-Worm boiler. It should sit between 1.0 and 1.5 bar when the system is cold. If the needle is below 1 bar, low pressure may be contributing to the fault and you can address this before attempting a reset.
- Top up system pressure via the filling loop if needed DIY safe
Locate the filling loop — usually a braided silver hose with one or two valves beneath the boiler. Slowly open the valve(s) and allow cold mains water into the system, watching the gauge rise. Once it reaches approximately 1.2–1.5 bar, close the valve(s) firmly. Do not overfill; pressure above 2.5 bar can trigger the pressure relief valve.
- Reset the boiler DIY safe
Press and hold the reset button (often marked with a flame and cross symbol) for around 3–5 seconds until you see the display respond. Allow the boiler a full minute to complete its start-up cycle. If the F7 or F75 code returns immediately or after a short time, do not keep resetting repeatedly — two or three attempts is the safe limit.
- Call a Gas Safe registered engineer to inspect the pressure sensor and pump Gas Safe engineer
If the fault persists after checking pressure and resetting, a Gas Safe registered engineer needs to take over. They will use diagnostic equipment to test the pressure sensor for electrical continuity and correct response, inspect the sensor port for sludge or limescale blockage, and assess the circulation pump for seizure or electrical failure. This is not work a homeowner should attempt — the components are part of the sealed pressurised system and gas-fired assembly.
- Engineer to replace a faulty or blocked pressure sensor Gas Safe engineer
If the sensor is found to be defective or irreversibly blocked, the engineer will replace it with a compatible part. On most Glow-Worm combi boilers this is a relatively straightforward job for an experienced engineer. Following sensor replacement, the engineer should also assess whether the system water is dirty enough to warrant a chemical flush, as sludge that blocked the sensor may be affecting other components.
- Engineer to repair or replace the circulation pump if required Gas Safe engineer
Should the pump be found to be seized or beyond reliable service, the engineer will replace it and run a full system pressure test to confirm that the boiler can now detect pressure changes correctly on start-up. A power flush of the system may be recommended at the same time if sludge is suspected as a contributing factor.
Parts you may need
- Water pressure sensor (differential pressure switch) · from £35
- Circulation pump · from £120
- Filling loop assembly · from £18
- System inhibitor (e.g. Fernox F1 or equivalent) · from £15
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–£380, depending on the underlying cause.
Frequently asked questions
What is the difference between the Glow-Worm F7 and F75 fault codes?
Both codes indicate that the boiler failed to detect a pressure change after the pump activated at start-up — in other words, they point to the same problem. F7 is the code displayed on older Glow-Worm models, while F75 appears on newer or more advanced versions of the range. If you see either code, the diagnostic and repair process is the same: check system pressure, attempt a reset, and contact a Gas Safe engineer if the fault persists. You do not need a different engineer or a different part depending on which variant your display shows.
Can I fix the Glow-Worm F7 fault myself?
The only safe DIY steps are checking and topping up system pressure via the filling loop, and resetting the boiler up to two or three times. Beyond that, diagnosing and replacing a pressure sensor or circulation pump involves working on a pressurised, gas-fired system and must be carried out by a Gas Safe registered engineer. Attempting these repairs without registration is both dangerous and illegal under UK law.
How much does it cost to fix a Glow-Worm F7 or F75 fault in the UK?
Most homeowners pay somewhere between £150 and £380 depending on what needs doing. A straightforward pressure sensor replacement including labour typically comes in around £150–£220. Pump replacement tends to cost more, usually £250–£380 with labour. If the engineer also recommends a full system power flush to remove sludge — which is sensible if blockage caused the fault — that can add £300–£500 on top. In rare cases where the boiler itself is ageing and uneconomical to repair, a full boiler replacement would cost £800–£2,500 depending on model and installation complexity.
How can I prevent the F7 fault from coming back?
The most effective steps are keeping inhibitor in the system (check and top up annually), checking your boiler pressure gauge monthly and topping up whenever it drops below 1 bar, and having a Gas Safe registered engineer service the boiler every year. An annual service includes inspecting the pressure sensor and pump — catching early signs of wear before they cause a lockout. If you live in a hard water area, ask your engineer about fitting a scale reducer or magnetic system filter, which catches sludge before it can block the sensor port.