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Glow-worm fault code · Wokingham & RG40 · Gas Safe 599015
Glow-worm F75 Fault Code
Water pressure sensor fault — what it means, what to check, and when to call an engineer
The Glow-worm F75 fault code is one of the most common Glow-worm lockouts Stuart sees across Wokingham and RG40. In most cases it's either low system pressure (which you can fix yourself in 5 minutes) or a failed pressure sensor (which needs a Gas Safe engineer). This guide tells you exactly which one you're dealing with and what to do.
F75
Glow-worm fault code
MeaningWater pressure sensor fault
SymptomBoiler lockout — no heating or hot water
Common causeLow pressure or faulty sensor
DIY?Repressurise only — sensor replacement needs engineer
UrgencyNon-emergency — but fix promptly
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What does the Glow-worm F75 code mean?
F75 is Glow-worm's fault code for a water pressure sensor fault. The boiler's internal pressure sensor monitors system pressure during pump start-up. When pressure fails to rise as expected — or the sensor cannot detect a rise — the boiler locks out with F75 as a safety measure.
There are two distinct scenarios that produce this code:
- The system pressure is genuinely low (below ~0.8 bar) and the sensor is correctly reporting a real problem
- The system pressure is normal but the pressure sensor itself has failed and is reporting incorrectly
Telling the two apart takes 30 seconds — look at your boiler's pressure gauge before doing anything else.
Step 1 — Check the pressure gauge first
Before touching anything, find the pressure gauge on your Glow-worm boiler (usually a small circular dial or digital display on the front panel). Normal operating pressure when cold is 1.0–1.5 bar.
- Pressure below 1.0 bar → Low pressure is causing the F75. Repressurise using the steps below.
- Pressure between 1.0 and 1.5 bar → Pressure is fine. The sensor itself is likely faulty. Call a Gas Safe engineer.
- Pressure above 2.5 bar → Overpressure. Do not repressurise. Call an engineer.
How to fix F75 caused by low pressure (DIY safe)
If your pressure gauge is reading below 1.0 bar, follow these steps to repressurise:
- 1Turn the boiler off and let it cool for 10–15 minutes. Never repressurise a hot boiler.
- 2Find the filling loop — a braided silver hose beneath the boiler, usually with one or two valves. On Glow-worm boilers it is typically an internal keyless filling link accessed via a small door on the boiler casing.
- 3Open the filling valve(s) slowly and watch the pressure gauge. You will hear water entering the system.
- 4When the gauge reaches 1.2 bar, close the valve(s) firmly. Do not go above 1.5 bar.
- 5Press the reset button on the boiler (hold for 3 seconds). The boiler should attempt to fire. If it starts normally and F75 clears, the issue was low pressure.
- 6Monitor the pressure over the next few days. If it holds at 1.2 bar the problem is resolved. If it drops again within days, there is a leak in the system that needs finding.
Stop and call if:
F75 returns after repressurising · The pressure drops again within a few days (there is a leak) · The pressure gauge reads normal when F75 appears (sensor fault) · You cannot locate or operate the filling loop · The boiler locks out again immediately after reset. Boiler repair from £96 →
All causes of the Glow-worm F75 code
Low system pressure
Pressure has dropped below the threshold for the sensor to detect a rise during pump start. Common after bleeding radiators or a slow leak.
Very common
Faulty pressure sensor
The sensor itself has failed and is not registering the correct pressure even when the system is fine. Needs replacement by a Gas Safe engineer.
Common
Blocked sensor pipe/port
Sludge or scale (common in Wokingham's hard water area) can block the small pipe or port connecting the sensor to the system, giving a false reading.
Occasional
Pump failure
If the pump is not creating sufficient pressure differential during start-up, the sensor may detect no rise even if static pressure is normal.
Occasional
Wiring fault to sensor
Damaged or corroded wiring between the sensor and PCB can cause intermittent F75 codes even when pressure and sensor are fine.
Less common
PCB fault
Rarely, the PCB misreads the sensor signal. Usually presents as an intermittent F75 that resets easily but keeps returning.
Rare
Wokingham hard water note: The blocked sensor port cause is more common in Wokingham's 330–380 mg/L hard water area than elsewhere in the UK. Limescale and magnetite can block the tiny sensing port, causing F75 to appear even when pressure is normal. Stuart checks sensor ports specifically when diagnosing F75 on Wokingham properties.
FAQ
What does the Glow-worm F75 code mean?
F75 means the water pressure sensor has detected a problem — either the system pressure is genuinely low, the sensor is faulty, its port is blocked, or the pump is not building sufficient pressure. The most common cause is simply low system pressure, which you can fix yourself by repressurising to 1.2 bar using the filling loop and resetting the boiler.
Can I fix the Glow-worm F75 code myself?
If the pressure gauge shows below 1.0 bar: yes — repressurise to 1.2 bar using the filling loop and reset. If the gauge shows normal pressure and F75 still appears: no — the sensor itself is faulty and needs replacing by a Gas Safe engineer. Do not attempt sensor replacement yourself. Book a repair from £96 →
Why does my Glow-worm keep showing F75 after I top up the pressure?
Recurring F75 after repressurising means either: (a) there is a leak causing pressure to keep dropping — the leak needs finding and fixing; or (b) the sensor itself is faulty and needs replacing regardless of pressure. Both require a Gas Safe engineer to diagnose properly.
How much does it cost to fix a Glow-worm F75 in Wokingham?
If it's just a case of repressurising — free, do it yourself. If the pressure sensor needs replacing, Thermotec charges from £96 for the diagnostic visit across Wokingham and RG40, with the repair quoted upfront before any work begins. A Glow-worm pressure sensor replacement typically costs £150–£250 parts and labour combined.
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