Glow-worm F.75 fault codewhat it means and how to fix it
Glow-worm's own documents give this code three different cause lists — pressure switch only, or sensor, pump or lack of water — and the fullest one is the Compact's eight-way diagnostic.
Take care
What is the pressure and what can you hear?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Pressure is below about 0.8 bar
Top up first. Glow-worm list water deficiency as a cause on the Ultimate3 and Compact.
Pressure fine, boiler hums when it should be pumping
Pump rotor stuck or the pump electrically faulty.
Book a Gas Safe registered engineer. The pump sits inside the casing.
Pressure fine, the pump sounds normal
The sensor is not seeing the change the pump makes. Sensor first, board second.
Engineer with a meter; do not fit a pump on the strength of the trade table alone.
What F.75 means
F.75 is the boiler starting its pump and failing to see the pressure change that proves water moved. Glow-worm publish it three ways. Their trade table for the Energy range says "Fault: Pressure sensor — pressure switch defective" and nothing else.
The Ultimate3 installation manual says "pressure sensor or pump defective or water deficiency", which adds the two possibilities an owner can do something about. And the 2026 Compact manual goes furthest, calling it "Fault: Pump/water deficiency" and attaching a diagnostic value an engineer reads from the board: 1 pump blocked, 2 electrical pump fault, 3 pump dry run, 5 pressure sensor fault, 6 no feedback from the pump, 7 incorrect pump detected, 8 flow insufficient at the end of the purge programme. So the narrowest Glow-worm reading points at the dearest part and the fullest one lists five things that are not that part.
Low system pressure and air are the two you can check before calling anyone; a pump that hums or does nothing, and a sensor that has stopped seeing it, are the rest. The code does not appear on Ultracom, Flexicom or Ultrapower boilers, whose equivalent is F73, and the Ultracom2 and Betacom2 tables have no 75 either — those boilers use F83 for a burner that lights and nothing warms.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump seized, stuck or not powered | Glow-worm's Compact values 1, 2 and 6 — blocked, electrical fault, no feedback. Older Glow-worm pumps that have sat all summer are the classic case; a tap on the body sometimes frees a rotor once and no more. | Common |
| Pressure sensor no longer registering the change | The Energy trade table's only cause and the Compact's value 5. The pump runs and the sensor output stays flat, so the board concludes nothing moved. | Common |
| Not enough water, or air around the pump | The Ultimate3 manual's "water deficiency" and the Compact's "pump dry run". Pressure under about 0.8 bar cold, or a system recently refilled and not bled, leaves the pump with nothing to push against. | Occasional |
| Wrong pump fitted after a repair | Compact value 7, "incorrect pump detected". Only relevant if the pump has recently been changed, but then it is the first thing to ask. | Rare |
What to try yourself
Check the pressure cold. If it is under about 0.8 bar, top up to the figure in your guide — 1.0 to 1.5 bar on the Ultimate3 and Betacom4 — as the F.22 page describes, then reset once.
If the system has been drained or refilled recently, bleed the radiators, starting with the highest, and top up again afterwards. Air sitting at the pump is one of Glow-worm's named causes.
Listen at the boiler when it is called on. A hum with no sound of water, or silence where there used to be a whir, is the pump, and that is where to stop and call an engineer rather than repeat resets.
Get a Gas Safe registered engineer to look at your Glow-worm F.75
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£120–£420
Time on site
1h 15m
Pressure sensor £120–£240 fitted. Pump £250–£420. Free if it was pressure or air.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Glow-worm water pressure sensor
£25–£60Authorised Glow-worm parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Glow-worm fault codes
- F.22Get it checkedGlow-worm's low-water-pressure shutdown, and the one code every Glow-worm user guide lets you clear yourself by topping the system up. The threshold differs by range.Related: water pressure
- F.9Get it checkedF9 on an Ultracom, Flexicom, Ultrapower or CI-range boiler is Glow-worm's water pressure sensor fault — on the hxi and Flexicom "or a blocked flow filter or pump fault". It is not the low-pressure code.Related: water pressure
- F.73Get it checkedThe water pressure sensor is reading too low to be real — open circuit or shorted — so the boiler has no pressure figure it trusts. On Ultracom boilers F73 is a pump fault instead.Related: water pressure
- F.74Get it checkedThe water pressure sensor is sending a signal too high to be real — a short to the 5 or 24 volt supply, or an internal fault. An engineer's code wherever it shows.Related: water pressure
- F.28Get it checkedThree ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Related: same brand
- F1Get it checkedOn Ultracom, Flexicom and Ultrapower boilers F1 is an ignition lockout after five failed attempts, and Glow-worm's guides let you reset it. On Energy and Ultimate boilers F.01 is the return sensor instead.Related: same brand
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.75
Glow-worm's trade page says pressure switch defective. Should I just have that replaced?
Their own installation manuals say otherwise — the Ultimate3 adds the pump and lack of water, and the Compact lists eight sub-causes of which one is the sensor. Ask the engineer to read the diagnostic value or test the pump before ordering anything.
Why does an Ultracom not show F.75?
Its list stops at F77 and uses F73, "unable to see pump activate", for the same event. Different generation, different numbering.
Can I run the hot water while I wait?
No. The boiler will not fire in either mode until it has proved circulation, because running a burner without water moving is how heat exchangers are ruined.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.