Viessmann F.75 fault codewhat it means and how to fix it
No flow rate detected through a 2020-on Vitodens. Viessmann start with top up and vent; if it recurs the engineer checks the boiler isolating valves, flow sensor and pump.
Take care
Get a Gas Safe registered engineer to look at your Viessmann 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 F.75 means
F.75 is a flow fault rather than a pressure one. On the B1HF, B1HG and 111-W B1LF/B1LG the row reads "System in a fault state — No flow rate — Top up with water. Vent the system. If the fault occurs repeatedly: Check/open the BDF valves. Check/replace the flow sensor (if installed). Check/replace the circulation pump." The B1GA and the 050-W B0HA/B0KA print the same cause as "Burner in a fault state" and order the measures differently — "Open BDF valves. Top up with water. Replace the flow sensor (if installed). Replace the circulation pump" — but it is the same fault: the control unit expected water to move when the pump ran and saw none.
How the boiler knows is worth a sentence. These units watch the temperature rise across the heat exchanger when the pump starts and the burner fires; if the flow sensor climbs as though the water were standing still, or a fitted flow meter reports nothing, the run is abandoned before the limiter has to act. The first measures are the householder-adjacent ones — pressure and air, because a heat exchanger with an air pocket in it passes no water — and the operating instructions cover both: check the pressure-gauge reading and top up below 1.0 bar, and bleed radiators. The repeat-fault measures are the contractor's: the BDF valves (the boiler's own isolators, occasionally left closed after work), the flow sensor, and the internal circulation pump, which on these boilers has two status LEDs and a red one that means "pump failure". F.89 is the pump reporting itself blocked; F.982 is the pump running dry; F.980 and F.981 are flow failures specific to cylinder heating.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Air in the heat exchanger | After a top-up, a radiator change or a low-pressure episode. The pump spins in air and moves nothing. Viessmann's first two measures. | Common |
| Circulation pump seized or failed | The pump's red status LED lit. Viessmann's last measure and the most expensive. | Common |
| BDF isolating valve closed | A boiler flow or return isolator shut after a service and not reopened. Instant F.75 on the first heat demand. | Occasional |
| Flow sensor faulty | Where a flow sensor is fitted, a dead one reports no flow through a system that is moving. "Check/replace the flow sensor (if installed)." | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£320
Time on site
1 hour
Air and pressure cost nothing. A flow sensor is modest; an internal circulation pump is the top of the range.
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.
Viessmann DHW flow sensor (combi)
£30–£80Authorised Viessmann parts distributor
Price checked 28 September 2026
Viessmann internal circulation pump
£150–£300Authorised Viessmann parts distributor
Price checked 28 September 2026
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Related Viessmann fault codes
- F2Stop — call an engineerThe temperature limiter has responded — the boiler water got hot enough to trip the safety chain. Viessmann want the fill level, pump and limiter checked before any reset.Related: overheat and circulation
- F.62Stop — call an engineerThe high limit safety cut-out has tripped on a 2020-on Vitodens: the boiler water got too hot and the burner is locked out. A Gas Safe engineer checks circulation, fill level and diverter valve.Related: overheat and circulation
- E3Stop — call an engineerA fault in the safety chain — the temperature limiter circuit has opened or its wiring has failed. Not a sensor fault; a Gas Safe engineer checks the limiter and control unit.Related: overheat and circulation
- F1Stop — call an engineerMaximum flue gas temperature exceeded. The flue gas sensor has seen the heat exchanger run too hot, almost always because water is not moving through it. Lockout; engineer.Related: overheat and circulation
- F.63Stop — call an engineerThe flue gas temperature limiter has tripped on a 2020-on Vitodens and the burner is locked out. Reset only once the flue has cooled; the cause is a circulation check for a Gas Safe engineer.Related: overheat and circulation
- F.89Get it checkedThe internal circulation pump of a 2020-on Vitodens is blocked (F.89), reporting an electrical fault (F.799) or running dry (F.982). No heating or hot water until it is freed or replaced by an engineer.Related: overheat and circulation
Own a Viessmann boiler? Save it and its codes come first next time.
Related guides
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
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.