Vaillant F75 Fault Code: Causes and How to Fix

What the Vaillant F75 fault code means

F.75 is Vaillant's code for a pressure problem the boiler spots at the exact moment the pump starts. On its own published fault code list, Vaillant describes F.75 as "The pressure change is too low when the heat pump is started up". In plain English: the boiler told the circulation pump to run, watched the water pressure sensor for the small jump in pressure that should follow, and did not see it. It cannot confirm that water is actually moving, so it locks out instead of lighting the burner.

That is why F75 is so often misread. The complaint is about movement, not level. Your gauge can sit at a perfectly normal 1.2 bar and the boiler will still show F75, because the reading never changed when the pump kicked in.

It is also not the same fault as F.22. Vaillant defines F.22 as "Low water pressure in the system", which is the straightforward top-it-up code. F75 sits one step further on, and it points at the pump, the sensor or the wiring between them. If your gauge is genuinely low, our Vaillant F22 page is the one you want. Our Vaillant fault code index covers the rest of the range.

While F75 is showing, the boiler will not fire, so you have no heating and, on a combi, no hot water. It is safe to leave the appliance switched off until an engineer can look at it.

The five causes Vaillant lists for F75

Vaillant publishes the possible causes of F.75 on its own fault code page. There are five. Only the first is something a homeowner can put right.

Why the sensor and the pump get blamed most

Those two sit at either end of the same test. The pump has to create the pressure change, and the sensor has to see it, so a fault at either end produces an identical symptom. A good engineer tests both rather than replacing one on a hunch.

Why air matters more than people expect

Air is compressible and water is not. A pocket of air sitting in the pump housing absorbs the pressure change the boiler is looking for, so the test fails even when everything is working. That is why F75 sometimes appears for the first time straight after a system has been drained, a radiator has been changed, or the heating has been off for months.

What you can safely check yourself

There are exactly three safe steps, and then it stops.

1. Read the pressure gauge

Vaillant's stated normal range is "the normal range of 1 to 1.5 bar", measured with the heating off. Read it cold, before you touch anything. Write the number down, because it is the single most useful thing you can tell an engineer about an F75.

2. Repressurise, but only if you are below 1 bar

Where the reading is low, Vaillant tells homeowners to "Locate your filling loop" and to "Slowly increase the water pressure to be between 1 to 1.5 Bar". Your model's own manual shows where the loop is, and Vaillant publishes those manuals to download. Overshooting means bleeding water back out again.

3. Reset the boiler once, and listen

Vaillant is honest about what a reset achieves: "Often resetting your boiler can resolve a fault or temporarily get your boiler working again." A reset makes the boiler run the pump start test again, so if air was the problem the test can pass. Stand next to it while it tries. A working pump gives a soft hum and a faint vibration through the casing. Silence is worth reporting.

If F75 comes straight back, stop there. Do not keep resetting, and do not take the casing off. Vaillant's own wording is blunt: "Don't try to solve it yourself as opening the boiler casing can be dangerous." That is not only advice. Under regulation 3(1) of the Gas Safety (Installation and Use) Regulations 1998, nobody may carry out work on a gas fitting unless they are competent to do so, and regulation 3(3) requires anyone doing that work in the course of business to be a member of a class of persons approved by the Health and Safety Executive, which in practice means a Gas Safe registered engineer.

If you smell gas at any point, this stops being a fault code question. Call the National Gas Emergency Service on 0800 111 999, free to call, 24 hours a day. National Gas advises you not to smoke or light matches, not to operate electrical switches, to open doors and windows, and to turn the gas off at the meter control handle unless the meter is in a cellar.

What an F75 repair involves and costs

The honest answer is that the price depends on which of the five causes it turns out to be, and nobody can tell you that down the phone. What we can give you is the one figure Vaillant itself publishes.

Vaillant's own fixed price repair for a boiler outside its guarantee starts "From £439 one-off payment (inc. VAT) payable at point of booking". That includes a gas safety check, parts guaranteed for 12 months with exclusions, and labour covered for 90 days. The heat exchanger and expansion vessel are not included. Where a boiler is judged uneconomical to repair, Vaillant says it will "only charge you for the fixed engineer time (£99)".

An independent Gas Safe registered engineer may quote above or below that, and we are not going to print an invented range for a pump or a sensor. Three things move the number: which part failed, whether it is still made for your model, and how old the boiler is. On a long-serving ecoTEC, a large bill deserves a straight comparison against replacing the boiler.

When you book, ask two things: whether the part is likely to be on the van, and what will be tested to separate a sensor fault from a pump fault. If the boiler is still inside its Vaillant guarantee, book through Vaillant first. Otherwise, our boiler repair page explains how we arrange one.

Common questions about the F75 fault

Can I get hot water while F75 is showing?

Not from the boiler. F75 is a lockout, so the burner will not light for heating or for hot water. An electric shower or an immersion heater runs off electricity and is unaffected. Never use a gas hob or oven to heat a room while you wait, because burning gas in an unventilated space can produce carbon monoxide.

Is a Vaillant F75 dangerous?

The code is the safety response, not the danger. The boiler has refused to fire because it cannot prove water is circulating, which is exactly what stops the heat exchanger being damaged. What is not safe is working around it with repeated resets or by opening the casing. If you smell gas, ring 0800 111 999 rather than treating it as a fault code.

Will topping up the pressure clear F75?

Only when low pressure was genuinely the cause, which means the gauge was under 1 bar before you touched it. If it was already sitting between 1 and 1.5 bar, adding water changes nothing and can push you over the top of the range. That is the most common wasted hour on this code.

How long does an F75 repair take?

Diagnosis is normally one appointment. Completion is not always, because pumps, pressure sensors and control boards for older ecoTEC models are often ordered in rather than carried. Ask about parts availability when you book, so a second visit is not a surprise.

Does an F75 affect my Vaillant guarantee?

A fault code does not void a guarantee. The condition that catches people out is servicing. Vaillant's domestic boiler guarantee terms state that "The boiler must be serviced annually by a Vaillant or other registered Gas Safe Registered engineer in accordance with the manufacturer's instructions", and that "The cost of annual servicing is not included in the guarantee". If your boiler is still in guarantee, go to Vaillant first, because a repair arranged elsewhere can put that cover at risk.

How to stop F75 coming back

No trick makes a pump or a sensor last forever, but three habits genuinely reduce how often this code turns up.

Keep the annual service. It is a written condition of Vaillant's guarantee, and it is the appointment at which a sticking pump or a drifting sensor gets noticed before it locks the boiler out on the first cold morning of the year.

Keep the pressure in the band and leave it there. Vaillant's stated normal range is 1 to 1.5 bar cold. Constant topping up is not maintenance, it is a symptom, because every refill puts fresh oxygenated water into a sealed system. If you are repressurising more than once or twice a year, the real question is where the water is going. Our guide to Vaillant boiler pressure walks through that.

Write down what the boiler did. Whether the pump made any noise, what the gauge read cold, how many times it has locked out, and in what weather. F75 has five listed causes, and an engineer who arrives with that detail narrows it before the casing comes off.

Where a cover plan fits

If the engineer finds a failed pump, pressure sensor or wiring harness, that is a boiler breakdown repair. Smart Plan is a service plan, not insurance, and it is modular, so you pick the parts of your home you want covered. With the boiler and central heating module on your plan, parts and labour are covered up to £500 in a 12-month period if the boiler is under seven years old, or up to £200 if it is over seven, and the plan's exclusions still apply. We aim to get an engineer out to you within two working days, with call-outs Monday to Friday, 08:00 to 18:00. A £95 call-out fee, paid in advance, applies in defined cases, including the first 30 days of a new plan, excluded issues, no access, and an early annual service. Faults that were already there before your plan started are not covered. See what boiler and central heating cover includes.

Sources: Vaillant boiler fault codes, Vaillant boiler repair, Vaillant domestic boiler guarantee and the Gas Safety (Installation and Use) Regulations 1998.