Vaillant Boiler Fault Codes Explained: F75, F28 and F22 (London Guide 2026)
Vaillant ecoTEC showing F.75, F.28 or F.22? What each code actually means, the checks you can safely do yourself in five minutes, and the exact point where you must stop and call a Gas Safe engineer.
F.22 means the boiler has shut down because system water pressure is too low, and it is the one code on this page a homeowner can usually clear alone by topping the system up to 1.2 bar through the filling loop. F.75 means the boiler started its pump but detected no rise in pressure, which points to a failed water pressure sensor, a failed or airlocked pump, or a system that is very low on water; top up to 1.2 bar and reset once, and if it returns it is a Gas Safe engineer job because the part sits inside the casing. F.28 means the boiler tried to light and no flame was established, so before calling, check the gas is on and other gas appliances work, check the boiler is not in credit lockout on a prepayment meter, and in cold weather check the external condensate pipe has not frozen. Never remove the boiler casing, and if you smell gas, stop and ring the National Gas Emergency Service on 0800 111 999 first.
A Vaillant that has stopped working and is showing a code on its display has already done you a favour. It has told you which of its internal safety checks failed. The problem is that Vaillant's codes are written for engineers, so F.75, F.28 and F.22 mean very little to a homeowner standing in a cold kitchen at seven in the morning wondering whether this is a five-minute fix or a call-out.
This guide translates the three codes we attend most often on London ecoTEC installations. For each one you get what the boiler is actually complaining about, the checks you can safely do yourself from outside the casing, and the clear point at which you stop and ring a Gas Safe registered engineer. Nothing in here asks you to open the boiler, touch a gas component or defeat a safety device, because none of that is legal or safe for anyone who is not on the Gas Safe Register.
⚠️ Before anything else: if you can smell gas, this is not a boiler repair problem. Turn the gas off at the meter's emergency control valve, open windows and doors, do not touch electrical switches, do not use naked flames, and ring the National Gas Emergency Service on 0800 111 999. It is free and staffed 24 hours. Repairs to the appliance happen after the supply has been made safe.
How Vaillant Fault Codes Work
Vaillant uses an F prefix for faults. Depending on the model and the display, the same fault appears as F.75 or F75, so treat the two as identical. Codes beginning with S are status codes rather than faults: they tell you what the boiler is currently doing, such as running a pump overrun or waiting on a demand, and they do not mean anything is wrong.
The critical thing to understand is that a fault code names the check that failed, not the part that broke. F.75 does not mean "your pump is dead". It means the boiler expected to see a pressure change when the pump ran and did not see one. That could be the pump, the sensor that reads the pressure, or the water in the system. Which of the three it is takes a meter and a set of eyes inside the casing, which is why the honest answer for two of the three codes below is that the diagnosis itself is engineer work.
💡 Write the code down before you reset. Some Vaillant models clear the code from the display once the fault resets, and an engineer who knows you had F.75 twice overnight will arrive with the right parts on the van. It is the single cheapest thing you can do to shorten a call-out.
F.22: Low Water Pressure (Dry Fire Protection)
What the boiler is saying: there is not enough water in the sealed heating circuit to fire safely, so I have shut down before I damage the heat exchanger by running it dry.
This is the friendliest of the three codes, because in most cases the cause is simply that the system pressure has drifted below roughly 1 bar and the boiler has locked out. It is also the only one of the three that a homeowner can usually resolve without an engineer, because everything you need to touch is outside the casing.
What you can safely check and do yourself
- Read the pressure gauge on the front of the boiler, or the digital pressure reading on the display. Below about 1 bar when the system is cold is low; below 0.8 bar is usually a lockout.
- Top the system up through the filling loop. On most Vaillant ecoTEC installations this is a short braided metal hose under the boiler, often with two blue lever handles that need opening together. Some newer models use a single silver handle or an internal filling key built into the casing rather than a loose braided hose.
- Stop at 1.2 bar. Do it with the boiler switched off and cool, open the valves slowly, and close them the instant the needle reaches 1.2. Releasing excess pressure is far more awkward than adding it. Our step-by-step repressurising guide walks the whole process with photos of the valve types.
- Reset once if the code has not cleared by itself when you switch the boiler back on.
When you stop and call an engineer
Call if the pressure will not rise at all when you open the filling loop, if the code persists at 1.2 bar, or if the gauge reads above 2.5 to 3 bar, in which case the answer is to release pressure rather than add it and that is not a homeowner job.
More importantly, call if F.22 keeps coming back. A sealed system that is behaving should hold pressure for six to twelve months between top-ups. If you are topping up weekly, the water is going somewhere: a weeping radiator union, a hidden pipe joint, a failed expansion vessel, or a pressure relief valve discharging out through the wall. Repeated topping up masks the fault and pumps fresh oxygenated water into the system, which accelerates corrosion in the radiators and the heat exchanger. We rank the four causes by how often we actually see them in why your boiler keeps losing pressure.
✓ Free 30-second diagnostic: step outside and look at the brass pipe that terminates low on the external wall near the boiler. If it is wet, dripping, or streaked with white limescale, the pressure relief valve has been discharging and your F.22 almost certainly traces back to a failed expansion vessel. Telling the engineer that on the phone can halve the diagnosis time on site.
F.75: No Pressure Change Detected When the Pump Starts
What the boiler is saying: I started the pump at the beginning of a demand and expected the water pressure to change as water began to move. It did not change, so I do not trust that the circuit is circulating and I have shut down.
F.75 is the most searched Vaillant code in the UK and also the most misunderstood, because people read it as "the pump has failed" and then find the pump is fine. The boiler is reporting a mismatch between two things: the pump running, and the water pressure sensor reporting a change. In practice, three causes cover the overwhelming majority of F.75 call-outs.
- A faulty water pressure sensor. The sensor reads system pressure and reports it to the boiler's control board. In London's hard water areas it scales up and stops reading changes accurately, so the boiler sees a flat line even though the pump is doing its job. This is the most common single cause we find.
- A failed, seized or airlocked pump. If the pump genuinely is not moving water, the boiler is correct to lock out. Pumps seize after long summer idle periods, and sludge in an older London system is a frequent contributor.
- Very low system water. If pressure is already near zero there is little for the pump to act on, and F.75 can appear alongside or instead of a low pressure code.
What you can safely check and do yourself
- Check the pressure gauge first. This is the only cause of the three you can address from outside the casing. If it reads below 1 bar, top up to 1.2 bar through the filling loop exactly as described for F.22 above.
- Reset once after topping up, and give the boiler a full start cycle.
- Listen. Stand near the boiler on a demand. A pump that hums but produces no flow sound, or a boiler that is silent where you would normally hear circulation, is a useful observation to pass on. Listening is fine. Opening anything to look is not.
⚠️ Stop here. Both the water pressure sensor and the pump sit inside the boiler casing on every Vaillant ecoTEC model. Under the Gas Safety (Installation and Use) Regulations 1998, opening that casing is restricted to a Gas Safe registered engineer. There is no homeowner test, no bypass, and no reset trick that resolves a genuine sensor or pump fault. If F.75 returns after one top-up and one reset, book the visit.
The good news is that F.75 is normally a same-visit fix. A water pressure sensor is an inexpensive part and a straightforward swap, and an engineer who knows the code before arriving will usually carry one for the common ecoTEC ranges.
F.28: Ignition Failure at Start-Up
What the boiler is saying: I ran my ignition sequence at start-up and no flame was established, so I have locked out rather than keep trying.
You can often hear F.28 happening: a run of clicks from the ignition electrodes, a pause, another attempt, and then the code. Because the fault is that the boiler could not light, the productive checks are all about whether gas is actually reaching the appliance and whether the flue and condensate paths are clear. Several of these are genuinely common causes, and all of them are outside the casing.
What you can safely check and do yourself
Confirm the gas supply is actually on
Find the emergency control valve at the gas meter. When the lever sits in line with the pipe the gas is on; across the pipe it is off. It is more common than you would think for a valve to have been turned off during other work and never turned back.
Test another gas appliance
Try the gas hob. If the hob will not light either, the problem is the supply rather than the boiler, and that is a very different phone call. If the hob lights normally, gas is reaching the property and the issue is at the boiler.
Check the meter is in credit
On a prepayment meter, a boiler that has run out of credit and been topped back up sometimes needs the emergency credit accepted at the meter before gas flows properly again. Worth 30 seconds before you pay a call-out fee.
In cold weather, check the condensate pipe
Find the white plastic pipe leaving the property, usually 22mm or 32mm and often the only white pipe on that wall. After a hard frost it can freeze solid, and a blocked condensate route commonly presents on Vaillant units as an ignition or lockout fault, frequently with a gurgling sound. Pour warm, never boiling water along it, concentrating on external bends and the exposed end. Boiling water can crack the pipe or its fittings.
Reset once, and once more at most
Hold the reset button for around five seconds. If the boiler fires and stays running, you have found it. If F.28 returns, stop resetting and book an engineer.
When you stop and call an engineer
If gas is on, other appliances work, the meter is in credit and the condensate pipe is clear, then the remaining causes all sit inside the appliance. The gas valve, the ignition electrodes, the flue and the boiler's own controls are Gas Safe territory without exception. Two resets is the limit. A lockout after failed ignition is the boiler doing precisely what it is designed to do, and repeatedly forcing it to retry an unsafe start is the one thing you should not do.
If a frozen condensate pipe turns out to be the culprit and it keeps refreezing every cold snap, ask an engineer to re-route or lag it properly. It is a cheap job that saves a winter of call-outs, and we cover the wider winter picture in our frozen pipes guide for London.
Other Common Vaillant Codes You Might See
A short reference for codes that turn up alongside the three above. The right-hand column is the honest answer about who can act on it.
| Code | What it indicates | Homeowner or engineer? |
|---|---|---|
| F.22 | Low water pressure / dry fire protection | Homeowner can top up to 1.2 bar via the filling loop |
| F.75 | No pressure change detected when the pump starts | Check pressure only, then engineer |
| F.28 | No flame established during start-up ignition | Gas, meter and condensate checks only, then engineer |
| F.29 | Flame established but then went out and would not re-light | Same external checks as F.28, then engineer |
| F.27 | Flame signal detected when there should not be one | Engineer only |
| F.20 | Safety shutdown on overheat | Engineer only |
| F.23 / F.24 | Safety shutdown on temperature behaviour, often linked to poor circulation | Engineer only |
| Any F.6x | Gas valve or electronics fault reported by the control board | Engineer only |
If your code is not listed here, do not guess at it. Note it exactly as displayed, including the dot if there is one, and quote it when you ring. Vaillant publish the full fault list in the installation and servicing instructions for each ecoTEC model, and an engineer will cross-reference it against your specific unit rather than a generic list.
Resetting a Vaillant Safely
On most ecoTEC models the reset is a dedicated button or the flame symbol on the control panel, held for roughly five seconds. That much is straightforward. The discipline is in how often you use it.
- Only reset after you have changed something. Topping the pressure up, thawing a condensate pipe or turning the gas back on are changes. Pressing reset again five minutes later is not.
- Two or three attempts, maximum. Vaillant's guidance across the range is to stop and seek qualified help once a fault persists through a small number of resets.
- A code that clears and returns the same day is a persistent fault. Book the visit rather than resetting through the evening.
The lockout is not the boiler being awkward. It is a safety mechanism that has decided a start sequence is not safe to repeat, and overriding it by habit is how a cheap sensor fault becomes an expensive one.
Where You Must Stop: The Gas Safe Line
This is the part that matters more than any code on this page. Under the Gas Safety (Installation and Use) Regulations 1998, work on a gas appliance in the UK may only be carried out by an engineer on the Gas Safe Register. Removing the casing of a sealed-system gas boiler counts as work on the appliance. It is a criminal offence for anyone else to do it, it invalidates the boiler warranty and often the home insurance, and it puts you alongside live mains electrics, a pressurised water circuit and a gas carrying component simultaneously. It also breaks the room-sealed integrity of the appliance, which is what keeps combustion products in the flue rather than in your home.
So the boundary is simple and it is physical: outside the casing is yours, inside the casing is not.
💡 Outside the casing, and yours to do: reading the pressure gauge, using the external filling loop, checking the gas meter's control valve and other gas appliances, confirming prepayment credit, thawing an external condensate pipe with warm water, checking the outside pressure relief discharge pipe for drips, and writing the fault code down.
⚠️ Inside the casing, and never yours to do: the pump, the water pressure sensor, the gas valve, the ignition electrodes, the expansion vessel, the pressure relief valve, the heat exchanger, the PCB and the flue. There is no exception to this for "just having a look", and there is no circumstance in which bypassing or defeating a safety device is acceptable.
When an engineer does attend, ask to see their Gas Safe ID card and check the registration number at gassaferegister.co.uk. The card shows which appliance types they are qualified to work on, and the number on the card should match the one on your invoice.
What These Repairs Typically Cost in London (2026)
Indicative London pricing. The call-out covers travel and the first 45 minutes on site including diagnosis, and any parts are quoted before fitting rather than after.
| Job | Typical London cost (2026) | Notes |
|---|---|---|
| Call-out + diagnosis (weekday daytime) | £60 | Includes first 45 minutes on site |
| Call-out + diagnosis (out-of-hours / weekend) | £90 | Same 45-minute window |
| Repressurise, reset or minor fix (no parts) | £60–£120 | Often completed inside the call-out window |
| Frozen condensate re-route + lag | £90–£160 | Stops the F.28 recurring every cold snap |
| Expansion vessel / pressure relief valve | £120–£280 | The usual cause behind a repeating F.22 |
| Pump replacement | £180–£380 | Part and labour, one of the F.75 outcomes |
| PCB (circuit board) replacement | £350–£650 | Often the repair versus replace tipping point |
Two things are worth knowing before you book. First, the call-out is not dead money: on most fault code jobs the diagnosis and a minor fix both happen inside that first window, so plenty of visits finish at the £60 to £120 mark. Second, if the diagnosis lands on a PCB or a heat exchanger on a boiler that is already 12 to 15 years old, pause and price a replacement before committing. Our emergency boiler repair guide for London sets out the full repair versus replace maths.
Landlords: A Fault Code Is a Repair Clock Starting
If you let the property, a locked-out boiler is a legal matter as well as a practical one. Under Section 11 of the Landlord and Tenant Act 1985 you must keep the installations for space heating and hot water in repair and proper working order. Under the Homes (Fitness for Human Habitation) Act 2018, a home left without heating or hot water, particularly in winter or where a child, an elderly or a vulnerable tenant lives, can be judged unfit for human habitation.
There is no single statutory deadline, but the accepted benchmark is to act within roughly 24 hours for a total loss of heating or hot water, and same day where a vulnerable person is affected. Two practical points: ask the tenant to photograph the display showing the code, which speeds up the dispatch and gives you a dated record, and never coach a tenant into opening the boiler. Separately, the annual Gas Safety Record (CP12) from a Gas Safe engineer is required for every gas appliance in the property regardless of whether anything has broken.
Frequently Asked Questions
What does F75 mean on a Vaillant boiler?
What does F28 mean on a Vaillant boiler?
What does F22 mean on a Vaillant boiler and can I fix it myself?
How many times can I reset a Vaillant boiler?
Is a Vaillant fault code an emergency?
Can I take the cover off my Vaillant to look inside?
Why does my Vaillant show F75 and F22 together?
The Bottom Line
Of the three codes, only F.22 is reliably a homeowner fix, and even then a repeating F.22 is a leak or an expansion vessel telling you something. F.75 gives you one check worth doing (the pressure gauge) before the diagnosis moves inside the casing. F.28 gives you four checks worth doing (gas on, other appliances, meter credit, frozen condensate) and a decent chance of fixing it yourself in a London winter. Past those points, the code stops being information you can act on and becomes information your engineer needs.
Note the code exactly as displayed, do the outside-the-casing checks, reset no more than twice, and if you smell gas ring 0800 111 999 before anything else.
Emergency Repairs London provides same-day and 24/7 emergency boiler repair with Gas Safe registered engineers across all 32 London boroughs, including Vaillant ecoTEC diagnostics and sensor, pump and expansion vessel replacement. Fixed £60 daytime call-out, quoted before work starts. Lines are open 24/7 on 0207 046 1363.
Key Takeaways
- Vaillant displays faults with an F prefix (shown as F.22 or F22 depending on the display). The code tells you which safety check failed, not which part has broken.
- F.22 is low water pressure, or dry fire protection. Top the system up to 1.2 bar through the filling loop and reset. This is the one code most homeowners fix themselves.
- F.75 means the pump ran but the boiler saw no corresponding pressure change. Usual suspects are the water pressure sensor, the pump, or a system with too little water in it. Diagnosis needs the casing off, so it is Gas Safe work.
- F.28 is ignition failure at start-up. Check the gas is on, other gas appliances work, the meter is in credit, and in winter that the external condensate pipe has not frozen.
- Do not press reset more than two or three times. Repeated resets on an unresolved gas fault are exactly what the lockout exists to prevent.
- Never remove the boiler casing. Under the Gas Safety (Installation and Use) Regulations 1998 that work is restricted to a Gas Safe registered engineer, and it is a criminal offence otherwise.
- Any smell of gas is not a boiler repair call. Turn the gas off at the meter, ventilate, and ring the National Gas Emergency Service on 0800 111 999.
- A London Gas Safe call-out is £60 weekday daytime and £90 out-of-hours, covering travel and the first 45 minutes including diagnosis.
