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Vaillant's ecoTEC range is among the most reliable on the UK market, but no boiler is fault-free. Here's a genuine guide to the faults that crop up most, what you can safely check yourself, and what repairs typically cost in 2026.
Most Vaillant boilers sold in the UK are part of the ecoTEC family (ecoTEC plus, ecoTEC pro and the budget ecoFIT pure), almost all of them condensing combi or system boilers. When something goes wrong, the boiler stops and displays an F-code — a letter "F" followed by a number — on its digital screen. That code is the single most useful thing you can give an engineer, because it points straight at the area at fault before anyone removes the casing.
Some F-codes describe a problem you can put right yourself in a couple of minutes. Others are protective lockouts around the gas, flue or sealed combustion circuit, and those are strictly for a Gas Safe registered engineer. Knowing which is which saves you both an unnecessary call-out and a dangerous DIY attempt.
Safety first. You can safely top up pressure via the filling loop, bleed radiators, reset the boiler once from the front panel, check the thermostat and fuse, and thaw a frozen external condensate pipe. You must never touch the gas valve, gas pipework, the flue, the sealed combustion circuit, the pressure-relief valve or anything behind the casing — that's Gas Safe work by law. If you ever smell gas, leave the property and call the National Gas Emergency line on 0800 111 999.
One of the codes Vaillant owners see most. F22 means there isn't enough water in the heating system, so the boiler shuts down to protect the heat exchanger from "dry firing". The standard fix is to repressurise via the filling loop back to 1–1.5 bar when cold — a homeowner-safe job. If it keeps returning, you have an underlying leak, a weeping pressure-relief valve or a failed expansion vessel that needs an engineer. Full walk-through in our Vaillant F22 guide.
F28 means the boiler failed to ignite on start-up; F29 means it lit but the flame went out and couldn't be re-established. Causes range from a simple gas-supply issue (a closed valve, an empty LPG tank, low pressure on the network) to a faulty ignition lead, electrode, gas valve or condensate blockage. A couple of homeowner checks are worth doing first — confirm other gas appliances work, check the condensate pipe isn't frozen, and try a single reset — but the boiler should not be repeatedly reset on an ignition fault. See our guides to the F28 and F29 codes.
F75 appears when the boiler can't detect a rise in water pressure when the pump starts — usually a faulty pump, a stuck or failed pressure sensor, or trapped air. It's often confused with F22 because the symptoms overlap, but the cause is different and the repair is a Gas Safe job, not a top-up. Our F75 guide explains the difference.
On a combi, the diverter valve switches flow between heating and hot water. When it sticks or wears, you get the classic symptom: hot water at the taps but lukewarm or cold radiators, or the reverse. It doesn't always throw a specific F-code, which is why it's frequently misdiagnosed. It's a behind-the-casing repair for an engineer. If your Vaillant gives heating but no hot water, our Vaillant no hot water guide walks through the likely culprits.
The PCB is the boiler's "brain". When it fails you can see erratic behaviour, a dead display, random lockouts or codes that don't match the symptoms. It's one of the more expensive parts and, on an older boiler, often the point at which replacement starts to make more sense than repair.
Not every Vaillant fault needs a professional. Before you book a call-out, rule out the simple, homeowner-safe causes:
Anything beyond this list — and certainly anything involving the gas valve, flue, sealed circuit or PRV — needs a Gas Safe registered engineer. You can confirm an engineer's registration on the Gas Safe Register. (CORGI registration was replaced by Gas Safe in 2009, so always check the Gas Safe number.)
There's no single price for a Vaillant repair — it depends on the part, your boiler's age, where you live (London and the South East run higher), and whether the engineer can fix it in one visit or has to order a part. The figures below are indicative UK ranges for parts and labour combined, not quotes. Vaillant is a premium German brand, so genuine spares can sit at the upper end of these ranges.
| Repair | Why it fails / what it does | Indicative cost |
|---|---|---|
| Call-out / diagnostic | Attending and finding the fault; often rolled into the repair if you go ahead. | £60–£120 |
| Repressurise (F22) | Topping up via the filling loop — usually a free DIY job if there's no leak. | £0 (DIY) |
| Ignition lead / electrode (F28/F29) | Worn ignition components stopping the burner lighting reliably. | £120–£280 |
| Pump (F75) | Circulates water round the system; a worn pump causes no heat or noisy running. | £250–£450 |
| Pressure sensor | Common cause of intermittent F75 and pressure-related lockouts. | £120–£280 |
| Diverter valve | Switches between heating and hot water; the classic "hot taps, cold rads" fault. | £250–£450 |
| Gas valve | Controls gas to the burner — a skilled, Gas Safe-only job. Never DIY. | £300–£550 |
| Fan | Drives flue gases out; a failed fan usually locks the boiler out on safety. | £250–£500 |
| PCB (circuit board) | The boiler's "brain"; expensive and often a tipping point on older units. | £350–£650 |
| Expansion vessel | Absorbs pressure changes; a failed vessel causes pressure to creep up or drop. | £150–£350 |
Smaller jobs — clearing an airlock, fitting a new auto air vent, replacing a thermistor — can come in under £150. At the other end, if more than one major part has failed on an ageing Vaillant, the cumulative cost is what tips many owners towards a new boiler. For wider context, see our UK boiler repair cost guide.
Vaillant boilers are built to last and routinely run well past a decade with annual servicing, so a one-off fault on a newer unit is almost always worth repairing. The picture changes when the boiler is over roughly 10–12 years old, a major part like the PCB or gas valve has gone, or you've had two or three repairs in a year. At that point a £500-plus repair on a tired boiler starts to look like paying for a replacement in instalments. Ask a Gas Safe engineer to quote both options before you decide.
The reason Vaillant repairs sting is that they're unpredictable — a pump one winter, a PCB the next. A boiler cover plan swaps that lumpy risk for a fixed monthly cost: the plan picks up the call-out, labour and covered parts when something breaks, subject to your excess, claim limits and exclusions. Because Vaillant spares sit at the premium end, the difference between a single big repair bill and a capped monthly figure can be meaningful on an older unit.
It isn't automatically the right call for everyone. Most plans exclude pre-existing faults, won't cover a boiler that's "beyond economic repair", and apply an initial waiting period before you can claim. Whether it pays comes down to your boiler's age, your appetite for surprise bills, and the plan's price against its excess and exclusions. Our best boiler cover guide and the comparison tool show indicative prices and cover levels side by side. This is information to help you weigh it up, not personal advice — and we show a selected panel of providers, not the whole market.
F22 (low water pressure) is one of the most frequently seen, because system pressure naturally drifts down over time. It's usually a homeowner-safe top-up via the filling loop. F28/F29 ignition faults and F75 pump faults are the next most common.
Only the homeowner-safe checks: topping up pressure, bleeding radiators, a single front-panel reset, checking the thermostat and fuse, and thawing a frozen condensate pipe. By law, anything involving gas, the flue, the sealed circuit or parts behind the casing must be done by a Gas Safe registered engineer.
It varies by part. Indicative UK ranges run from around £120–£280 for ignition components up to £350–£650 for a PCB, parts and labour combined, plus a £60–£120 call-out if it isn't rolled in. Vaillant being a premium brand, genuine spares sit towards the upper end.
Vaillant's ecoTEC range has a strong reputation for reliability and longevity, and many run well past 12 years with annual servicing. No boiler is fault-free, but spares and engineer familiarity are widely available across the UK.
A parts-and-labour plan typically covers the call-out, labour and covered parts, subject to your excess and exclusions. Most plans won't cover pre-existing faults, very old boilers or replacements deemed "beyond economic repair", and there's usually a waiting period before you can claim.
Compare indicative prices and cover levels from our selected panel of UK providers, then buy direct on their site. Information, not advice — a chosen panel, not the whole market.
Compare boiler coverThis article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs and pressure figures are indicative UK guides for 2026. Vaillant is a trademark of its owner; this is an independent guide and we are not affiliated with Vaillant.