Read Time: 10 minutes
If your Vaillant boiler has lost pressure, thrown up an F code or stopped making hot water, you’re in good company. Most Vaillant faults trace back to three things: water pressure, water circulation or ignition. A few can be checked safely at home in five minutes. The rest need a Gas Safe engineer, and knowing which is which saves you money.
This guide covers the 10 faults we see most in London homes, what each fault code means, what to check before you pick up the phone, and what repairs typically cost in 2026.
Safety first: If you smell gas or suspect carbon monoxide, turn off the gas at the meter, open the windows, leave the property and call the National Gas Emergency Service on 0800 111 999. Never remove the boiler casing or touch gas components yourself.
Quick answer: what is your Vaillant telling you?
| What you see | Most likely cause | Can you check it yourself? |
|---|---|---|
| Pressure below 1 bar, or F22 | Leak, recently bled radiators, tired expansion vessel | Yes, repressurise once and watch it |
| Pressure drops every few days | Slow leak, expansion vessel or pressure relief valve | No, book an engineer |
| Heating works, no hot water | Diverter valve or blocked plate heat exchanger | No |
| F75 | Water pressure sensor or pump | Reset once, then call if it returns |
| F28 or F29 | Ignition electrode, gas supply or condensate pipe | Check gas meter and condensate pipe, reset once |
| Banging or kettling | Limescale or sludge restricting flow | No |
| Radiators cold at the bottom | Sludge | Bleed first, then call if no change |
| Boiler switches on and off | Poor circulation, pump or overheating | No |
Why Vaillant boilers struggle in London homes
Vaillant boilers are well built, and the ecoTEC Plus sits in a huge number of London flats and loft conversions. The boiler is rarely the real villain. London is a hard water area, so limescale builds up inside heat exchangers and sensors. Older properties often have long pipe runs, mixed old and new radiators, and years of sludge. All of that restricts flow, and restricted flow sits behind many of the faults below.
[From the field: add 2 to 3 sentences from a real recent job here. Give the boiler model, the symptom, what you found and how long the repair took. A genuine example is the strongest trust signal on this page.]
The 10 most common Vaillant boiler problems
1. Pressure keeps dropping (F22)
Your boiler needs water pressure to run. When it falls too low, the boiler shuts itself down and shows F22. A healthy cold reading is 1 to 1.5 bar. A small rise when the heating is on is normal, but pressure should never climb past 2.5 bar.
Likely causes: a leak at a radiator valve, pipe joint or the pressure relief pipe; an expansion vessel that has lost its charge; or radiators that were bled recently.
A simple test: switch the heating on and watch the gauge. If pressure shoots up then falls back when the boiler cools, suspect the expansion vessel. If it creeps down slowly over days, suspect a leak.
Call an engineer if you are topping up more than once a month. Every top-up pulls in fresh water carrying oxygen and minerals, which speeds up corrosion and scale, and it hides the real fault.
2. Heating works but there is no hot water
On a combi, the diverter valve decides whether water goes to your radiators or your taps. If it sticks, you get heating but lukewarm or cold taps. In London, a plate heat exchanger clogged with limescale is the other usual suspect. A faulty flow sensor or thermostat can also play a part.
Check: that the hot tap is fully open, that boiler pressure is above 1 bar, and that the temperature dial hasn’t been knocked down. If those are fine, this is an internal repair.
3. F75: pump or pressure sensor fault
Vaillant describes F75 as the boiler failing to detect a pressure change when the pump starts. The two usual culprits are a faulty water pressure sensor or a faulty internal pump.
Here is the practical way engineers tell them apart. If the pump sounds and feels normal but you still lose heating and hot water, the sensor is the more likely cause. Sensors can clog with dirt, debris and limescale, which is why F75 turns up so often in hard water areas and on older systems with sludge. If the pump is silent, noisy or running hot, the pump itself is the suspect.
A reset may clear it once. If F75 returns, book a repair before it becomes a full breakdown.
4. Ignition failure (F28) and flame loss (F29)
F28 means the boiler tried to light and failed. F29 means it lit, then lost the flame. Common causes are a worn ignition electrode, an interrupted gas supply, or a blocked condensate pipe.
Check before calling:
- Is your gas meter on credit if you are on a prepayment meter?
- Do other gas appliances, such as your hob, work?
- In freezing weather, is the external condensate pipe frozen? Pour warm (never boiling) water over it.
If none of that helps, don’t keep resetting. Repeated failed ignitions need an engineer.
5. Boiler keeps switching off (short cycling)
When the boiler fires up and shuts down every few minutes, water is not moving through the system fast enough, so the boiler overheats and cuts out. Causes include a weak pump, sludge, a partly closed radiator valve or a bypass set incorrectly. Short cycling wastes gas and wears out parts, so it is worth fixing early.
6. Banging or kettling noises
Kettling sounds like a kettle about to boil. Limescale or sludge creates hot spots in the heat exchanger, and water boils locally and bangs. Left alone, it cuts efficiency and can damage the heat exchanger. A descale or system clean usually solves it, and fitting a magnetic filter helps stop it coming back.
7. Radiators not heating properly
If the top of a radiator is cold, it needs bleeding. If the bottom is cold and the middle is warm, sludge is the likely cause. Bleed the radiator, check boiler pressure afterwards (bleeding lowers it), and if the cold patches stay, a professional system flush is the next step.
8. Boiler is dead or shows no fault code
No display and no code is usually something simple. Check the boiler is switched on at the fused spur, the programmer is calling for heat, the room thermostat has fresh batteries, and the gas is on. If the display is still blank, the problem may be the circuit board or a power fault, which needs an engineer.
9. Cold-weather lockouts
When temperatures drop, the condensate pipe from a condensing boiler can freeze and shut the boiler down. This is one of the most common winter callouts. If the pipe runs outside, thaw it with warm water and ask your engineer to lag it or re-route it so it doesn’t happen every cold snap.
10. Water leaking from the boiler
A drip from the pressure relief pipe outside usually means pressure has climbed too high or the relief valve has failed. A leak from inside the boiler could be a failed seal, pump or heat exchanger. Switch the boiler off, don’t top up the pressure, and call an engineer.
Vaillant fault codes explained
| Code | What it usually means | What to do |
|---|---|---|
| F.22 | Water pressure too low | Repressurise once; call if it drops again |
| F.23 | Temperature difference too large, low circulation | Call an engineer |
| F.24 | Temperature rising too fast | Call an engineer |
| F.25 | Flue gas temperature too high | Call an engineer |
| F.27 | Flame detected when there shouldn’t be one | Reset once; call if it returns |
| F.28 | Ignition failed on start-up | Check gas supply, reset once |
| F.29 | Flame lost during operation | Check gas supply and condensate pipe |
| F.32 | Fan fault | Call an engineer |
| F.61 / F.62 | Gas valve control fault | Call an engineer |
| F.73 / F.74 | Water pressure sensor fault | Call an engineer |
| F.75 | No pressure change when pump starts | Reset once; call if it returns |
| F.77 | Flue flap or condensate fault | Check condensate pipe; call if it persists |
How to reset a Vaillant boiler: press and hold the reset button for about three seconds until the code clears. If the same code comes back, the boiler is telling you something real. Don’t keep resetting it.
How to repressurise your Vaillant boiler
Always check your model’s manual first, as filling loops differ.
- Switch the boiler off and let it cool.
- Find the silver filling loop under the boiler and check both ends are connected.
- Open the valves slowly. You should hear water flowing.
- Watch the gauge and stop at 1 to 1.5 bar.
- Close both valves fully, and remove the loop if yours is detachable.
- Switch the boiler back on and check that pressure holds.
If you overshoot, bleed a radiator to bring it down. If pressure falls again within days, you have a leak or a failing component.
Can you fix it yourself or should you call an engineer?
| Problem | DIY check | Call an engineer |
|---|---|---|
| Low pressure, first time | Yes, repressurise | If it keeps dropping |
| Air in radiators | Yes, bleed them | Only if cold spots remain |
| Frozen condensate pipe | Yes, warm water | If it keeps freezing |
| Gas on prepayment meter | Yes, check credit | No |
| F75, F28, F29 repeating | No | Yes |
| No hot water | No | Yes |
| Kettling or banging | No | Yes |
| Any gas smell | No | Emergency: 0800 111 999 |
What Vaillant repairs cost in London (2026)
Prices vary by model, part availability and whether you call out of hours. These are typical ranges for London, including parts and labour. Always ask for a fixed quote before work starts.
| Repair | Typical London cost |
|---|---|
| Repressurise and leak check | £95 to £250 |
| Ignition electrode (F28) | £120 to £240 |
| Pressure sensor or pump (F75) | £180 to £450 |
| Diverter valve | £180 to £450 |
| Expansion vessel | £120 to £380 |
| Circuit board (PCB) | £300 to £650 |
Repair or replace?
Vaillant says its boilers typically last 10 to 15 years. If yours is older than that, repairs keep coming back, or one repair costs more than about half the price of a new combi (entry-level installs start around £1,750), replacing is usually the smarter spend. If it is under 10 years old and the fault is a single part, repair is almost always better value.
How to prevent Vaillant boiler problems
- Book an annual service with a Gas Safe engineer.
- Check your pressure gauge monthly.
- Bleed radiators at the start of each heating season.
- Fit a magnetic filter and use inhibitor to fight sludge and limescale.
- Insulate any external condensate pipe before winter.
- Consider a system flush if your heating is more than 10 years old.
When to call a Gas Safe engineer
Call a professional if your pressure keeps dropping, the same fault code keeps returning, you have no hot water, you hear banging, or you see any leak. Always ask to see the engineer’s Gas Safe ID card and check them on the Gas Safe Register.
Need Vaillant boiler repair in London? Our Gas Safe engineers offer fast fault diagnosis, same-day repair where available and emergency support.
📞 Call 07877 767776
Frequently asked questions
A reset only clears minor errors. If the boiler still won’t run, the cause is usually low pressure, an ignition fault or a failed part such as the diverter valve or pump. It needs inspecting.
A slow loss points to a small leak, a failing expansion vessel or a worn valve. Topping up repeatedly only hides it.
It means the boiler didn’t detect a pressure change when the pump started. The cause is usually a faulty pressure sensor or pump, and sludge or limescale can contribute.
Low water pressure. Repressurise to 1 to 1.5 bar once. If it drops again, there is a leak or component fault.
1 to 1.5 bar when cold, rising slightly when hot, and never above 2.5 bar.
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