In This Guide
Intro
Vaillant is one of the most widely fitted boiler brands in the UK, and most Vaillant faults fall into a handful of familiar patterns. Sometimes the problem shows as a symptom, no hot water, radiators staying cold, pressure that keeps dropping, or a leak. Other times the boiler displays an F-code and locks out to protect itself.
This page helps with both. Start with the common problems section if you have a symptom but no code, or go straight to the fault-code directory if your Vaillant is showing one. Either way you will find the likely cause, what you can safely check yourself, and when it needs an engineer. 0800 Homefix repairs Vaillant boilers across London and the South East seven days a week, 365 days a year.
Common Vaillant Boiler Problems
Not every Vaillant fault shows a code. If the boiler is misbehaving but there is no F-code on the display, use the symptoms below to find the likely cause.
Symptom | Likely cause | What to do |
No hot water but heating works, or the reverse | Diverter valve sticking, a failing diaphragm, or a control fault | Some diverter faults need an engineer; check the boiler is set to both heating and hot water first |
Boiler pressure keeps dropping | A leak somewhere in the sealed system, a bleeding radiator, or a failing expansion vessel | Top up once via the filling loop to around 1.2 bar on a cold system; if it drops again, book a diagnosis rather than repeatedly refilling |
Radiators cold at the bottom or not heating | Sludge build-up, trapped air, or poor circulation | Bleed the radiators; if it persists, the system may need a powerflush |
Boiler makes banging or kettling noises | Limescale or sludge on the heat exchanger restricting flow | Needs an engineer to diagnose; often resolved by a powerflush |
Boiler locks out and shows an F-code | A specific fault the boiler has detected | Find the code in the directory below for the exact cause and fix |
No heating or hot water at all, no code | Power, gas supply, thermostat, or a frozen condensate pipe | Check power, gas and thermostat; thaw a frozen condensate pipe if safe to reach |
If a symptom keeps returning after a basic check, that is the boiler telling you the underlying fault is still there and needs an engineer.
Vaillant Fault Code Directory
If your Vaillant is showing an F-code, find it below for what it means and a link to the full fix guide.
Code | What it means | Full guide |
F1 | Low water pressure detected in the system | https://www.0800homefix.com/heating/vaillant-f1-low-water-pressure-causes-and-solutions/ |
F20 | Overheat safety lockout — water temperature beyond the safe range | https://www.0800homefix.com/heating/vaillant-f20-error-what-it-means-and-how-to-fix-it/ |
F22 | Low water pressure — protective shutdown so the boiler cannot fire | https://www.0800homefix.com/heating/vaillant-f22-error-code-causes-and-solutions/ |
F23 | Temperature difference between flow and return — a circulation fault | https://www.0800homefix.com/heating/vaillant-f23-error-flow-and-return-sensor-fault-explained/ |
F24 | Pump failure or air in the system causing poor circulation | https://www.0800homefix.com/heating/vaillant-f24-error-code-pump-failure-or-air-in-the-system/ |
F28 | Ignition failure — the boiler could not establish a flame at start-up | https://www.0800homefix.com/heating/vaillant-f28-failure-in-gas-supply-causes-and-solutions/ |
F29 | Flame loss during operation — the flame went out after igniting | https://www.0800homefix.com/heating/vaillant-f29-error-code-causes-and-solutions/ |
F32 | Fan speed variation — an air or flue fault, safety lockout | https://www.0800homefix.com/heating/vaillant-f32-fan-speed-variation-causes-and-solutions/ |
F62 | Gas valve shutoff fault — gas detected when it should be off | https://www.0800homefix.com/heating/vaillant-f62-gas-valve-shutoff-fault-causes-and-solutions/ |
F65 | Electronics temperature too high — a PCB or wiring fault | https://www.0800homefix.com/heating/vaillant-f65-electronics-temperature-too-high-troubleshooting-guide/ |
F75 | Pump or pressure sensor fault — no pressure rise when the pump runs | https://www.0800homefix.com/heating/vaillant-f75-pump-water-pressure-fault-causes-and-solutions/ |
F76 | Heat exchanger overheating — thermal fuse triggered | https://www.0800homefix.com/heating/vaillant-f76-error-code-heat-exchanger-overheating/ |
How To Use Your Fault Code
Note the code exactly as it appears on the display, then find it in the directory above. As a general rule, a single reset is worth trying once: turn the boiler off at the power for a few seconds, then back on. If the code clears and does not return, it was likely a one-off. If it comes straight back, or the code is a safety lockout, that is the boiler telling you the fault is still present and needs diagnosing rather than repeated resetting.
Anything involving gas, ignition, the flue or a persistent lockout is a Gas Safe job by law, not a homeowner fix. If you can smell gas at any point, do not attempt a reset. Leave the property, open a door or window if it is safe to do so, and call the National Gas Emergency Service on 0800 111 999.
When To Call 0800 Homefix
If the reset does not hold, if F2 keeps returning, or if your gas supply is clearly fine, the fault is almost certainly internal and needs professional diagnosis. Flame-sensing electrodes, gas valves, fans and control boards are not homeowner repairs. By law, any work on the gas side of a boiler must be carried out by a Gas Safe registered engineer, both for your safety and to keep any warranty intact. Every 0800 Homefix engineer is Gas Safe registered.
If your boiler is also throwing other codes, it can help to read across them. Our guides to the Ideal L1 fault, the Ideal F3 fan fault and Ideal error code 0 cover the codes that most often appear alongside a flame-loss fault.
Spotted Any of These Warning Signs?
Our leak detection specialists cover London and the South East using acoustic detection, thermal imaging and moisture meters — no unnecessary damage, full written report, available 7 days a week.
Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
