A heat generator fault on your Vaillant boiler usually means no heating and no hot water — and it tends to happen at the worst possible time. Most of these faults come with a clear error code, so you can work out roughly what's wrong before anyone books an engineer.
The heat generator is the core of your boiler: the heat exchanger, burner, ignition system, and the sensors that keep everything running safely. When something in that chain isn't right, the boiler shuts itself down and shows an error code rather than risk running unsafely.
The code on the display tells you which part of the system has the problem. You can check what each code means on our full guide to Vaillant boiler fault codes — that's the starting point for any fix.
- Content Guide
- » Common Causes of Vaillant Heat Generator Faults
- - Water Pressure Issues (F22)
- - Pressure Sensor and Pump Faults (F75)
- - Ignition and Flame Detection Faults (F27, F28, F29)
- - "Heat Generator 1 Requires Maintenance" Message
- » How to Troubleshoot a Heat Generator Fault
- - Check the Code First
- - Fix F22 Pressure Faults
- - Fix F75 Sensor and Pump Faults
- - Reset Your Boiler
- » When to Call a Professional
- » Keep Faults from Coming Back
Common causes of Vaillant heat generator faults
Water pressure issues (F22)
F22 is the fault you'll see most often. It means the water pressure in your system has dropped below about 1 bar, which stops the heat generator running safely. Full detail on this one is on our F22 fault page.
Usual causes are a small leak, radiators that have recently been bled, or pressure just dropping off gradually over time. F22 is the boiler protecting itself rather than a sign anything's broken.
Pressure sensor and pump faults (F75)
F75 is more involved. It shows up when the boiler expects to see a pressure change once the pump kicks in, and doesn't. See our F75 fault guide for the full breakdown — usual causes are a faulty pressure sensor, a pump that isn't circulating properly, or an airlock somewhere in the system.
Ignition and flame detection faults (F27, F28, F29)
F28 means the boiler tried to ignite and failed. F29 means the flame lit and then dropped out mid-cycle. F27 is different again — it's a phantom flame fault, where the boiler detects a flame signal when the gas valve is shut, which shouldn't be possible. All three usually trace back to gas supply, dirty or faulty electrodes, a blocked gas valve, or moisture on the board. See how often boiler electrodes need replacing if that's the likely culprit — this isn't something to test yourself.
“Heat generator 1 requires maintenance” message
Some Vaillant models show a maintenance message rather than a fault code — “heat generator 1 requires maintenance” is the common one. It's not an emergency. It's the boiler flagging that a service is due, usually based on running hours rather than an actual fault. Booking an annual boiler service clears it.
How to Troubleshoot a Heat Generator Fault
Check the code first
Note the exact code on the display before doing anything else. Vaillant models vary slightly in how they use each code, so check your manual if you're not sure.
Fix F22 pressure faults
Find the filling loop and top up the pressure slowly, aiming for around 1.5 bar when the system is cold. Reset the boiler once it's back in range. If the pressure drops again within a day or two, you've likely got a leak — that needs a professional look.
Fix F75 sensor and pump faults
Start by bleeding your radiators to clear any airlocks, working from the ground floor up. If the fault comes back, the pressure sensor or pump itself is the likely cause, and that's a job for an engineer.
Reset your boiler
Hold the reset button — usually marked with a flame symbol — for a few seconds and let the boiler run its full startup cycle. Full steps are in our boiler reset guide. If the fault reappears, don't keep resetting it — repeated faults mean something needs looking at properly.
When to call a professional
Water pressure faults you can usually sort yourself. Sensor, pump, ignition and flame faults need a Gas Safe registered engineer — these involve gas safety and components that shouldn't be opened up without the right training.
If your boiler keeps tripping the same fault after a reset, stop resetting it and book a boiler repair instead.
Keep faults from coming back
Most heat generator faults are avoidable with regular servicing. An annual boiler service catches worn parts, corrosion and pressure issues before they turn into a breakdown, and keeps your gas safety certificate current if you're a landlord.
Got a heat generator fault that won't clear? Book a Gas Safe registered engineer with Serviceteam and get it sorted properly.
