What to check, in order
- Check the gas is on
Make sure the gas stopcock is open.
- In cold weather, check the condensate pipe
It's the small white plastic pipe that runs from the boiler to a drain outside. A bubbling or gurgling sound from the boiler is another sign it's frozen.
- Thaw it with warm water
Pour warm water along the frozen part, from a watering can for example. Never use boiling water: it can crack the pipe.
- Reset the boiler
Older ecoTEC plus and ecoTEC pro: press the fault clearance key for one second. The 2023 ecoTEC plus: hold the reset button for more than three seconds. Try it no more than three times.
When to call a Gas Safe engineer
If F28 is still there after three resets, or the pipe wasn't frozen, call a Gas Safe registered engineer. It may be the gas supply, an electrical fault or an ignition problem.
Good to know
- Older Vaillant manuals say the boiler locks out after three failed attempts to light. The 2023 ecoTEC plus manual says five.
- Once the pipe has thawed, Vaillant suggests insulating the outside section so it doesn't freeze again.
Parts people end up buying
Only if the checks above point to a part. Match it to your model number first: it's on a label on the data badge, usually under the boiler or behind the front cover.
- Condensate pipe insulationSearch on Amazon
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Quick answers
What does the Vaillant F28 fault code mean?
The boiler tried to light but didn't detect a flame, so it has locked out. In winter, Vaillant says the most common cause is a frozen condensate pipe.
Can I fix Vaillant F28 myself?
Easy (about 20 minutes). Start with: check the gas is on; in cold weather, check the condensate pipe; thaw it with warm water. If F28 is still there after three resets, or the pipe wasn't frozen, call a Gas Safe registered engineer. It may be the gas supply, an electrical fault or an ignition problem.
Is Vaillant F28 worth repairing?
Usually free. In winter it's most often a frozen condensate pipe, which you can thaw yourself. If not, it needs an engineer.
Checked against
- Vaillant: understanding an F28 fault code
- Vaillant ecoTEC plus operating instructions, 2023 edition (PDF)
- Vaillant ecoTEC plus VU, VUW, VUI ..6/5-5 operating instructions (PDF)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.