On this page
Quick answer
On Vaillant ecoTEC boilers, F28 means the boiler tried to light several times at start-up and did not detect a flame, so it locked out. F29 means the flame was established but then lost during operation. Both usually trace back to the gas supply, a frozen or blocked condensate pipe, or a worn ignition or flame-sensing electrode. You can check the gas supply and the condensate pipe yourself and try one or two resets. If the code returns, you need a Gas Safe registered engineer.
Key takeaways
- F28: ignition failure at start-up. F29: flame lost during operation.
- Check other gas appliances and your meter first; a supply issue is a common cause of both.
- In cold weather, a frozen condensate pipe frequently triggers these codes.
- Worn electrodes, a faulty gas valve, flue problems or PCB faults need an engineer.
- Do not keep resetting the boiler if the code keeps coming back.
What F28 means in practice
When the ecoTEC gets a call for heat, it runs the fan, opens the gas valve and sparks the ignition electrode. If it cannot detect a flame after its set number of attempts, it shows F28 and locks out. Customers often describe clicking from inside the boiler with no burner lighting.
What F29 means in practice
F29 means the boiler did light, but the flame signal dropped out while it was running. This can happen if the gas supply falters, if condensate backs up into the boiler, or if the flame-sensing electrode is failing or dirty. Some customers see F29 only when the hot water runs, when gas demand is highest.
What you can check
Gas supply. Can you light the hob? Is the emergency control valve by the meter fully open? On a prepayment meter, is there credit? See prepayment meter and boiler not working.
Condensate pipe. If it has been below freezing, check the plastic pipe where it leaves the house. Our guide to thawing a frozen condensate pipe explains how to do it safely with warm, not boiling, water.
Reset. Press the reset button once and let the boiler go through its start-up. Our post on how to reset a boiler safely covers how many attempts are sensible.
What needs an engineer
Ignition and ionisation electrodes. These wear over time and can be damaged or coated. Replacement is a routine repair.
Gas pressure and the gas valve. An engineer will check the working gas pressure at the boiler. Low pressure can come from undersized pipework or a meter fault; a failing gas valve is also possible.
Flue and air intake. A blocked or disconnected flue can disturb combustion and flame detection.
Condensate trap. A blocked internal trap can cause the same symptoms as a frozen external pipe.
PCB. Less common, but a control board fault can produce ignition errors.
Why an annual service helps
A Vaillant service includes inspection of the burner and electrodes, combustion analysis and a check of the condensate trap. Many F28 and F29 faults develop gradually and are picked up before they cause a winter breakdown. Our Vaillant boiler service guide explains what is included.
Book with Gasmaster
Vaillant showing F28 or F29? Gasmaster Plumbing & Heating, Gas Safe registered (220932), repairs Vaillant boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service or book through our boiler service booking page.
Frequently asked questions
Can I fix Vaillant F28 myself?
You can check the gas supply, thaw a frozen condensate pipe and reset the boiler. Anything inside the casing must be done by a Gas Safe engineer.
Why does my Vaillant show F29 only when I run hot water?
Hot water demand makes the boiler work at high output. A marginal gas supply or a weak flame signal is more likely to fail under that load.
Is F28 dangerous?
The code shows the boiler has shut down safely. The danger would come from tampering with it. If you smell gas, call 0800 111 999.
Does F28 mean I need a new boiler?
No. F28 is usually a repairable fault. Age and repair history are what decide whether replacement makes sense.
Sources
- Vaillant, ecoTEC installation and servicing instructions (vaillant.co.uk)
- Gas Safe Register (gassaferegister.co.uk)
- Health and Safety Executive, gas safety guidance (hse.gov.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
