Vaillant ecoTEC pro 28 - F29?

If you have checked the fan,flue and aps switch then most likely to be the electrode i had this on aquaplus after intial flame the electrode starts sparking but does not ignite the flame this ignition sequence goes through this several time before f28 or f29 shows on lcd screen
*Always isolate electric gas and water before attempting any kind of check or repair and if your not competent get someone is*

soyeb
November 2014

F29 with gas available would generally mean the flame is starting, and being 'baffled out', this would lead to two suspects:
The flue (break down in the flue seals meaning the boiler is burning already spent flue gasses).
Or: (and more likely)
The condense is blocked (the gasses can't escape through the flue because the condense water is backed up into the boiler).
Listen for a gurgling sound while the boiler is firing, which will indicate blocked condense.

Carefree
January 2012