If your Vaillant boiler is showing error code F.28 in very cold weather it is most likely that your condensation pipe is frozen outside. Pour hot water over the pipe and reset the boiler (Or turn it off and on at the spur switch). If possible put some pipe insulation on condensate pipework. Condensate pipework will be in white, grey or black plastic and around 21mm or 35mm in size.
I got called to Vaillant EcoTec Plus 825 with intermittent problems with ignition. I went to fault history to check errors that boiler was showing at the time of failure. This can be done in diagnostic menu. I found two codes: F.54 and F.28 in the fault history. Both codes are gas supply related and can show up together. According to Vaillant 825 manual:
Gas meter defective or gas pressure monitor has triggered, air in gas,
gas flow pressure too low, thermal isolator device (TAE) has triggered,
condensate duct blocked, incorrect gas restrictor, incorrect spare part
gas valve, fault on the gas valve, multiple plug on PCB incorrectly
plugged in, break in cable harness, ignition system (ignition transformer,
ignition cable, ignition plug, ignition electrode) defective, ionisation
current interrupted (cable, electrode), incorrect earthing of product,
When I got to site boiler was igniting and not showing any errors. Quick checks of gas pressures on the boiler and at the meter confirmed faulty part. Gas supplier got contacted and gas supply was repaired the same day. This time it turned out to be a problem on gas meter / gas supply side not the boiler itself.
If your Vaillant boiler is suffering from F.28 and F.54 errors you can contact me through this website to arrange for a repair. I cover most of London.
If your boiler is showing error code F.28 on it’s own please make sure that your gas supply is on or your pre paid meter is in credit.