Lock out problem with vaillant turbomax plus?

My turbomax plus 828e locks out under the following circumstances.1 If I draw hot water when the boiler is cold 2 If I try to heat up the central heating too quickly. It ignites and burns for 2 minutes and then the yellow light starts to go on and off 2 or 3 times then the red lock out happens. With the DHW demand lockout is similar but much quicker. If I increase the temperature in small increments I can get it up to temperature OK. The fault code is F29. The problem started after replacing the fan after it had burnt out. The problem is not due to faulty thermistors. i replaced all 3 with no improvement. The system pressure is well over 1.5 bar at all times. The diverter valve was replaces 12 months ago