F75 is a well known and documented fault with Ecotec boilers.
It can be made worse with dirty system water, but more often than not it is a faulty pressure sensor.
Call Vaillant out under warranty. You still have 6 months left yet
f75 is a faulty pump (jammed) or pressure sensor. If the system is full of sludge this will cause the pump/sensor to fail prematurely. If you have a pink coloured pressure sensor it may be faulty but it sounds like you have a dirty system (and a very oversized boiler!) I would range rate the boiler down to 26kw and check for an external autobypass.
The System was completly new, as the house was a shed when we bought so all new pipe work rads etc.....as there was nothing here! but was still flushed with Senitol X300 and Senitol Inibitor X100 2 bottles added.
With regards to the comments about the system been too large the house is a victorian detached 5 bed, 2 bath ( 3 kids!) and 3 large downstairs rooms, kitchen and 3 cellar house!! which is to be extended ( god it sounds as if im in buckingham palace!) to add a large dining kitchen down stairs bathroom and 2 ensuites and 2 loft beds, so the the system was added knowing this work is to be carried out . So thats why the system was put in.
ANYWAY>>>>>>>> I called Valliant back out after reading other plumbers comments spoke with my uncle as he landed! and demanded a 2nd opinion, they came today, took a water sample .............which was clear!, and the engineer this time said he could smell the inibitor in the water, and he knew that it was ok, said immediatley it was the pump, took the pump out where there was a slight build up of Ferrous iron, checked the balancing etc.... and suggested investing in a MagnaClean. Checked the sensor, air etc... all ok, we turned the room termostat and boiler on and BINGO fired straight up, rads boiling hot and hes been gone 4 hours and were still running, I was lucky if I got 20 mins of luke warm radiators, before the F75 Code came and locked it out for the last week.
He says whilst he wont slag other engineer off..... he did say he knew him, and that he should of let the water run for more than 1 second not take from the pressure realease valve, checked the rads water, which was clear.....and said "maybe he needed to leave work early!" and smiled!
So I'm all up and running..............lesson don't trust them........and ask for a second opinion .............and get your Uncle to move closer to you!!
