Danmat2332

The Code is F75

Hi all, First of all my apologies if I have not approached this correctly, it is my 1st time ever on a forum. Im having issues wIt’s my approx 7 year old Vaillant Ecotec 624 boiler, it heats our water cylinder from approx 2pm-3pm, has 3 bedroom rad, 1 small hall rad & a towel rail rad on the system along with 2 zone underfloor heating, all set to come on at staggered times. During the summer when it’s heating the water there is no problem at all, during the colder months the system seems to lose water pressure Then it shuts down and a code comes up on the digital display (F something, I have forgotten but can find out) and I keep needing to top it up, this happens every approx 2 weeks. Sometimes it won’t let me top it up until I turn on a tap. there seems to be no leak anywhere, we have also replaced the pressure relief valve but to no luck. My local plumbers call, twiddle a few things in hope that it’s fixed to no prevail. Would anyone out here have any solutions? Thanks again for reading and to anyone that can help! If you need any info or pics please let me know. dan