Losing Pressure vaillant ecotec 624

[Danmat2332]

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

[Danmat2332]

The Code is F75

[Nickfromwales]

You have probably lost the pre-charge pressure in the heating expansion vessel. When running hot water there is minimal expansion, but when the heating is on for a duration it is problematic, plus you have a very small heating system so minimal adventitious expansion available there. 
The heating system needs to be drained to zero pressure, then ( a plumber ) will bring a pump ( like a bike tire pump which you stand on and pump ) which will have a long hose. The hose goes onto the Schroeder valve on the top of the expansion vessel and you pump it up to the manufacturers specification ( usually around 1 bar ). 
The boiler and heating warm up and the system water expands, then the PRV let’s tiny amounts of water escape ( disappears as steam due to the heat in the boiler ) and then the system cools down again. Upon cooling the contraction draws air IN through the air admittance valve of the boiler pump and bye bye to the pressure after prob one or two heating cycles. 

[Pocster]

1 hour ago, Nickfromwales said:

You have probably lost the pre-charge pressure in the heating expansion vessel. When running hot water there is minimal expansion, but when the heating is on for a duration it is problematic, plus you have a very small heating system so minimal adventitious expansion available there. 
The heating system needs to be drained to zero pressure, then ( a plumber ) will bring a pump ( like a bike tire pump which you stand on and pump ) which will have a long hose. The hose goes onto the Schroeder valve on the top of the expansion vessel and you pump it up to the manufacturers specification ( usually around 1 bar ). 
The boiler and heating warm up and the system water expands, then the PRV let’s tiny amounts of water escape ( disappears as steam due to the heat in the boiler ) and then the system cools down again. Upon cooling the contraction draws air IN through the air admittance valve of the boiler pump and bye bye to the pressure after prob one or two heating cycles. 

I agree 

[Temp]

+2

 

Although i would expect to see some evidence of water coming out of the PRV. Perhaps pressurise the system when cold then fire it up. If the Expansion vessel has lost pressure I would expect some water to come out of the PRV as it heats up. Perhaps temporarily put a plastic bag over the outlet so you can catch it.

 

If sure there is nothing coming out of the PRV then it might be a leaving rad valve somewhere.