Vaillant ecoTEC plus 837 R1 with persisting F75 fault

[yossibac]

Can someone help diagnose Vaillant ecoTEC plus 837 with persisting F75 fault please?
We already replaced the pressure sensor, installed F75 kit, replaced the main heat exchanger, and the pump, and the PCB, and flushed the system and installed a filter on the return, but still sometimes we get F75. The boiler still reset and run, but we would like to get to the bottom of it if at all possible. Thanks in advance.

 

[ShaunCorbs]

Who did the work ?

 

[Noxer]

Blocked flow sensor?

 

[Riley]

Wow that’s some repairs

 

[yossibac]

All those parts has been replaced in the course of 5 years not all at ones, and from different reasons, but the F75 has been creeping out all this time, some times the fault would be gone for months so you think that it was cured. The boiler is otherwise working well. There are two questions: 1) if there is a fault why the boiler always reset and continue to work sometimes for months? 2) if there isn't a fault why is the F75 keep creeping out? Thanks for your thoughts.

 

[Nico]

Do you have to top up the pressure in the system often?

 

[yossibac]

Yes, every three months or so, and it does seem to keep F75 away for a while, we are now on 1.8 bar when cold and 2.1 bar when hot

 

[Nico]

I believe F75 is a fault code for when boiler pressure is too low/ also doesn’t detect an increase in pressure when pump kicks on. Have you ever noticed what pressure your boiler is sitting at when this code is showing?

 

[yossibac]

I noticed that if the F75 starts kicking and if after the first reset F75 returns, it makes it calm down if I top up. Last time boiler was at 1.7 bar before the first F75 kicked in. Generally speaking Vaillant have different fault code for low pressure and the boiler can work very well with 1.2 bar or even a bit lower, but that only if the boiler is not faulty.

 

[Nico]

ah Right could be a number of things. Sound like a defect in the sensor or it’s partially blocked if it needs such a high pressure to pick up anything. Has your expansion vessel ever been investigated as well as everything else you had mentioned?