Vaillant Gas Boiler - Rads and UFH - Expansion Tank

[wozza]

Hi All,

 

We had UFH fitted to our existing heating system around three years ago. AT the time I asked the installer if we needed an expansion vessel and he said nope you should be ok - I questioned him again a few times to be told no, it should be fine - so we never fitted one, even though lots of people on here said we should have one.

3 years later and all has been fine until today.

I fired up the heating for the first time since March - everything was working ok - Rads got warm, UFH all working and warm - I then checked the boiler and we had suffered a small leak of water from inside the boiler - I estimate around a cup full, and no more has leaked out - heating still working fine.

I then noticed that the system was running at 2.1 bar - it used to run at around 1.6 to 1.7.

So I drained off some water to reduce the pressure but anything lower than 1.9 bar and the boiler stops working and displays an F75 fault.

I have topped the system back up to 2.1 bar the the fault has reset and everything works great.

The boiler is a Vaillant Ecotec Plus 831 and it was fitted in 2010 and it has never missed a beat - we have 9 rads and 6 UFH loops - I have an engineer booked to come and look but thought I would ask the brainiacs here if a few questions so I could be better armed.

 

Do you think the boilers expansion tank is knackered?

Do I need an extra  expansion tank? If so then how do I size it and can it be fitted anywhere on the return?

If I do need an expansion tank, does anyone have any suggestions for a wall mounted one

 

Thanks all, Wozza.

 

 

[JohnMo]

We had similar issues in our last house, the diaphragm in the expansion had failed. The plumber should just replace like for like.

[wozza]

Turns out that is was the expansion tank (only a small leak though as there wasn't much water in it), and the pressure switch was sticking - both replaced and working like it should.