I have an unvented system with underfloor heating on three floors and a towel radiator loop. So a fairly large system. I have a ten year old Vaillant Ecotec Plus 637.

 

I understand that the boiler has an expansion tank in it already. I have seen the pressure on the boiler go from 1.7 bar at rest to 1.9 bar when heated. I'm not sure if it has been higher or if water has been ejected (where does this happen?). Do I need another external expansion tank? I've seen online calculators that help you work this out but I have no idea how much water there is in my system so not sure how to use those.

Get your manual out or download it. It will have explicit instructions on system volume and the required additional size if required. You will need to calculate the approximate system volume.

 

If you are getting bid swings in pressure and your expansion vessel is sized ok, you may need to look at the pressure setting in the accumulator, it may need blowing up.

 

UFH pipes will be approx 14mm bore.