You need to understand exactly how it is setup. If it isn’t as per the diagram, you need to know that and get a diagram of the actual setup, else the rest is just guesswork.
[edit]
Are you sure you asked about the Buffer Tank? The DHW bit will be a coil.
[edit2]
This is the sort of UFH manifold I am suggesting. The valve at the bottom of the pump controls the temperatre of the water entering the UFH pipework