This is NOT OFFICIAL or anything but I always assume at least a 5 degree lose when going thru a HX. I know the whole conservation of energy thing (from school & science classes), but in the real world there is this thing that is called resistant, it affects everything
...but then again 95% of the time I am running a mixing valve on the secondary side of the HX before I get to the manifold, so I don't really need 180F water anyway. Cause the underfloor plates do not need more than about 140F MAX, and the in slab closer to 120F MAX