I would start with emissivity. That is the main issue.
The total hemispheric emissivity was conflated normal spectral emissivity, which ultimately causes an error that almost exactly cancels the excess heat reported when corrected.
There are other minor issues that confound exact solutions, but they can be ignored in the first pass.
The Lugano authors headlined excess heat and acceleration of excess heat with temperature as the two facts that could not be explained.
The error above catches the excess heat to within experimental errors. More interestingly (to me) it also catches the acceleration in excess heat exactly (to within 0.5% of heat. That just falls out of the calculations without fudging and is strong validation that there is no exothermic reaction. In spite of this the meme that Rossi-like systems generate exothermic LENR survives, as far as I can see on no solid evidence.
Pages here document all of this.
