I am currently performing an acceptance test for a newly installed gas fired shell boiler. I am following the British Standard "BS EN 12953-11:2003". I have encountered a problem with equation (8.5-2) found on page 16 of the document described above. This problem being that a value of enthalpy is subtracted from itself, thus equating it to zero. Surely if this was correct then a value of zero would simply suffice within that section of the equation. The value i decribe is highlighted in the two images attached. I have contacted the British Standard however it needs to run through a committee before they can produce an answer. Part of me does doubt my thoughts as it published ten years ago and a revised copy has not yet been published. Thought i'd best check with you guys though.