you know, Ive had crappy luck with heat gain/loss programs in the past and about decided to continue doing my sizing from the sidewalk but decided to buy "just one more" and picked up hvac calc. I was so sure it wouldn't work that I didn't try it for a couple of months so if it didn't perform, I would have already forgot what I paid for it. If any of you have ANY doubts, trust me, all the promises are not sales pitches, this program WORKS. and when i had problems don picked up on the first ring and stayed on the line till it was easy. And I am a comfusser (computer) idiot. the program asked all the right questions and I totally trusted the answer. it actually asks if the occupants are sitting, standing, moving around, working, or excersizing. it doesn't just say 200 btus per person. it knows what to ask and why. I can't recomend it enough. jack spotts. spotts bros. furnace co. since 1957