Ignorance is bliss... or a curse. I am not an HVAC pro, hence have no idea what Manual J calcs look like. I had several contractors come out to give estimates for system replacement. Some based new size on existing - I learned to pass on those. Three others (so far), when I asked them what the size is based on, told me it's based on heat load calculations. Great!
1st contractor: only walked around the outside of the house, but said that he has access to public records with sq footage. He did not measure anything inside. His conclusion is 2.5 and 3 tons (I have two systems to replace).
2nd contractor: also walked around the outside and quoted two 4 ton units (w/ 2-stage compressors). Told me his calcs yielded 36,000BTU for one floor and 48,000BTU for another.
3rd contractor: came yesterday. He's the first one to measure all rooms and windows inside. Took notice of skylights too. Told us the house is not standard colonial, but contemporary with lots of irregular and open spaces. He would need some time to do a heat load calc, which is okay by us. I don't have his numbers yet, but now I started wondering whether the other two did the calc correctly, by just walking outside.
If I ask the contractors to show me the calculations, what should I look for to tell if they were done right? The difference in suggested size between 1st and 2nd contractor is big. I don't want to over- or under-size the system. How could they both be right? I'd like to be able to determine who's done it right.