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HVAC Heat/Cool Load calculation example ?

I'm posting a Heat/Cooling load calculation that I did using a fantastic program posted here on this website (posted by permission).

Just about everyone here has stated the importance of doing a heat/cooling load calculation by experts who know how to do them. Just to understand how they might work I tried the program and came up with the following results.

To me (with the untrained eye) my results below look low. Especially when two of the possible HVAC companies that I contacted that did do a load calculation came up with much higher values:

Company One: 2 stage 78K/50K BTU with a 4 ton H.P.

Company Two: 2 stage 37K/58K BTU with a 3 ton H.P.

Currently Have: mid 90's Single stage 100K/80% with a 3ish H.P.

2800 SqFt

My Results: Heating: 35,826 Cooling: 40632 (sorry about the small size of the graphics below)

Attachment 344021

Attachment 344031

I realize that these load calculations are only one "tool" and require some artistic application, but again----just for my curiously---why would there be such a large difference ?

Thanks for the comments