HVAC Heat/Cool Load calculation example ?

[vstech]

to answer your question, the difference may be in the numbers you vs the contractors used for the software. wall construction, duct leakage, window size/awnings etc all affect the load. shade, roof design, also the web numbers may not precisely relate to your design condition.

[ADillon]

As far as efficiency loss from outside air goes, it is pretty minimal. If you view it as a concer, then you should have the nit installed single pipe so that you are pulling combustion air from the conditioned space. (you must have adequate free area to pull from)