Need to replace two gas furnaces and two AC in my house. Already have two contractors came to the house but each spent about 15 minutes looking at the existing units and looked at me like I have two heads when I asked about doing the manual J load calc. Their reasons for not doing the load calc was that it had already been done when the house was built. The house is 15 years old. This is an expensive undertaking so I just want to make sure that everything is done correctly the first time. I am in Northern Virginia.