I am getting estimates on replacing the furnace in my house. I have had 3 different HVAC contractors give me pricing and one of them said that the house had nowhere near enough return air for any heating or air conditioning unit to be efficient and that I should re-size my main ducts in the basement, add more return registers on the first level and add supply registers in the 2nd level. I then had another of the contractors, who originally never mentioned his concern over this lack of return or supply air, come back and re-evaluate the situation and he still maintained that although I was maybe a bit short, he did not think it was necessary to re-configure or additional ducting. I'm wondering how two contractors can have such a different view on the situation and if there is an exact fromula used to determine required return and supply air amounts. Please advise.