Is there an "off-season" for HVAC installs? I'm looking to replace a 17-yr old furnace and a/c. Nothing wrong with it yet, so I can time my purchase to get the best deal. I have read that later winter/early spring is a good time (March-April). Is this true? What about the fall, when we're sort of in-between summer heat and the winter cold? And can I expect to pay less if I go off-season, or will I just get more attention from the contractors because they have more time on their hands? Thanks.