I have a 1489 SQ Foot house - Two Story Pittsburgh, PA - (basement is 744 sq feet) which were adding another 1536 SQ Feet to it (basement is 768 sq feet). We just finished enclosing it. All new windows 2x6 walls are highly insulated.
When we bought the house, I gutted it and had a new Trane XV90 installed 80K BTU and all new duct work.
I ran the HVAC Calc for the entire house and addition which came out to 69K Heat Loss. Now I am thinking that the current furnace is way oversized for the existing house, but undersized for the addition.
I have also noticed that in the summer, the upstairs is significantly hotter than the downstairs even if I shut the downstairs vents.
Question would it be more efficient to leave the existing furnace/ac and add a second unit just for the addition, or should I just replace what I have with a bigger unit that will condition the whole house. Im thinking if I add another system, maybe I could use the existing furnace to condition the basement or 3rd floor attic space so it isnt so oversized.
Final question - Is two separate HVAC systems more or less efficient than one system. I really dont like the idea of two separate systems, but maybe it makes sense.