You can take the results of a Manual J loadcalc if the rooms are separated & figure the percentage of air needed for the upstairs & downstairs to get an idea. If the same percentage of the load works out for both heating & cooling then with properly sized duct work one system can work. Lets say the heating load was 50,000 btus & the cooling load was 40,000 btus for the whole house. If the cooling load for the upstairs was 16,000(40%) & the heating load was 20,000(40%) then a proper duct design that puts 40% of the air upstairs should work in both seasons.
Originally Posted by lithnights
But if the upstairs load is 25,000(62.5%)cooling and 20,000(40%) heating then there is no way one system can balance the temp in both seasons
without being zoned. There is also the factor of how much air drops & rises between floors depending on the openings between them which I don't think even ManualJ takes into account. So whether one system will work right really depends on the design of the house itself along with the design of the ductwork. That's why I say it's possible but very unlikely you'll be happy with one unzoned system. Even if one system would work right, there may be times when you want to regulate different temperatures between the two floors. Whatever deal you can make with your contractor is one thing but its a lot easier to bite the bullet & get it done right in the construction phase then it will be after the house is finished.
The best things in life are free but not everyone is willing to pay the price.
I think Manual J looks at a static scenerio and doesn't fully take stack effect, reverse stack effect and convection into account. In my experience in my home at least, even with a assymetrical 1500 up/1700 down floor plan and higher ceiling downstairs, during normal occupancy and not cooking in the kitchen, upstairs cooling is about 60/40, and in winter heating is about 70/30 including partially heating a full unfinished basement.
Therefore airflow varies dramatically all season.