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Admitted amateur here on this subject.
Condition, small, ~1100 sq ft house pretty well insulated. Currently heated with a 2 stage 40 k btu goodman 2 stage forced air furnace. Currently all heating duct terminations are equal sized off a main trunk in the basement 6” round with floor/base grills. With open floor plan, the heat is very even without undue hot or cold spots. When it is generally running it runs on low stage nearly all the time except when bringing up rooms from overnight reset. Low fan on heat is 731 cfm. High is 990. There are currently 9 warm air outlets and one return air, all located downstairs.
Plan, to add a goodman heat pump to provide some heat when conditions are right, but to also provide A/C. Planning on using 2 ton HP unit. Moderate cooling climate, coastal Maine. The goodman air handler blows 403 cfm on low cool, nearly 600 on high.
Problem, up stairs attic style bedroom, ~200 sq ft with sloped ceilings, quite well insulated but subject to heat gain from the roof. (The room keeps warm without any ducting simply from natural convection as long as the stair top door stays open).
I can easily run a ~6“ Duct to the upstairs through a downstairs closet. I could even run two, one on each end of the room. I would probably control this duct (or duct) with thermostatic controlled dampers so as not to cool or heat the room if not needed.
Question then...is on 6” duct likely to be enough to cool this room? Is two? My intuition is that one would be minimal, two probably enough.
Any advice appreciated.