replacing two package AC heat pumps that are 20+ years old, going to convert it to a split system with an air handler inside an insulated closet style box on the outside of the house. Ducts are in the attic with very limited space, then come outside the attic and down the side of the house inside this boxed in structure I will expand a little and put the upflow air handlers in. I was shocked to pull the 2.5 ton heat pump and find it connected 12 inch round metal pipes wrapped in insulation. So can I get by with the 12 inch pipes till I can figure what to do? I will be running them in 2 ton mode. Later I would look to come back and redo the duct work. Its in south western NC so not super cold or hot. I just cannot redo the duct work at this time and need to get the heatpump installed and working.
Yes, my “Ductulator” says a 12” diameter duct (with insulation on the exterior of the duct) is a little small, but barely adequate for a 2 ton unit.
If I was sizing it I’d use a 14” diameter with a friction loss of .09 per 100 feet of duct on the SUPPLY side and .07 on the RETURN side.
However, your 12” duct will make the air travel a little faster than recommended, but it will not “whistle”.