Q: I have a 1000 sq ft apt with a 1 1/2 ton ac and 40K BTU heat unit. 4 rooms with 6 in. duct 2 rooms with 4 in. duct. What size return supply and grill should I use.

The minimum free air return is around 144 square inches per ton. Most common return grill size for 1.5 and 2 ton systems is 12 x 24 filter grille.  Supply grill size and style is usually determined by where the grill is installed in the ceiling. If the grill is closer to the outside wall, use a 3-way grill. If the grill box sets closer to an inside wall, use a 1-way grill.  Most common sizes are 10×6 or 10×8.

We don’t use 4” flex duct and recommend increasing to 6” for proper airflow.

To retrofit to an existing home’s application, take the existing grill down. Measure the grill box opening to determine correct grill size.  Note that all opening sizes are in round number. Example: 10 ¼ x 5 7/8 would be a 10 x 6 register.