I’m installing a small (18k), central ducted inverter heatpump, with the air handler in a very small attic of an otherwise cathedral ceiling house. It’s an unvented cathedral roof with all the insulation in and above the rafters, hence the attic area is within the structure envelope.
My plan is a single, main return air grille from a high point in a hallway that’s right on the other side of the attic wall. It’s going to be cozy in that attic, so a return duct/plenum is tricky to accommodate. My concern is, do I require it at all? If I simply have a grille in the wall, the return air must simply pull through the grille, into the attic, and from the attic to the air handler. Clearly the space needs to be well sealed, however there’s actually no place else for it to pull air from (any leak would just originate from other conditioned areas).
Does that work? Any other factor I need a duct? Thank you.