You guys probably hate questions like this, but I need to have some idea of whether I'm getting BS from the contractor.
I have a section of my house that is 624 sq ft (outside dimensions) which consists of one large room and one tiny bathroom. The main room has a ten foot ceiling. This room is less than two years old and seems reasonably well insulted: 2x4 walls with fiberglas batting and fiberglas in the ceiling that is at least 16" deep. Three of the four walls are outside walls.
During the afternoon, with outdoor temps in the high 80s or low 90s, the temp of the main room will not drop below 76-77F with the heat pump running continuously. The heat pump is a 1.5 ton unit installed less than a year ago. The air coming out of the ducts is about 58 degrees or so. The filter is new.
Given the duct temp, I'd say the unit is probably working ok which makes me think it might be under sized, but 624 sq ft doesn't seem too large for a 1.5 ton unit. I'm curious as to how much additional capacity is required for a 10 foot ceiling versus an 8 foot ceiling.
I'd be grateful for comments that might help me get a little perspective on this.