Not quite 1 year old Carrier single-stage 80-percenter, 66k BTU. Can't recall the model and serial. The house is an older 1100 sq. ft. ranch with a single return in the hall and two more in bedrooms.

Unit was consistently going out on primary limit. I found the temperature rise to be well over 80F when I first got there. 40-70 is the correct range.

After verifying the correct blower speed setting and that the capacitor was good, I checked the gas pressure. Was set at 3.7"wc. I pulled it back to 3.2". Data tag says 3.2 to 3.8. A-coil was only three years old, and clean. Venting was fine. And it wasn't the vent limit switch that was tripping.

The filter is a 16x20 pleated that was new. There was also a patch panel over a roughly 10x10 hole on the drop where a humidifier used to be. I pulled both of those just out of curiosity to see what I would get. Temp rise dropped to a bit over 70. Still high, though.

I think the return duct is a contributor to the problem. The bottom of the drop is basically a 16x16x20 box with the filter in it. There's a 16x8 drop on top of that. On one side of the drop, there's 8" round that goes off to a single return (can't tell exactly how far - basement is finished). On the other side of the drop, 16x10 ductwork goes about 8 feet, making two 90-degree turns in the process, before it stops and another several feet of 8" round connects to the panning for the other two returns.

I'm only describing the ductwork from memory. And I'm sure my descriptions of it sound goofy...I'm greener than green on the whole tin side of things.