The registers are adjustable, so I could just close them. They don't seem to have been moved. :)

The basement level is mostly finished off, but there is a section that could be examined. Thanks!

I suspect that if you have a heat pump and use nighttime setbacks or if the temperature differential between the set point of the thermostat and the house becomes 2˚F or greater that your backup is coming on just before you’re getting up and the higher output temperature from heat strips or a gas furnace is heating up the that BR more than just the HP that operates with lower temperature output.

IMPO

SR

Every house I’ve lived in has required some amount of fidgeting with the dampers on the supply registers to balance airflow. Closed a tad or more in smaller rooms and those closest to the blower, more open at registers most distant from the blower. My current house is a string bean with separate equipment at each end and still dampers needed to be adjusted as described.

No heat pump, no setbacks. We will fidget with the registers and see if that helps.

How did the outdoor temperatures compare on the two days? That can make a significant difference in the overall heating load and the heating load of individual rooms.

@Charles Ross Homes Nothing different really. It was in the teens those nights, but we've had many days in single digits or even below zero this year and it didn't do this. Oddly, it has now returned to it's normal function and is staying in the mid-60's all night. No idea....