The upstairs or one back room never cools down

Delivery or capacity

Hold a hand at the register in the hot room and at one in a room that cools. If the hot room gets a weak trickle, or air that is barely cool, the problem is delivery. If every room gets strong, cold air and the hot room still loses, the room gains heat faster than it gets cooling: a west-facing wall of glass, a bonus room over the garage, thin attic insulation over that spot.

What happened after it was fine

A room that cooled for years and then stopped points at a change in the attic. Cable, satellite, solar and insulation work all put people on the attic joists, and a flex duct under a knee collapses without a sound. A taped joint can also let go after years in an attic that runs far hotter than the outdoors.

What a technician measures

Airflow at the registers, static pressure across the system, and the temperature of supply air at the hot room against air leaving the coil. Then a walk of the attic runs to that room. Those numbers show whether the fix is a reconnected duct, a larger run, a return path, or a capacity question for the whole system.

What to have ready when you call

• Which rooms are hot, on which floor, and since when
• Whether anyone has worked in the attic recently
• The install year of the system
• Your address, ZIP code and attic access

Common questions

Will closing vents downstairs push air upstairs? Usually less than hoped, and it raises duct pressure, which makes leaks worse. Ask a technician before doing it.