Weak airflow from vents, or one room hotter or colder than the rest

Weak airflow everywhere usually starts at the filter or a frozen coil. One room that never gets comfortable is more often a blocked register, a closed door, or vents closed somewhere else in the house. A few questions sort out which.

Also searched as: weak airflow from vents, one room hotter than the others, one room colder than the rest, upstairs not cooling.

A hand resting on a floor register in a sunlit bedroom feeling for airflow

Every question and answer on this page, in order

Question 1: Is the airflow weak at every vent, or in one room or area?

Hold your hand up to a few vents in different rooms while the system runs.

Every vent in the house: go to question 2
One room, or one floor: go to question 3

Question 2: How does the air filter look?

Slide the filter out and hold it up to a light.

Gray or dusty, and little light comes through: Likely cause: a clogged air filter
Clean, or changed in the last month: go to question 4

Question 3: In that room, are the registers open and clear of furniture, rugs and curtains?

No, they're closed or covered: Likely cause: closed or blocked registers
Yes, open and clear: go to question 5

Question 4: Do you see ice on the copper line or the indoor unit?

Check the larger copper line where it enters the indoor unit. Look only; don't open panels.

Yes, there's ice or frost: Likely cause: ice on the coil is blocking air
No ice: Likely cause: the blower, coil or ductwork

Question 5: Which of these is true?

The door to that room usually stays closed: Likely cause: closed doors or closed vents elsewhere
I've closed vents in other rooms: Likely cause: closed doors or closed vents elsewhere
Neither: Likely cause: dampers, duct leaks or balancing

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.