Fans and Airflow in Your Home: The AC Fan Setting, Closed Vents, Hot Rooms and Whole-House Fans
Moving air is cheap. Moving it the wrong way costs money. The airflow habits worth changing.
By Mark Curant·Updated
Quick answer
- Set the thermostat fan to Auto, not On. "On" can put moisture back into the house.
- Don't close vents in unused rooms. It increases energy use and can freeze the coil.
- Ceiling fans cool people; use them to raise the thermostat, and switch them off when you leave.
- Window and whole-house fans work when it's cooler outside than in, best in dry climates with cool nights; not in hot, humid ones.
- Powered attic fans usually cost more to run than they save.
Fans and airflow: the series
- Ceiling fan vs. AC: how much a fan saves
- How much it costs to run a fan
- Ceiling fan buying guide
- Ceiling fan direction and settings
- Fans and airflow in your home (this page)
Five kinds of fan, five different jobs
| Fan | What it does | When to use it | Climate |
|---|---|---|---|
| Ceiling fan | Moves air over people in one room | Whenever someone is in the room; lets you raise the thermostat | Everywhere |
| AC fan set to "On" | Runs the AC's indoor blower nonstop | Rarely; can raise humidity | Avoid in humid climates |
| Window fan | Pulls cool outdoor air through the house | When it's cooler outside than in, e.g. evenings | Mild nights; less useful when it's humid |
| Whole-house fan | Pulls outdoor air in through windows and out through the attic | Evenings and nights cooler than indoors, AC off | Dry climates with cool nights |
| Powered attic fan | Exhausts hot air from the attic | Generally not recommended | Rarely pays off anywhere |
Your AC's fan: Auto, not On
The "Fan" switch on your thermostat controls the indoor blower in the air handler or furnace, a much bigger motor than a ceiling fan. On Auto it runs only while the AC is cooling. On On it runs all the time.
That matters for humidity. While the AC runs, water condenses on the cold indoor coil and drains away. When the compressor stops, the coil is still wet. With the fan on Auto, it stops too and the water drains. With the fan on On, air keeps blowing across the wet coil and carries that moisture straight back into the house. In one test by building scientist Allison Bailes in his own home, switching from Auto to On raised indoor humidity from about 58% to about 70%, the level where mold starts to thrive.
"On" also costs more, because the blower runs 24 hours a day. There are reasons to use it (some homes use it to even out temperatures or run a filter), but in a humid climate, Auto is the safe default. If your house feels clammy, see house humid with the AC on.
Don't close vents to save energy
It seems logical: close the vents in rooms you don't use and cool less house. But a central system is designed to push a set amount of air. Close vents and the pressure in the ducts rises, so more air leaks out of the ducts, often into an attic or crawlspace. A Lawrence Berkeley National Laboratory study found that closing registers increased energy use in typical houses with ducts outside the living space, and recommended against it. Closing more than about 60% of registers can even freeze the cooling coil.
If you must close one, close the vent farthest from the indoor unit; that does the least harm. Better still, leave them open and use a thermostat schedule. If the coil does freeze, see AC frozen up.
Fixing a room that's always hot
Check the supply
Hold your hand to the vent with the AC running. Weak airflow compared with other rooms points to a long or leaky duct run, a crushed or disconnected flex duct in the attic, or a balancing damper partly closed. A technician can check and seal ducts; sealing leaky ducts typically costs $500–$2,000.
Give the air a way out
Many homes have one central return. When a bedroom door is closed, the air pushed in has no easy way back, so less cool air goes in. If the room cools better with the door open, the fix is a return path: a larger gap under the door, a transfer grille, or a jumper duct to the hallway.
Cut the heat coming in
Sun on west-facing windows, a room over a garage, or one under a hot attic gains more heat than the rest of the house. Blinds or shades, window film, and attic insulation help more than anything you do to the AC. A ceiling fan then makes the people in the room comfortable at a higher setting.
Window fans and whole-house fans
These fans don't cool you with a breeze; they replace hot indoor air with cooler outdoor air. That only works when it's actually cooler outside, so they're for evenings, nights and mild seasons, with the AC off and windows open.
Window fans
The Department of Energy suggests putting a fan in a window to blow hot air out, ideally upstairs and on the side away from the wind, and opening windows on the cooler, shaded side, ideally downstairs, so fresh air is drawn through the house.
Whole-house fans
A whole-house fan is mounted in the ceiling and pulls air through open windows and out through the attic, changing the air in the whole house quickly. Guidance from the Department of Energy's Building America program says they suit climates where summer nights fall below about 67°F with a day-to-night swing of 25°F or more, typically dry regions. Before installing one:
- Only run it when it's cooler outside than inside, and never with the AC running.
- Open windows first. Running it against closed windows can pull exhaust from gas appliances back into the house; homes with a standing pilot light or gas appliances that draw room air need special care.
- The attic needs enough vents (about 1 square foot of free vent area per 750 CFM) or hot attic air gets pushed back into the house.
- Get an insulated, sealed cover so it doesn't leak air in winter.
In humid climates
Where nights stay warm and muggy, as in Florida, along the Gulf Coast and across the Caribbean, outdoor air rarely gets cool and dry enough to help. A whole-house fan would bring in moisture the AC then has to remove. There, ceiling fans with the AC are the better combination.
Powered attic fans: usually skip them
An attic fan on the roof or gable exhausts hot attic air. It sounds sensible, but research from the Florida Solar Energy Center found these fans generally cost more to run than they save in cooling. They can also pull cooled air out of the house through gaps in the ceiling. Sealing air leaks in the attic floor and adding insulation does more for less.
Frequently asked questions
Should I set my thermostat fan to On or Auto?
Auto, for most homes. On "On", the indoor blower runs nonstop, and after each cooling cycle it blows across the wet coil and puts moisture back into the house. In one test by building scientist Allison Bailes in his own home, switching from Auto to On raised indoor humidity from about 58% to about 70%. It also uses more electricity.
Does closing vents in unused rooms save energy?
No. A Lawrence Berkeley National Laboratory study found that closing registers increased energy use in typical houses with ducts outside the living space, because higher duct pressure makes the ducts leak more. Closing many vents can also freeze the AC coil. Leave them open.
Do whole-house fans work?
In the right climate, very well. They work best where summer nights fall below about 67°F with a big day-to-night temperature swing, typically dry regions. They should only run when it's cooler outside than inside, with the AC off and windows open. In hot, humid climates they bring in muggy air and are a poor fit.
Are attic fans worth it?
Usually not. Research from the Florida Solar Energy Center found powered attic ventilators generally cost more to run than they save in cooling, and they can pull cooled air from the house up into the attic through ceiling gaps. Sealing and insulating the attic floor does more.
Why is one room always hotter than the rest of the house?
Common causes are too little supply air reaching it (a long or leaky duct run, a crushed flex duct, a partly closed damper), no easy way for air to get back out when the door is closed, extra sun on windows, or being over a garage or under a hot attic. A ceiling fan helps the people in the room; fixing the airflow fixes the room.
Sources
- Fan On vs Auto humidity test: Allison Bailes, Energy Vanguard.
- Closing registers: Walker, Register Closing Effects on Forced Air Heating System Performance, Lawrence Berkeley National Laboratory, 2003.
- Whole-house fans: DOE Building America Solution Center.
- Window fans and ceiling fans: U.S. Department of Energy, Energy Saver.
- Attic fans: Green Building Advisor, citing Florida Solar Energy Center research.
- Duct sealing cost: our AC repair cost guide.