Whole-house fans and night flushing: cooling for pennies
Contents (8)
Before central air conditioning became standard, American houses were cooled by the daily temperature swing: closed up through the afternoon, thrown open at dusk, flushed with cool night air until dawn. The machine that industrialized that habit is the whole-house fan, and in the right climate it still does most of a summer’s cooling on roughly a tenth of the electricity. In the wrong climate it does nothing useful at all. Knowing which one you live in takes a single number.
What a whole-house fan actually is
It is a large fan mounted in the ceiling of a central hallway, drawing air out of the living space and discharging it into the attic, from where it escapes through the roof vents. Open windows on the cool side of the house, switch it on, and the entire volume of the house is replaced with outdoor air several times an hour. Two things follow from that description. First, it cools the house and the attic, which matters because a 140 °F attic keeps radiating into your bedrooms long after sunset. Second, it does not condition anything: it moves outdoor air indoors, unfiltered and undehumidified, which is the whole basis of both its efficiency and its limits.
It is not an attic ventilator, which exhausts the attic only and, in a house with a leaky ceiling, can pull conditioned air up out of the living space. Nor is it a window box fan, which cools the room it sits in rather than the building.
The number that decides it: dew point, not temperature
Night air only helps if it is genuinely drier and cooler than the air it replaces. The practical test is the overnight dew point, printed on every weather forecast:
| Overnight dew point | What night flushing does | Typical US regions |
|---|---|---|
| Below 55 °F | Excellent — can replace AC most nights | Mountain West, Great Basin, inland California, high desert |
| 55–60 °F | Very good — AC becomes an afternoon-only machine | Pacific Northwest, northern New England, northern Plains |
| 60–65 °F | Useful early and late in the season | Mid-Atlantic, Midwest, inland Northeast |
| Above 68 °F | Counterproductive — you import latent load | Gulf Coast, Florida, Deep South in midsummer |
The rule behind the table: a house cooled to 76 °F with 65 °F night air ends the night cooler and no wetter. The same house flushed with 72 °F air at a 70 °F dew point ends the night barely cooler and distinctly damper, and the air conditioner then spends the morning removing the moisture you invited in. Humid-climate readers are better served by the setpoint discipline in our thermostat guide than by a fan.
Sizing: CFM, and the vents that are usually forgotten
Two figures matter, and the second is the one that gets installations wrong.
- Fan capacity. A common design target is 2 to 3 CFM per square foot of conditioned floor area — roughly 4,000–6,000 CFM for a 2,000 sq ft house, which is fifteen to thirty air changes an hour. Below that you get a breeze; above it you get noise for little extra benefit.
- Attic exhaust area. The Department of Energy’s guidance is one square foot of net free vent area per 750 CFM of fan capacity, which typically means two to four times the vent area a house already has. Undersized venting is the classic failure: the fan pressurizes the attic, air backs up, the motor labours, and the promised cooling never arrives.
Window opening matters just as much. Open a total window area at least equal to the fan’s throat, distributed towards the coolest and quietest side of the house, and close the windows in rooms you do not want the air pulled through. A single cracked window with a 5,000 CFM fan running produces a shrieking draft and very little air change.
The power arithmetic
| Machine | Draw while running | Cost per hour at $0.18/kWh |
|---|---|---|
| Whole-house fan, 4,500 CFM, low speed | ≈ 0.2 kW | ≈ 4 ¢ |
| Whole-house fan, high speed | 0.4–0.7 kW | 7–13 ¢ |
| Central AC, 3 tons, 14.3 SEER2 | ≈ 2.5 kW | ≈ 45 ¢ |
Eight hours of night flushing on low costs about a third of a dollar. Eight hours of compressor time costs several dollars, and there are US markets where evening rates make that comparison starker still. The saving is real but bounded: the fan replaces the cooling you would otherwise buy at night and in the shoulder months, not the peak-afternoon load, so plan on a supplementary machine rather than a replacement unless your climate sits in the top row of the dew-point table. For scale, put the numbers against your own tariff and running hours using our AC running cost breakdown.
The routine that makes it work
- Close up at breakfast. Windows shut, blinds down on the east and south, and nothing opened until the outdoor temperature drops below the indoor one — often 7 or 8 p.m. in July.
- Open, then start. Windows first, fan second, always. Starting the fan in a sealed house pulls air down whatever chimney or flue is available.
- Run high for the flush, low for the night. Twenty to thirty minutes on high dumps the day’s heat out of the structure; low speed overnight keeps the attic and the framing cooling quietly.
- Close before sunrise. The building’s thermal mass is your storage battery; leaving windows open past 8 a.m. discharges it.
Safety and the things that go wrong
Backdrafting is the serious one. A large exhaust fan depressurizes the house, and a depressurized house can pull combustion gases back down the flue of a gas water heater, furnace or fireplace. Never run the fan with windows closed, never run it while a natural-draft appliance is firing without adequate make-up air, and fit working carbon monoxide alarms on every level — advice that applies to any powerful exhaust appliance, as EPA’s indoor air quality guidance sets out.
Outdoor air is outdoor air. Pollen counts, wildfire smoke and traffic pollution all come in with the cooling. During a smoke event the fan is exactly the wrong machine, and a household with severe allergies may prefer filtered mechanical cooling in peak pollen weeks.
Winter heat loss. An uninsulated louvered fan opening is a hole in your ceiling insulation for eight months of the year. Modern units with motorized insulated damper doors solve it; older louvered models need a fitted winter cover.
Noise. Belt-driven and ducted models on low speed are the quiet option; a direct-drive fan at high speed in a hallway ceiling is loud enough that households stop using it — the most common reason a correctly specified fan saves nothing.
Where it fits alongside everything else
Night flushing works best behind the passive measures. Shade the west-facing glass, insulate and air-seal the attic plane and cut internal gains first; then flush the residue with night air; then let the compressor handle the design-day afternoons that remain — the load that decides how much central cooling you actually need.
Frequently asked questions
Can a whole-house fan replace air conditioning entirely?
In a dry climate with a large day-to-night swing — much of the Mountain West and inland California — many households cool a well-insulated house on the fan alone for most of the season, keeping AC for heatwave afternoons. In humid regions it cannot: the fan removes no moisture, and importing 70 °F dew point air undoes what the air conditioner achieved.
What does one cost to buy and install?
Indicative market bands for US residential work run roughly $600 to $1,600 for the equipment depending on capacity and damper quality, plus installation — commonly $400 to $1,200 where the ceiling joists and attic venting cooperate, more where vent area has to be added. Treat those as reference ranges, not quotes.
Do I still need the attic vents if I have a whole-house fan?
More than ever. The fan discharges into the attic and that air has to leave; one square foot of net free vent area per 750 CFM is the DOE benchmark. Adding a fan without adding vent area is the single most common installation error.
Is it safe to run all night with bedroom windows open?
From a mechanical standpoint yes, with functioning CO alarms and no natural-draft appliance firing. Ground-floor window security and rain are the practical constraints, which is why most households flush hard for half an hour, then run the fan on low with upstairs windows only.
How does it compare with an evaporative cooler?
Both need dry air, and neither works in a humid climate. A whole-house fan exchanges air and adds no moisture; an evaporative cooler cools the air it delivers by adding moisture, so it can drop the temperature further but raises indoor humidity. In the arid Southwest the two are often used together — the fan at night, the cooler by day.
Sources
Read next
- Cooling a garage or workshop: what works and what the code forbids
- Through-the-wall air conditioners and PTACs: the sleeve decides everything
- BTU sizing guide: matching cooling power to real rooms
- Air conditioning in a rental: what a landlord owes you, and what you can install
- Reading an HVAC quote: how to choose a contractor without guessing