Venting and Relief Air for Swamp Coolers: How Much to Open, Where, and Why
How to vent a house with a swamp cooler: the 1 to 2 sq ft per 1,000 CFM rule, worked window math, ceiling relief dampers, attic venting and pressure symptoms.
Short answer
A swamp cooler needs an exit for every cubic foot of air it pushes in. Plan roughly 1 to 2 sq ft of open window or relief damper area per 1,000 CFM, placed in the rooms you want cooled and away from the supply, so air sweeps through the house and leaves before it turns humid.
Key takeaways
- Relief area of about 1 to 2 sq ft per 1,000 CFM keeps relief velocity around 500 to 1,000 fpm, low enough for quiet, easy flow.
- Too little relief raises indoor pressure and humidity, cuts airflow and makes the house feel muggy even though the cooler is working.
- Open windows in the rooms you want cooled; air flows from the supply toward the openings, so they steer the cooling.
- Ceiling relief dampers into a well-vented attic let you cool with windows closed, and they cool the attic too, but only if attic vents can pass the airflow.
- Screens cut effective opening area, so open a bit wider than the bare arithmetic suggests.
On this page
Venting is the part of a swamp cooler system people skip, and it is the part that most often makes a good cooler feel bad. The cooler pushes thousands of cubic feet of moist air into your house every minute. That air has to leave. Give it roughly 1 to 2 sq ft of open window or relief damper area for every 1,000 CFM of cooler airflow, and put those openings where you want the cooling to go.
Below is the reasoning behind that rule, the window math, the attic relief option, and how to read the symptoms of a house that is not venting properly.
Why a swamp cooler needs relief air
An air conditioner recirculates indoor air through a cold coil and removes moisture. An evaporative cooler does the opposite: it pulls in outdoor air, adds water vapor, and blows it in. It works only because the air it delivers is constantly replaced. (For the mechanism, see how evaporative coolers work.)
If the house is closed up, three things happen at once:
- Pressure builds. The blower is rated at low static pressure, so as indoor pressure rises, airflow falls sharply.
- Humidity accumulates. Moist supply air lingers instead of being flushed out, so indoor relative humidity climbs and the air feels clammy.
- Cooling stalls. Less airflow and higher humidity both cut the cooling you actually feel, even though the supply air temperature at the register may look fine.
Those effects are why "my swamp cooler makes the house humid" is often a venting problem, not a cooler problem. See swamp cooler too humid for the full diagnosis.
The 1 to 2 sq ft per 1,000 CFM rule
The rule comes straight from airflow velocity. Air leaving through an opening moves at a speed equal to the flow divided by the area:
With 1 sq ft per 1,000 CFM, air leaves at 1,000 fpm (about 11 mph). With 2 sq ft per 1,000 CFM, it leaves at 500 fpm (about 6 mph). That range is gentle enough that the house barely pressurizes and the blower delivers close to its rated airflow. Go much smaller and velocities climb fast, so pressure and noise rise.
| Cooler CFM | Minimum relief (1 sq ft per 1,000 CFM) | Comfortable relief (2 sq ft per 1,000 CFM) |
|---|---|---|
| 2,000 (small window unit) | 2 sq ft | 4 sq ft |
| 3,500 | 3.5 sq ft | 7 sq ft |
| 4,500 | 4.5 sq ft | 9 sq ft |
| 5,500 | 5.5 sq ft | 11 sq ft |
| 6,500 | 6.5 sq ft | 13 sq ft |
Aim toward the 2 sq ft end in more humid weather, such as monsoon season, when you want the moist air flushed out faster. See swamp coolers during monsoon season.
Calculating window openings
A window opening's area is simply width times height of the gap, in feet. The catch is screens: a standard insect screen blocks part of the opening, so its effective free area is noticeably less than the gross opening. A practical habit is to open screened windows about half again wider than the arithmetic says.
Worked example: venting a 4,500 CFM cooler through windows
Target: 4.5 to 9 sq ft.
A 36 in wide double-hung window opened 8 in: 3 ft x 0.67 ft = 2.0 sq ft gross.
A horizontal slider 48 in tall, slid open 12 in: 4 ft x 1 ft = 4.0 sq ft gross.
One slider plus one double-hung = 6.0 sq ft gross. Allowing for screens, that is near the low end of the target. Open a third window or widen the openings on the hottest afternoons.
Pro tip: Use a thin tissue or a strip of paper at a doorway. If it is pushed hard toward the open window, airflow is moving. If doors push shut on their own or whistle when nearly closed, the house is over-pressurized and needs more relief area.
Where to open: steering the cooling
Air takes the path of least resistance from the supply to the openings. That means you control where the cool air goes by choosing which windows to open.
- Open in the rooms you use. In the afternoon, open windows in the living room and kitchen. At night, close those and open bedroom windows.
- Open away from the supply. A window right next to the diffuser short-circuits air out before it does any work. Prefer openings on the far side of the house.
- Spread the openings. Several modest openings in different rooms cool more evenly than one wide-open window.
- Keep interior doors open between supply and relief, or add transfer grilles over doors that must stay closed.
- Adjust with the weather. On very dry days, smaller openings are fine. On humid days, open wider so moisture leaves faster.
Ceiling relief dampers and attic venting
Ceiling relief dampers (often called up-ducts or barometric relief vents) are spring-loaded or gravity flaps installed in ceiling openings. When the cooler pressurizes the house slightly, they open and let air flow into the attic, then out through the attic vents. When the cooler stops, they close.
Advantages
- You can cool with windows closed, useful for security, dust, noise and pets.
- Exhaust air at 70 to 80 F flushes the attic, which can be well over 130 F in summer. A cooler attic means less heat radiates through the ceiling.
- Relief is placed permanently in the rooms that need it, such as each bedroom.
The attic has to pass the air
All the air entering the attic must exit through the roof and soffit vents. Apply the same 1 to 2 sq ft per 1,000 CFM rule to the attic vents' net free area (the published effective area, not the overall size). If attic vents are undersized, the attic pressurizes, the dampers barely open, and moist air lingers against roof decking and insulation.
Worked example: can the attic handle it?
A 5,500 CFM cooler with three ceiling relief dampers needs at least 5.5 sq ft of net free attic vent area for the relief path alone. A home with ten soffit vents of 0.4 sq ft net free area each plus two gable vents of 1.0 sq ft each has 4 + 2 = 6 sq ft. That just meets the minimum. Keep a window cracked on humid days or add attic vent area.
Warning: Do not install relief dampers into an unvented (sealed) attic or a cathedral ceiling without a vented path. Moist air trapped there can lead to condensation and mold. Close or cover relief dampers in winter, the same as the supply drop.
Diagnosing venting problems by symptom
| Symptom | Likely cause | Fix |
|---|---|---|
| House feels muggy, indoor RH climbs over 60 to 70 percent | Too little relief area | Add open area toward 2 sq ft per 1,000 CFM |
| Doors slam or whistle | Pressurized house, openings in one area only | Open windows behind closed doors, add transfer grilles |
| One room cool, others warm | Relief concentrated in one room | Distribute openings to the rooms you want cooled |
| Strong draft near diffuser but little elsewhere | Relief window close to supply | Close nearby window, open far-side windows |
| Weak register airflow despite clean cooler | High back pressure from closed house or attic | Open windows; check attic vent area if using dampers |
| Musty attic or damp insulation | Relief dampers into undersized attic venting | Increase attic vent net free area |
A cheap hygrometer helps. Indoor RH that stays in roughly the 40 to 60 percent range is a sign the venting is in balance; for more on humidity targets, see swamp coolers and indoor humidity.
Combustion appliances, dust and security
A swamp cooler slightly pressurizes the house, which generally does not cause backdrafting of gas water heaters or furnaces the way exhaust fans can. Still, keep gas appliance closets and vent paths clear and functional, and have a carbon monoxide alarm on every level. If you also run large exhaust fans while the cooler is off, the situation changes.
For dust, open windows on the side of the house away from roads and unpaved yards, and keep screens in place. For security, use window stops or locking pins that let a window open only a few inches; several partly opened windows meet the area rule as well as one wide open window.
To estimate how much cooling the incoming air can deliver on a given day, use the cooling temperature calculator.
Frequently asked questions
Should I open windows when running a swamp cooler?
Yes. An evaporative cooler is a once-through system, so the air it brings in must leave. Without open windows or relief dampers the house pressurizes, airflow drops and humidity rises.
Which windows should I open with a swamp cooler?
Open windows in the rooms you want to cool, ideally on the side of the house away from the cooler supply. At night, open bedroom windows; during the day, shift openings to the living areas.
Can I run a swamp cooler with windows closed?
Only if the house has another relief path, such as ceiling relief dampers into a well-vented attic. With everything sealed, the cooler will mainly raise indoor humidity.
Why do my doors slam when the swamp cooler is on?
Doors slamming or whistling is a sign the house is pressurized because relief openings are too small or are concentrated in one area. Add relief area or open windows in the rooms behind those doors.
Does venting a swamp cooler into the attic cause problems?
It can if the attic vents are too small to pass the airflow, because moist air then lingers in the attic. With adequate attic ventilation, relief into the attic is common practice and helps cool the attic.
Sources and further reading
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- Evaporative Coolers (fact sheet), Colorado State University Extension
- ASHRAE Handbook: HVAC Applications (evaporative cooling chapter), ASHRAE
- Indoor Air Quality (IAQ), U.S. Environmental Protection Agency