Part VI: Staying CoolChapter 10. Passive and Low-Energy Cooling
Night Flushing and Whole-House Fans
Using cool night air to drop the temperature of the mass inside the house.
- Difficulty
- Intermediate
- Cost
- $50–400
Night flushing works when the outdoor air is cooler than the house and dry enough to invite in. You open the windows, you move a lot of that air through the rooms, and the floors and walls that got hot during the day give some of the heat up. In the morning you close up and keep the coolth you bought. Shade during the day, from the shade guide, is what keeps you from needing a heroic fan.
A window fan is the small version. A listed whole-house fan in the ceiling, exhausting into a vented attic and then outdoors, is the large version. Both fail the same way if you run them in a shut house, or against an atmospheric water heater, or on a muggy night that makes the indoors damper than it was.
Difficulty
- Level
- Intermediate
- Time
- An evening to learn the window habit, or a careful install for a ceiling fan
- Skills
- Opening enough inlet area, and leaving combustion appliances alone
Cost estimate
$50–400
A window fan or a pair of box fans is the low end. A listed whole-house fan with a shutter that seals in winter is the high end, and a quiet one with a proper attic vent area can cost more. Do not price a hole in the ceiling as if a box fan and some screws were the product.
| Piece | Typical cost |
|---|---|
| Window fan used on cool nights | Near the low end |
| Listed ceiling fan, shutter, and attic vent work | The high end or above |
| A carbon monoxide alarm if you do not have one | Small, and not optional in a house with combustion equipment |
Materials
| Item | Qty | Notes |
|---|---|---|
| A thermometer indoors and out | 2 | Run the flush only when the outdoors is usefully cooler. Five degrees is a weak reason. Fifteen is a real one. |
| Openable windows on the inlet side | More than you think | The fan maker will have a minimum free area. Shut windows do not count. |
| Listed fan and a shutter, if you cut the ceiling | 1 | The shutter must close and stay closed through the heating season. A box fan over a homemade hole is not this item. |
| Attic vents to outdoors | Matched to the fan | The air has to leave. Stuffing it into a tight attic wets the insulation and loads the roof. |
How much air
The sketch divides the volume of the house by the number of minutes you want one exchange to take. A fan that replaces the air every few minutes can pull the stored heat out of the surfaces before dawn. A bath fan that replaces the air once an hour will not. Window fans sit between those, and they are enough for one room or a cross breeze.
Open the windows before you start the fan. If it labors, whistles at the doors, or makes the house smell like the flue, you do not have enough inlet, or you are pulling on a burner. Stop.
Mass is the point of a long flush. Air temperature drops in minutes. A slab or a masonry wall takes longer. The mass page is about storing sun. The same mass stores a hot afternoon and can store a cool night if you give it hours of cooler air. Carpet over the slab slows that down, which is the same complaint as in winter.
Combustion, humidity, and winter
An atmospheric furnace, water heater, or fireplace uses the room as its air supply. A large fan that pulls harder than the flues can draw combustion products into the house. Sealed combustion appliances are built to avoid that. Atmospheric ones are not. Turn them off before a whole-house fan runs, or do not install the fan until a technician has said the combination is safe. You need a carbon monoxide alarm regardless, but it does not test the flue.
Humid nights are a poor flush. Cool surfaces will sweat when damp air hits them. In a sticky climate, a fan on a person, a dehumidifier, or air conditioning may be the better tool. Night flushing is at its best in a dry place with a large day-to-night swing.
In winter the shutter has to seal like the rest of the ceiling. If it leaks, you are heating the attic. Check it the first cold night. Insulation must not block the fan or the attic vents. Plan the winter closed position when you install the fan, so you do not discover a hole in the ceiling in January.
Plan for security. Leave ground-floor windows open on purpose, with latches or stops that still let air through, and not out of habit during a rough week.
Confirm the night is actually cooler and not soggy
Indoor and outdoor thermometers. Skip the flush when the outdoors is warmer or dripping.
Open inlet windows before any fan
Use the area the fan maker asks for. A ceiling fan running with the windows shut depressurizes the house.
Protect the flues
Atmospheric burners off, or sealed combustion you have confirmed. Alarm in the hall. Stop if you smell combustion.
Run long enough to touch the mass
Air will feel better in minutes. Floors and walls need longer. Close up in the morning, shades down, before the day climbs back in.
Close the shutter for the heating season
A listed shutter, not tape. Attic vents stay clear. No box fan screwed to the drywall.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/Cooling/passive_cooling.htm
- https://www.builditsolar.com/Projects/Cooling/Shading/Shading.htm
- https://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm
- https://www.builditsolar.com/Projects/Cooling/NoAC/ComfortWithNoAC.htm
- https://www.builditsolar.com/Experimental/RoofCooling.htm
- https://www.builditsolar.com/Experimental/RadiationCool/EvaproRad.htm
- https://www.builditsolar.com/Experimental/VentSize/
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.