Twin window fan vs single window fan

Nobody certifies window fans, so every figure below is a manufacturer claim from their own product page, and the efficiency column is our division of one by the other.

Two things in that table are worth saying plainly. The first is that the gap is a factor of four, not a few percent. The second is that the two most convenient fans in the list — the cheap twin and the low-profile Vornado — publish no airflow figure at all, so nobody can tell you where they sit.

Width gets all the attention because that is what the accordion panels adjust. Height is what rules fans out.

• A twin needs roughly 13 inches of clear opening height. Most sashes clear that.
• A 16-inch single needs 22 inches, and a 20-inch single needs 26.25. Plenty of windows — and nearly every apartment window with a sash stop — do not open that far.

Measure the clear opening with the window fully open, not the glass. If the answer is under about 22 inches, the choice is made for you, and the twin is not a compromise so much as the only format that fits. If your window slides sideways or cranks out, neither format may fit at all, and the casement and slider problem is its own page.

Neither format publishes much here. Air King gives 64 / 57 / 50 dB for the 20-inch model and nothing for the 16-inch; Lasko publishes no figure for the W09550 at all.

What is knowable is the shape of the problem. A twin’s 9-inch blades have to spin fast to move any air, and small fast blades make higher-pitched noise, which people find more intrusive at the same measured level. A 16 or 20-inch blade at low speed moves far more air for a lower, duller sound — which is why the quietest useful setting is usually a big fan turned down, not a small fan turned up. The general version of that argument is in how loud a fan actually is.

The twin format has absorbed every convenience feature in the category. The Lasko has three speeds, an eight-hour timer, a remote, an adjustable thermostat that cycles the fan against room temperature, and independent control of each motor. The single-blade whole-house fans have a rotary switch.

That matters more than it sounds for overnight running: a thermostat is what stops a fan pulling in air at 4 a.m. that is colder than you wanted, and a timer is what stops it running until noon. If those are the features you are shopping for, the wider field is in fans with remotes and timers.

• Buy a twin if the opening is under 22 inches high, if the room has only one usable window, or if you want a thermostat and a remote. It is also the only format light enough to move between rooms without thinking about it.
• Buy a single if the opening takes it and the job is a whole floor rather than one room — overnight flush-outs, a house that holds the day’s heat, or anywhere you would otherwise be running two twins.
• Buy neither if outdoor air is not cooler than indoor air for some part of the day. A window fan can only deliver the air outside; when that air is 88 degrees, the honest answer is an air conditioner.