Can You Run a Dehumidifier With the Window Open? 3 Easy Tips

Last Updated on October 24, 2026 by Umar Farooq

Can you run a dehumidifier with the window open? Yes, nothing will break, but it is usually wasted electricity, because the machine ends up trying to dry the outdoors. Manufacturers and Energy Star all say the same thing: close the openings while it runs.

I have no objection to a window being open. I have an objection to a window being open and a compressor humming away beside it, which is the domestic equivalent of bailing out a boat with the plug still out.

Living room with a tall glass door open onto a riverside deck, white curtains drawn back, an example of airing a room before closing up and running a dehumidifier — Airing a room is fine. Doing it while the dehumidifier runs is the part that wastes energy. Stock photo of an open door, no dehumidifier in shot.

Can you run a dehumidifier with the window open? Technically yes, practically no

A dehumidifier does not know which air is yours. It pulls in whatever reaches its intake, strips out some water and blows the drier air back out. Open a window and a steady stream of outdoor air replaces the air it just treated.

Dehumidifier makers are consistent about this. Energy Star advises that doors and windows to the space being dehumidified should be closed while the unit is running. The Dri-Eaz DrizAir owner’s manual puts it more bluntly: operate the machine in an enclosed area, as this creates a drying chamber, and close all doors, windows or areas that open to the outside to maximise water removal efficiency. That is a restoration-trade machine, but the physics is identical for a household one.

So the machine will run. It just stops being a dehumidifier for your room and becomes a dehumidifier for a small patch of your street.

Why the machine never catches up

Three things happen, and they feed each other.

It never reaches target. A dehumidifier cycles off when its humidistat reads the level you set. With fresh outdoor air constantly arriving, the reading keeps being dragged back up, so the humidistat is never satisfied. (Energy Star notes the unit is meant to cycle on and off automatically once the room reaches the desired level; with the window open, that moment may simply not arrive.)
It runs continuously. No cycling off means the compressor and fan run for as long as you let them. I have not measured the extra consumption on any particular machine, so I will not invent a figure; work it out from the label wattage and your tariff.
The tank keeps filling. If the outdoor air is damp, the water collected is outdoor water. A full tank then switches the machine off, usually at the moment you have gone out, and the room is no drier than when you started.

If you are watching a tank fill in an afternoon and the room reading has not moved, check the window before you check the machine. How long a dehumidifier should run covers what a normal run looks like when the room is properly closed.

When the outside air really is drier

There is one genuine exception, and it points the opposite way. If the air outside holds less water than the air inside, then opening the window dries the room better than any machine, and for nothing.

That does not mean running both. It means swapping them: window open, machine off. The comparison that matters is moisture content (dew point), not the relative-humidity percentage on a weather app, and the windows and humidity guide explains how to tell the two kinds of day apart without any arithmetic here.

Rain, mist and warm summer evenings are the usual days when the outdoor air is wetter, and those are the days the open-window-plus-dehumidifier combination does most harm.

Cracking a window while drying washing

This is the one I would call a judgment call, and I want to label it clearly as my opinion rather than a tested result.

Person standing at a clothes airer loaded with washing in a small bedroom with a window, the situation where people wonder whether to crack the window open — A small room, a loaded airer and a window. The pull to open it is strong; usually the machine does better with it shut.

Wet washing puts a lot of water into a small room fast, and some people crack a window “to let the damp out”. If the day is dry and the outdoor air is genuinely drier, a short airing before you start is sensible. A window left open for hours while the machine runs is not: you are still feeding it outside air, and in a small room with the door shut a closed-up machine does the job faster. The full method is in drying clothes indoors with a dehumidifier.

My rule: if you must have a gap, make it brief and do it before the machine goes on, or once the washing is dry. I would not leave it open for the run.

Ventilate first, then close up and run: 3 easy tips

The sequence that avoids the waste is simple.

Check the air outside. If it is drier than indoors (use a cheap hygrometer indoors and a weather app dew point outdoors), open the windows wide and let it swap the air for a while.
Close everything. Windows, external doors, trickle vents you control. Leave internal doors in the arrangement you want: open for whole-floor maintenance, shut for a one-room job. Where to put a dehumidifier covers that, including why rule 4 (shut the external ones) comes last in the list but first in practice.
Then switch it on. The room is now a sealed batch of already-drier air, and the machine only has to deal with the moisture you add.

If after a week of this the room still will not reach target, the problem is more likely size than windows. What size dehumidifier you need is the place to check.

What if it is stuffy and you need air?

People sometimes open a window because the closed room feels airless, not because of damp. Cooking, bathing and sleeping all add moisture and stale air, and a sealed room with a dehumidifier does not replace oxygen or remove cooking smells.

The fix is scheduling, not compromise. Ventilate in short bursts when you are in the room and the outdoor air suits, then close up and run the machine when you leave or go to bed. Use a trickle vent or the bathroom extractor to handle the moisture-making moments directly, so the dehumidifier is only mopping up what is left.

Open the window first, run the machine second

So: you can run a dehumidifier with the window open, and you will get a loud, thirsty machine that never finishes. Ventilate when the outdoor air is drier, close up, and only then let the machine do its job.

FAQ

Does a dehumidifier work with the window open?

It runs, but it works poorly. The outdoor air keeps replacing the air it has dried, so the room rarely reaches the target level and the machine does not cycle off.

Is it bad for the dehumidifier to run with the window open?

It will not damage it, but it runs longer than it should and fills the tank with water that came from outside. The cost is electricity and wear, not an immediate fault.

Should I close the doors and windows when running a dehumidifier?

Yes, the external ones. Energy Star advises closing the doors and windows to the space being dehumidified while the unit runs. Internal doors depend on whether you are treating one room or a whole floor.

Is it better to open a window or run a dehumidifier?

Whichever of the two suits the day. If the outdoor air holds less water than the indoor air, opening the window is free and effective. If it is wetter outside, keep it shut and use the machine.

Can I run a dehumidifier with a window open in winter?

You can, but cold air entering will make a compressor unit work in poorer conditions, and the room loses heat. Shut up and run it, and ventilate in short bursts instead.