Where to put a dehumidifier

A dehumidifier pulls air across a cold coil, condenses water out of it, and blows the drier air back out. It has no influence on air it never touches. So the machine’s effective range is set entirely by how well air circulates back to it, and that is a property of the building rather than the appliance.

Two consequences people find counterintuitive:

• Standing it next to the damp wall does nothing special. Moisture evaporating from that wall disperses into the room air within minutes. What matters is whether the room air keeps reaching the machine.
• A closed door is a hard boundary. Relative humidity equalizes readily through open space and very slowly through a closed door. A unit in the hall is not drying the closed bedroom.

Every portable unit specifies a minimum clearance around the intake and the discharge, commonly in the range of six to twelve inches, and it is the single most-ignored line in the manual. Blocking either side does two things at once: it starves the coil of air, and it lets dried, warmed discharge air loop straight back into the intake, so the machine measures its own output and cycles off while the room stays damp.

1. Check which faces are intake and discharge on your unit — top-discharge models can sit closer to a wall than back-discharge ones.
2. Keep furniture, curtains and stored boxes out of both.
3. Stand it on a hard, level surface. Deep carpet restricts the underside on units that draw from below, and level matters for the tank float and the drain.
4. Leave room to get the tank out, unless it is permanently drained — which it should be, per continuous drainage.

Humidity moves with air, so a dehumidifier serves whatever volume is genuinely open to it. In a typical basement that means the main open area rather than the utility room, with interior doors left open.

• Basement: central, on the main slab area, not in the furnace closet. Keep it clear of the boiler or water heater draft hood.
• Whole floor: near the middle of the circulation path — a hallway or stair landing — with doors open. One machine cannot serve two floors; warm moist air rises and does not come back down on its own.
• Laundry or bathroom: only as a supplement. These rooms have a moisture source and usually an exhaust fan; ventilation beats dehumidification there, which is the argument in bathroom exhaust fans.
• Crawl space: a dedicated unit, sealed space, ducted or positioned to circulate the full run. Portables in a vented crawl space are drying the outdoors — see crawl space dehumidifiers.
• Garage: temperature decides the machine before placement does. Below about 60 F you want a desiccant unit, wherever you stand it.

Warm air holds more moisture and rises, which makes a case for raising the machine. Two things argue back: water drains downward, and a dehumidifier on a shelf is a dehumidifier that will eventually be knocked off one.

In practice, floor level is right for almost every portable, for the practical reason that gravity drainage needs the outlet above the drain point. The exception is a space with a pronounced temperature difference top to bottom — a tall basement, a stairwell — where a few feet of height puts the intake in the warmer, wetter air. If you do raise it, use something stable and rated for the weight, and remember it will now need a pump only if the drain is above the new outlet height.

1. Hard against a wall or in a corner. Recirculation plus restricted intake. The most common error by a wide margin.
2. In a closed utility room. The unit dries eighty square feet beautifully and the rest of the basement not at all.
3. Next to a heat source. A furnace, a water heater or a dryer changes the air it samples, so the humidistat reads a microclimate rather than the room.
4. Under a shelf or a stair. Discharge air hits the underside and comes straight back. The machine short-cycles and the humidistat is satisfied by its own exhaust.

Capacity and placement are different problems, and buying more capacity does not fix a placement problem. Work out which you have:

• It reaches the set point and cycles in the space it is in, but other rooms stay damp: a placement or air path problem. Open doors, add a fan to circulate, or move the unit.
• It runs continuously and never reaches the set point: a capacity problem, or an unsealed space letting outdoor moisture in as fast as it is removed. Sizing is in the sizing guide, and what continuous running actually costs is in how long one should run.

A cheap box fan moving air between rooms is often a better second purchase than a second dehumidifier — where to put a fan applies exactly the same reasoning to a different machine.