It will not get below 60 percent

1. It is undersized

The most common cause by a distance, and the easiest to diagnose: the machine runs continuously, never cycles off, and the reading sits stubbornly in the low sixties. That is exactly what a unit does when the room's moisture load exceeds its extraction rate.

It is also expensive. A 30-pint unit running permanently costs about $42 a month at the US average rate, which is more than a correctly sized 50-pint unit at twelve hours a day. The cheaper machine is the more expensive one. 30 versus 50 pint.

2. Humid air is getting in

A dehumidifier can only dry the air it can reach, and if that air is being replaced from outside, it is drying the outdoors. The usual culprits, in rough order of how much they matter:

• An open basement window through a humid summer. This is the big one and it is free to fix.
• A garage door with a failed bottom seal. A large gap along a wide opening.
• An unsealed crawl space hatch, or crawl space vents left open. In a bare-earth crawl space, ground moisture alone will beat any machine. Crawl spaces.
• A dryer venting indoors, which is a substantial and continuous moisture source that people forget entirely.
• Doors left open to a larger unconditioned space. The machine is now sized for the whole volume rather than the room.

3. The room is too cold

Below about 60F a compressor unit's output falls sharply, and near 41F it spends more time defrosting than working. It is running, it is drawing power, and it is removing very little. The reading will not move because the machine is not really operating.

Check with a thermometer rather than by feel - a basement that feels merely cool can easily be 58F. If the space is genuinely cold and needs to be dried, a desiccant unit is the machine that works there. Desiccant versus compressor.

4. The filter is clogged

Reduced airflow means less air over the coil, which means less water condensed per hour while the machine draws its full power. In bad cases it also ices the coil, at which point extraction drops close to zero. Pull the filter and look at it; most are washable. Cleaning and maintenance.

5. The two readings disagree

Built-in humidistats sit in the intake airstream and are not calibrated instruments. If the machine's display says 50 and your hygrometer says 62, the machine has stopped because it believes it is done.

The fix is not to establish which is correct - it is to set the machine lower until the hygrometer reads what you want. Trust the instrument in the place you care about.

And the fault, which is last for a reason

If the room is warm, the space is sealed, the filter is clean, the unit is correctly sized and both readings agree that it is humid - and the exhaust air is not noticeably warm and dry after ten minutes of running - then the refrigeration system is the remaining explanation.

That is a warranty call. A sealed system needs recovery equipment and certification to open, and on a low-cost portable the economics generally favor replacement. Most portables carry one year; AlorAir publishes six on its crawl space units.