Technology Connections recently posted a detailed video about dehumidifiers that’s worth watching. But a key takeaway is that on an AC, the hot side is outside (making the inside cool), while in a dehumidifier they are directly next to each other (condensing the moisture).

Without looking into the specifics, my guess is that it’s not routing the cold air over the hot side to keep things inside the same temperature.

Sounds like either a) the dehumidifier is broken or confused b) your apartment has more moisture in it than you thought or c) the moisture is being replenished from somewhere.

I stayed in a friends house a few months ago and she had 4 dehumidifiers running since she bought it. The house was built before damp proofing was a thing and set into a hill, so the place is basically one massive wick for groundwater. It will never be over for her.

My guess is your heat pump doesn’t have a good dehumidify mode. The thermostat does, but it’s just turning on the A/C. A good dehumidify mode is a very slight A/C and moves a lot of air. So, if your compressor doesn’t handle enough steps down, you’re spending a lot of electricity for not much effect. A dehumidifier with a continuous drain might be a better solution for you.

The way an AC works is that by condensing water, a surface is cooled. By blowing air through this cold surface, the room is cooled. Well, at least sort of. Let’s leave it with that explanation.

Now if you set it to dehumidify, it basically does exactly the same, but might aim for target humidity rather than target temperature.

Cold air can take less moisture, so depending on circumstances, temperature and humidity, it might get colder but the humidity actually goes up.