“I’ve just turned on my new dehumidifier. How long will it take to reduce the humidity below 50%?”
This is the classic ‘how long does it take for a dehumidifier to work’ question. We are going to look into how quickly the dehumidifier starts to work and how many hours (or even days) we have to run it to turn a humid room (50%+ relative humidity) into a dry room (30-50% relative humidity).
Namely, dehumidifiers are used to reduce indoor relative humidity levels. Here is the 2-step principle of how dehumidifiers work; it’s very similar to air conditioners. We have these 3 cases of how to answer how long does it take for a dehumidifier to work:
- Dehumidifier starts working 5-10 minutes after you turn it on (at a 50%+ dehumidification rate). This is akin to AC units and connected to the refrigeration cycle. When you turn the AC on, it takes about 5 to 10 minutes before it starts blowing cold air. The dehumidifier similarly needs about 5 to 10 minutes to start working (refrigeration cycle cooling coils in order for indoor moisture to be able to condensate on the coils, thereby removing humidity).
- You will “feel” that dehumidifier is working after 1-8 hours. In that time, the dehumidifier will reduce humidity levels by at least 10%, and we will feel that the room/house is less damp.
- Dehumidifiers will take anywhere from several hours to several days to reduce the humidity levels below 50%. There are 3 key factors here: 1) Size of the room (lowering humidity levels in a small 10×10 room is faster than in a bigger 16×16 room, for example), 2) Dehumidifier capacity (20-pint, 30-pint, 50-pint), and 3) Initial relative humidity levels (dehumidifier will get to below 50% humidity levels much quicker if we start at 60% humidity levels than at 90% humidity levels). We also have to note that warm air (let’s say warm 80°F air can carry more air moisture than colder 60°F air, for example).
Quick Example: Let’s take a 12×12 room (8 ft ceiling height) that is at 70% relative humidity levels and at 70°F temperature. How long will it take for a 20-pint dehumidifier to reduce the humidity levels to 50%?
There are a lot of calculations to answer this adequately. Here’s the short version: 12×12 room contains 1,152 cubic feet of air. At 70% humidity and 70°F temperature, 1,152 cubic feet of air contain about 92.97 pounds of air and 12.90 pounds of water. At lower than 50% humidity, the same amount of air will contain 9.10 pounds of water; that’s 3.80 pounds (or 3.64 pints) less moisture than before.
A 20-pint dehumidifier is capable of removing 20 pints of water per day (at ideal conditions); that’s 0.83 pints per hour dehumidification rate.
If we have to remove 3.64 pints of water, and we have 0.83 pints per hour dehumidification rate, it will take a minimum of 4 hours and 23 minutes for the dehumidifier to make this decrease from 70% to 50% relative humidity. In practice, however, it can take up to 10 hours since the 20-pint/day specified dehumidification rate is measured at ideal conditions (usually at 80% humidity).
Now, as we can see, the calculation of how quickly the dehumidifier can reduce humidity levels is quite complex. We had to use these Engineering Toolbox resources and even data from Nasa just for the quick example.
Now, let’s look at what happens when you turn the dehumidifier on and how long it takes actually to start reducing humidity levels:
Dehumidifier Starts Working In 5-10 Minutes
In order for the dehumidifier to start working, it needs to use cold refrigerant to cool the coils. When the coils are cool enough, the indoor water moisture will start condensing on the coils (much the same as water droplets condense on a cold window).
When you turn the dehumidifier on, you start its refrigeration cycle. Here is a short description of what happens:
- The compressor and refrigeration flow start right away.
- In about 5 minutes, we will get cold refrigerant cooling the evaporator coils (cold coils), and the condenser coils (hot coils) will be warm enough to heat the air up to indoor temperature.
- Simultaneously, the fan will start within 5 minutes. This will produce the airflow of moist air over the evaporator coils (water condensation, decreased air temperature) and proceeding airflow over the condenser coils (increase air temperature back to normal).
- Within 10 minutes, you should see the first water droplets channeled from the capillary tube to the drip tray. The water extracted from the humid indoor air will accumulate in the water tank, or be directly channeled outwards if you have a dehumidifier with gravity draining or a pump.
After 10 minutes, we will not yet “feel” that dehumidifier has started working. We will also feel the airflow is being produced (fan in operating) and hear the sound of the compressor.
We will only start to notice that dehumidifier is working (making a change in humidity levels) after a few hours (it can take only 1 hour is small rooms or up to 8 hours in big rooms). That’s when the humidity levels will fall by more than 10% (from 80% to 70%, 70% to 60%, and so on).
Namely, our skin, hair, and lungs cannot detect a 1% difference in humidity levels. We will need at least +/- 10% relative humidity difference for our body to detect that the humidity levels have decreased. That’s when (after few hours) we can actually feel that the dehumidifier is working.