Dehumidification deconstructed: Frequently asked questions

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What is relative humidity?

Relative humidity, or RH, is the amount of water in the air compared to how much water the air can hold before it condenses and turns into droplets. Once 100% RH is achieved, the air is fully saturated, which often means rain. 

What is dew point?

Given that warmer air can hold more humidity than cooler air, dew point is the temperature to which air must be cooled so it can no longer hold all the water in the air in a vapor state. That means when the air is cooled to the dew point, condensation will form.

How does a refrigerant dehumidifier work?

Essentially, air flows over a cool evaporator coil, which causes condensation to form because the coil temperature is below the dew point of the air. Refrigerant dehumidifiers work best when the dew point is above freezing so condensation can form without creating ice.

How does a desiccant dehumidifier work?

Desiccants use an absorbent media, two air streams and heaters to remove moisture. The process air stream passes through an absorbent desiccant wheel, drawing water out of the air. A separate reactivation air stream is heated and passes through the now saturated desiccant wheel, drying it out and sending the moist air outside of the conditioned space.

What relative humidity should I target?

In a commercial setting, target relative humidity is often dictated by the process, application or environmental conditions. For instance, a glass manufacturer may want incredibly dry air—as low as 10% relative humidity—whereas a baker may desire 65% to keep their product from drying out. While there are many variables, a general rule of thumb for inhibiting mold growth and optimizing comfort is to aim for between 50-60% relative humidity.

Why do I need a dehumidifier if I have A/C?

While an air conditioner does dehumidify to an extent, it doesn’t do so effectively or efficiently. In addition, you can run into an over-cooling situation, and cooler air cannot hold as much water as warmer air. That means condensation is more likely. So, even though A/C removes water as it cools, the relative humidity can actually rise from cooling a space.

If A/C cannot dehumidify without a heat load, should I follow it up with reheat?

A/C will remove approximately 1 – 2 pints of water from the air per kwh.  When you reheat, you double the energy usage, so now you can figure you’re removing ½ – 1 pint per kwh. Quest dehumidifiers, however, can remove up to 8 pints per kwh. The energy efficiency and cost savings is significant.

Now, when there is a natural heat load that drives the temperature above the desired temperature, let the A/C do its thing and take the 1 – 2 pints per kwh as a by-product of necessary cooling.  Dehumidifying should never be the product of cooling.

Can’t I just ventilate my way out of a humidity problem?

When the dew point outside is lower than the dew point inside the space being dried, ventilating will dehumidify. However, when the dew point outside is higher than the dew point inside, ventilation will raise the humidity.