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Why Over or Undersized Equipment Causes Humidity Issues

It’s a random Tuesday and your relative humidity is at 80%. The next day? 50%. What the heck, right? Don’t worry – it’s not as mysterious as it seems. 

So, what’s the answer to the mystery? Often the issue is your air conditioning is oversized or undersized, it’s not operating correctly – or both.

Let’s break down each of the causes, so you can get a quick understanding of what you may need to address before adding additional dehumidification (If you don’t feel like reading, episode 7 of Dansplainin’ lays this out for you – just click the video link above). 

It’s all about the evaporator coil

When we’re talking about oversizing and undersizing your AC, we’re specifically referring to the evaporator coil. 

If you’re not familiar, the evaporator coil is the workhorse inside an AC. Filled with cold refrigerant (40-50 degrees F or lower), it lowers the air temperature as the airstream flows over the coil. As that air cools off, the temperature falls below the dewpoint. When that happens, water is pulled out of the air – helping drop the relative humidity. 

 But because AC units are built primarily to cool the air, moisture removal becomes an inconsistent byproduct.  You can’t rely on the A/C to pull out all the moisture you need.

Undersized AC = uncontrolled RH 

This is straightforward, folks. If the evaporator coil isn’t large enough, it can’t pull out enough moisture from the air to get your humidity to the desired level.  

When this is the issue, you have two options: 

Oversized. Seriously, it’s a problem.

If you’re thinking how is that possible, hang on because things are about to get crazy. 

First, the background. AC units run on thermostats, so they’re focused exclusively on temperature (for reference, dehumidifiers run on, you guessed it, a dehumidistat). 

Here’s why that’s a problem if the AC unit is oversized: 

In this situation, the coil never got cold enough to properly remove moisture from the air. It would’ve needed at least 5 minutes of run time to affect the RH. 

The result from a scenario like this: You’re left with a cold, clammy space that’s uncomfortable and has an RH too high for any application or process you may have running in that space. 

Operational changes to consider

  1. Slow down the amount of airflow across the coil. In other words, reduce the CFM. The less air going across the coil equates to more contact time for that air with the coil. This will push the air to a colder temperature (ideal in comfort cooling applications). That leads to a lower dewpoint, which equates to more moisture being pulled from the air. Ta-da! You get a lower RH.
  2. Avoid defrost cycles. This is a common issue for our friends in industrial facilities, such as cold storage warehouses.  

Once the evaporator temperature dips below freezing, you lose capacity to remove moisture because the water turns to frost as it’s pulled from the air. That significantly reduces the capacity of the evaporator and, you have to put the unit into a defrost cycle. That means taking it out of service for a period of time and losing heat and moisture removal capacity.

**One fix: If possible, raise your loading-dock temperature to 40-42 degrees F and do the same in your cooler. This will allow you to run the coil wet, meaning water will continue to cascade off the coil. You’ll eliminate the need for a defrost cycle and therefore increase capacity. 

Still have questions? Need help determining the specifics? Give us a call or send us an email