That dehumidifier you see in the photo above is in the Energy Vanguard office here in Decatur, Georgia. The compressor doesn’t come on often, but this past week I’ve heard it running quite a bit. Depending on where you’re from, you may be thinking, “Yeah, it’s always humid in the South. I don’t know how you people can stand it.” Or you may be thinking, “Wait a minute, it’s January. You shouldn’t need a dehumidifier in the Atlanta area in January.”
The latter statement is the one that aligns with our reality here. Our average high temperature in January is 53° F, and our average low temperature is 33° F. Now, those numbers don’t describe how much water vapor is in the air, but they do tell us about the limits.
Our ventilating dehumidifier
Before we get into the details of why our dehumidifier has been running, though, let me explain the setup. We changed out the furnace and air conditioner in our office in 2018 with Mitsubishi mini-split heat pumps and an Ultra-Aire ventilating dehumidifier. A local contractor we’ve worked with on consulting projects, PV Heating & Air, did the installation for us.
You can’t really tell from the photo above, but the ventilating dehumidifier actually has three ducts. The two ducts you see are the return air at the top, which brings air from the office into the dehumidifier, and the supply air at the bottom, which sends dehumidified air back into the office.
The third duct is also at the top, behind the return air duct, and it brings in outdoor air to mix with the indoor return air. The combined air—from the office and from outdoors—goes through the dehumidifier and gets sent back into the office through the supply duct. Since it’s a ventilating dehumidifier, it can operate as a supply-only ventilation system with no dehumidification or as a supply-only ventilation system with dehumidification.
Why we’ve needed dehumidification
We know that cold air is dry air, not in terms of relative humidity—which could be 100%—but in the actual content of water vapor in the air. I like to use dew point as a proxy for that (although it’s imperfect for reasons we can explore over drinks sometime), but there are others, too, like humidity ratio and absolute humidity.
So on a day with a high temperature of 53° F, the dew point won’t be higher than 53° F. We can bring outdoor air into our office, heat it up to 70° F and not have to worry about humidity. According to the psychrometric chart, the relative humidity will be about 55%.
But this January in Decatur has been more like January in Florida. We’ve had six days in the past week with a high temperature of 65° F or higher, including two days in the 70s. And those days weren’t accompanied by sunny, dry weather. It’s been raining…a lot. We’ve had almost 4 inches of rain from 10-16 January.
When the outdoor temperature is 70° F with a relative humidity of 98%—and we’ve had those conditions— he ventilation air coming in would make our office too humid. So the compressor on our ventilating dehumidifier comes on and removes the moisture so our office stays comfortable.
The Goldilocks condition for dehumidification
Here’s what it all boils down to:
When it’s cold outdoors, you don’t need a dehumidifier because you can control the indoor humidity with ventilation. Cold air is dry air, you know.
When it’s hot and humid outdoors, you may not need a dehumidifier because the air conditioner will be running and it should do all or most of the dehumidification you need. But that’s changing as homes get more airtight and better insulated. When we build a house with a really good building enclosure, we reduce the amount of time the air conditioner needs to run and that makes controlling humidity with just the AC a bit harder. This is especially true for smaller homes, like apartments and condos.
Mild humid weather
When the temperatures are in the 60s and 70s Fahrenheit and the outdoor humidity is high, keeping the indoor conditions comfortable will probably require a dehumidifier. It’s too cool for the air conditioner to run but too warm for the outdoor air to be able to dry out the indoor air.
We have plenty of these dehumidifier Goldilocks days in the Atlanta area. They just usually don’t happen in January. I wonder what could be causing this. Hmmm….
–Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard. Photos courtesy of the author.