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The Perfect Weather for a Dehumidifier

A Whole House Ducted Dehumidifier

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.”

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 the 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 but 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 both a simple 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 — the 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:

Cold weather

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.

Hot weather

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 here.  I wonder what could be causing this.  Hmmm.


Related Articles

Cold Air Is Dry Air

What Is the Relative Humidity When It’s Raining?

Controlling the Humidity in Your Home in Winter


Disclosure:  Mitsubishi and Ultra-Aire are sponsors of Energy Vanguard, and PV Heating & Air donated the labor for the installation of the equipment.


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This Post Has 21 Comments

  1. “When we build a house with a
    “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.”

    NOT if the AC is sized properly!

    1. Curt, yes, you raise a good

      Curt, yes, you raise a good point.  As I mentioned in the part after your quote, this is more of a problem in smaller homes.  When the total load for a house or a zone is lower than the smallest equipment put in to serve it, runtime will drop.  That’s why I’m not a fan of fixed capacity equipment.  Most of our designs specify multi-stage or variable capacity equipment. 

      1. Your comment on multi-stage
        Your comment on multi-stage or variable capacity equipment makes sense. But the building user may have a different perspective. In my house, for example, the geothermal heat pump is inside the house. Despite sound insulation in the heat pump cabinet, and around the compressor, the sound level is objectionable, especially at night when competing sounds are absent or diminished. Since my two stage compressor operates at the same RPM at both stages, the sound level is much the same at either level. But the running time is longer on the lower capacity stage than if the system had only one stage, high. My new system is economical, but I would trade some of the efficiency for a shorter run time and fewer hours per year of compressor noise.

    2. Did you know that a properly
      Did you know that a properly sized air conditioner is actually over sized 90% of the time. (single stage, which by far are the most common)

      Why? They are sized to handle heat of day normally between the hours of 3pm – 6pm. Under more extreme heat waves this can be expanded from 3-7pm.

      (another note: I am not talking about the desert south west)

    3. I know for a fact AC can’t
      I know for a fact AC can’t always control humidity. Our well insulated 2.4ACH50 home will have indoor temps in the 60s through spring and fall thanks to Ohio’s 50s – 70s outdoor temps. Our furnace/AC doesn’t run for weeks during these shoulder seasons but the exterior humidity comes in (in part) through our HRV. An independent humidifier would make my system perfect. At the moment I use a portable dehumidifier because I have it on hand and it gets the job done. I am hoping the next few years brings a dehumidifying HRV for when I replace my HRV.

    1. Chuck, there was no engineer

      Chuck, there was no engineer of record for this project.  The city didn’t require stamped plans for the design.

  2. Absolutely echo this from
    Absolutely echo this from here in Austin TX. We also had the same mix installed (Minisplits and an UltraAire 98H ventilating dehu). And similar weather the last few weeks. Over last weekend I had the UltraAire off completely because we were making dust in a renovation (concrete houses, man), and I forgot to turn it back on. Horror of horrors, my beer showed condensation when sitting inside and I had that recently unfamiliar clammy feeling.

    Checking we had 75% relative humidity at 72° inside. No desire for AC to operate (brrrr), but turning the dehu sorted that out in about 45 mins.

    Definitely the best bit of equipment in the house.

  3. When I started out in the
    When I started out in the HVAC biz in muggy Florida 11 years ago I did my best to avoid central dehumidifiers, believing them a costly band-aid to oversized and otherwise improperly selected, configured and commissioned HVAC systems as well as sloppy leaky houses and duct systems.

    Along the way we developed and implemented an ever-longer checklist of ways to control indoor humidity…15, 16 ,and now up 21 separate items ranging from the obvious (proper HVAC) to less obvious items such as putting out panting dogs and sweaty children.

    Lately, the climate here was worn down my resolve. During my 25 years here, average dewpoint has steadily risen, and we experience more weather like what Allison describes. If it is too cool to cool and foggy or rainy, there is no other solution than separate dehumidification.

    Ducted units are best, but spendy. A less expensive solution (though with its own irritants) is to park a portable dehu near a main return…that at least corrects immediate problems and introduces owner / occupants to the concept of dehumidification.

    1. Do you do this while the
      Do you do this while the heater or the A/C is running? My assumption would be A/C only. Please clarify?

      Thank you,

  4. I took a different approach
    I took a different approach and installed an Inverter Heat Pump, not a mini split though.

    Mini splits in my opinion are a spot cooling / heating appliance. While you could use them as a way to zone, the cost to maintain, repair and ultimately replace leaves you equipment poor.

    So I turn my Inverter heat pump, which is also zoned into a dehumidifier by dropping the temp a couple of degrees while running in cool mode. So in turn, 1 less piece of equipment to buy.

    High humidity — foiled again. Love it when that happens.

  5. Ran into the same problem in
    Ran into the same problem in Greenville, SC. I don’t have a dehumidifier, but I do have a humidity sensing vent in the master bathroom. The house is tight (beyond Energy Star, but not PH) and the HVAC pulls outside air periodically to keep things fresh.

    During the period Allison is describing (we have the same weather as Atlanta, just a hour or so later) the bath fan would run after a shower and never turn off. The reason, which should be obvious, is that it was creating negative pressure and bringing in the humid outdoor air via the HVAC.

    My temporary solution was to turn off the bath fan and wait it out. Low humidity re-established itself outside not long thereafter and I turned the fan back on.

    This is also a problem during the shoulder seasons, but, so far, has not gotten to the point of causing any observable issues, such as condensation and mold. I am watching the relative humidity in the bathroom, the main floor, the basement and outside, and no worries so far, but I am keeping an eye on it.

  6. The technique of reducing
    The technique of reducing cooling temperature setpoint to reduce humidity sometimes works, but sometimes the overcooling causes discomfort. It also causes those unfamiliar to call in: “I set my thermostat for 76*F, and it is 74*F and my system is still cooling”

    Then we have to explain how “cool to dry” works (and sometimes how to turn it off)

    “Cool to dry” if taken too far, can seriously backfire – whenever indoor temperature gets below outdoor dewpoint, serious moisture and mold problems are at risk.

  7. This is a nice and simple way
    This is a nice and simple way of framing it for my clients who are easily confused about this.

    Do you have to set those ultra-aire units up to be in one mode or the other (supply-only vs dehumidifying-supply-only), can you toggle between them as you want, or does it have some kind of automatic toggle? What is the energy use like on those things? I remember doing an energy model for a client who had one and being unsure how to report the operating wattage for the ventilation system.

  8. I am just an ignorant
    I am just an ignorant homeowner struggling to wrap my head around this crawlspace humidity science. It seems if not for these warmer and wetter than usual fall, winter, and spring seasons an average homeowner just trying to maintain 14% wood moisture here in Greensboro, North Carolina could conceivably install the temperature controlled louvered crawlspace vents however he would cover them up during the warmer and wetter summer months. The wood moisture may increase a little during the summer season however it would decrease to acceptable levels during the other seasons. Now that the fall, winter, and spring seasons are in fact warmer and wetter I think sealing all of the vents and installing an Atmox system would be a whole lot more economical than a dehumidifier. I would like to believe a system like the Atmox could dry a crawlspace rather quickly after a 4 day rain spell in the 70s like you mentioned in your post.. At least here in my region. I own a rental and my tenant cannot afford to pay the electricity bill for a dehumidifier.
    I am probably missing some key facts about crawlspace humidity science but this is where I am right now in my knowledge journey on this topic.


  9. Great topic and one I am
    Great topic and one I am quite familiar with here is coastal NC. Without debate, the only acceptable solution is a dedicated dehumidifier like the Ultra-aire or Aprilaire (my preference as the Ultra-aire can add an annoying low frequency hum to the building). I put them in every house I build. One can design all the HVAC systems you want and still not completely solve the humidity issue on a year round basis. We have four shoulder months here where very little if any HVAC is running. Still need humidity control. Even in winter, a very tight house can need de-humidification as we do things that create moisture in the air. Even government studies have shown the dedicated option as best. Lastly, I have installed HVAC heatpumps with dehumidify cycles and they are still not enough. Systems properly sized for low sensible loads in energy efficient homes, also drop the latent capability. The ones I have tried are limited to a 3 degree drop in sensible settings before they stop dehumidifying. The house can still be damp. I install mine in the conditioned attic space with separate ducting from the HVAC system. Makeup air is then drawn into the attic space to be mixed there before being introduced in the living space via an intake duct on the dehumidifier intake. I use an Air King QuFresh air exchanger with temperature and humidity limiting controls to bring in make-up air to the attic space. I never want my houses to go negative if possible. When bath fans or the clothes dryer is running, makeup air is naturally drawn in by the same system. We keep my personal home set at 45% RH year-round and it never deviates more than a couple of percentage points.

  10. Do you have a ultra-aire rep
    Do you have a ultra-aire rep/contact person that you could share with me? I would like to know all the possible sequences of operation that can be performed with this unit. Thanks in advanced.

    1. Michael, two people who can

      Michael, two people who can help you are David Treleven and Nikki Krueger.  I sent you their contact info via email.

  11. That squashed connection on
    That squashed connection on the bottom hurts to look at after reading your excellent series on ductwork design.

  12. Hello,

    I have a Sante Fe Compact 70 dehumidifier in the crawlspace of my rental home. The instruction manual has a Caution! message which states
    “The dehumidifier will run continuously until the RH is reduced to the humidity control dial setting. It is not recommended to set the humidity control to “dry” in rooms under 65-degrees; doing so will result in long periods of ineffective dehumidifier run time.”
    Would you recommend that I purchase a dehumidifier that will turn it self off if the crawlspace temperature falls below 65-degrees?

    Thank you,

  13. The hvac 2.0 program suggests using residential dehumidification with electric reheat. Heat pumps with already in-place heat strips can potentially provide this. I assume this is needed only in regions with high relative humidity in cooler weather. Is this something that is needed in particular areas of the country? How can I tell if my region needs it or not? Interested because I have an old system due for replacement.

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