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Dehumidification: Get Good

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This summer is shaping up to be warm and wetYour customers may turn down the thermostat, and still feel uncomfortable, or even warm.  The problem isn’t the room temperature, it’s the humidity. With a dedicated (or whole housedehumidifier (DHU), ACCA’s application guidance, and installation instructions, you can control moisture levels and create comfortable conditions for your customer.   

Dehumidifiers: Warm Dry Air  

DHUs produce dry air, but they do not cool. DHUs have a compressor, evaporator, condenser, and fan.  Air enters the DHU, passes through the filter, over the cold evaporator, condensate forms, and runs down the drain.  The cold, drier air leaves the evaporator and then passes over the condenser.  In addition to heat from the condenser, the air also absorbs some compressor and fan motor heat.  Almost all of this heat goes into the discharge air.  That is why, for example, a DHU can produce 100˚F discharge air.  

The combination of drier air and increased air temperature creates air with very low relative humidity.  When it’s done right, the distribution of that warm, dry air lowers the indoor RH and creates comfortable customers (ACCA members see Dehumidification: ACCA’s How-To).  When it’s done wrong, the only thing that may happen is a higher utility bill. To learn more about other duct configurations and what not to do, ACCA recommends a study by the Florida Solar Energy Center.) 

What A Dehumidifier Can Do 

Dehumidifiers are a great tool in the hands of a competent professional.  Here are some challenges a dehumidifier can resolve: 

  • Pretreat Ventilation Air: Outdoor air brought into the cooling system, especially in humid climates, adds a considerable load to the equipment and impacts how well the system cools the space. Manufacturers of dedicated DHUs, or whole-house DHUs, offer models with ports to accept ventilation air as well as air from the home (or zone).  Dehumidified ventilation air that is properly distributed through the home (or zone) eases or removes the burden on the cooling system. (See the   Dehumidification: How-To for application guidance).   
  • Infiltration Loads:  Thermodynamic equilibrium causes wet things to go where it’s dry.  When it’s humid outside and dry inside, moisture seeks to flow through every hole, gap, crack, and crevice in your home.  DHUs are great at treating this moisture… up to a point.  ACCA recommends the home be evaluated with an envelope pressure test (blower door).  Homes with five air changes per hour (5 ACH50), or less should enjoy the benefits of what a DHU can offer.  
  • Occupant’s Behavior: bathing, cooking, cleaning, etc., all add moisture to the air. DHUs are the perfect tool to address these loads and create a dry environment.   

What a Dehumidifier Can’t Do 

  • Leaky Return Ducts: Leaky return ducts in unconditioned spaces draw in any moisture from a hot, humid attic or a damp crawlspace.  DHUs will help.  However, to use a DHU to overcome these losses is a wasteful misapplication. Test the ducts to know, then seal the ducts.  In humid climates, return ducts should be sealed to less than 6% leakage.  In moderate climates, return ducts should be sealed to less than 9% leakage.    
  • Leaky Homes: Homes with more than five air changes per hour (5 ACH50) need to seek remediation to tighten the dwelling.  A DHU in this application will spend most of the time drying the surrounding neighborhood. Like ventilation loads, these moisture increases impact how the equipment operates. 

Leaky ducts are at least one problem a dehumidifier cannot fix.  Nor can it reduce the humidity in the attic or crawlspace, or at least it shouldn’t.  Leaky (as well as poorly insulated ducts) must be sealed (and properly insulated) to prevent bringing in. 

  • Extreme Occupant’s Behavior: DHUs may not resolve extreme circumstances.  
  • Plants. A dozen large plants (over 24”) generate about 1 pint of water about every three hours. Several dozen large plants create a significant latent load.   
  • Aquariums. A small bowl for the goldfish is fine. A 40-gallon aquarium will evaporate one pint of water every six hours or four pints per day.  Several aquariums or large aquariums create larger latent loads.  

Whether plants or aquariums, alone they are easily addressed by a DHU.  However, when combined with infiltration, ventilation or other loads it could exceed the DHU’s capacity. 


DHUs are a great tool in the hands of an educated HVAC professional (who got good). However, they are not a cure-alldon’t be stupidCarefully consider all circumstances. Incorrect application of a dehumidifier may do nothing more than raise the electric bill and upset your ex-customer. Follow ACCA’s guidelines and the installation instructions for a cool, dry, happy customer.
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