Condensation – It Isn’t Where Hot Meets Cold

Photo by Stephen Rardon

I hear it all the time. Someone will talk about undesired condensation on an air handler cabinet, on a supply air duct, in a ventilation duct, or on a vent like the one above. In reply, someone will inevitably say, “Condensation occurs where hot meets cold.”

Early on in my career, I believed that. So, when I saw a vent like the one above, I would either increase the airflow to warm up the supply air, seal around the vent, or even pile insulation on top to make the ceiling “less warm.”

The trouble was that the problem almost never went away just by trying to separate hot and cold.

Then, in 2003-2005, we had some of the most active hurricane seasons in Florida on record; we had numerous land strikes and tons of power outages. We also had weeks with high latent (humidity load) and low sensible load (low outdoor temperatures).

Condensation and mold growth EXPLODED!

In the summer of 2004, I had a few things happen that opened my eyes to the reality of condensation.


When I had to clean a vent in an unfortunate location…

First, I kept going back to the home of pro golfer and former Masters champion Mark O'Meara and wiping down his vents and ceiling, all while frantically attempting to solve the root issue.

Eventually, the vent DIRECTLY over his large, heavy, king-sized bed started growing mildew. I tried moving it, and it wouldn't budge, so I ended up STANDING on his bed, reaching with my tiptoes to wipe down the vent and the ceiling. The whole time, I was PRAYING he didn't walk in and see me that way. That event got me to the point where I understood that simply sealing the boot and vent and insulating above and around it wasn't doing the trick.

Luckily, a month or so later, I was able to help install an AprilAire whole-home dehumidifier on a test house where they tracked the results vs. a typical home with a variable speed air conditioning system.

The results were incredible. The house with the dehumidifier had no issues with condensation and was able to maintain target relative humidity no matter the latent or sensible load on the space.

This is what I learned:

Condensation occurs whenever air hits the dew point. Period.

Dew point is simply the temperature at and below which air containing a particular amount of moisture can no longer contain that moisture. So, the air will begin to give up water in the form of condensation. Saying “dew point” is the same as saying the 100% humidity point. (For a more in-depth look at dew point as it relates to psychrometrics, check out THIS article.)

Air can reach the dew point without coming into contact with a surface at all (see clouds). However, often we observe that air hits the dew point when it contacts a surface of a lower temperature than the air itself. So, condensation on surfaces is a function of:

  • The moisture contact of the air
  • The temperature of the surface
  • Contact time on the surface

So, what causes air to hit the dew point and condensate in undesigned places? It is either colder or more humid than it is designed to be in those places.


Sweating air handlers

In Florida, we have many air handlers (fan coils) located in unconditioned garages. That is not a great design right off the bat. Also, let's add in the fact that we ALSO have a high latent (humidity) load. So, we run the blowers at low CFM output, and we have a recipe for sweating (condensating) air handlers.

The only way to resolve the issue is to warm up the air handler cabinet by running the system at higher CFM (warmer), decrease the humidity in the garage through supplementary dehumidification (add a dehumidifier), or ventilate the area better, which keeps the air in contact with the cold air handler surface for a shorter period of time.


Supply register condensation

Common knowledge about sweating vents tells us that we should seal a vent when it sweats. That's true because it's just good practice, but it's also because it prevents unconditioned, moist air from entering around the boot or can and condensing moisture around the vent and on the ceiling. In my experience, sweating registers more often result from high humidity in the space, poor air velocity, low air temperature caused by low system CFM output, or a combination of all three.

The problem is that many techs will try to solve this problem by increasing system airflow. While increasing airflow will increase the registered temperature, it will also reduce the ability of the system to dehumidify, resulting in high relative humidity in the space.

The best way to reduce sweating registers is to reduce or eliminate the effects of moisture “drivers” that introduce new moisture into the space in the first place. That can be done by properly ventilating bathrooms and kitchens, keeping doors and windows shut, improving the airtightness of the conditioned space, and using an ERV to keep the space under neutral or slightly positive pressure.

Obviously, proper cooling system sizing and duct design will help extend system run times and decrease indoor humidity.

When designing fresh air systems in humid climates, it's also helpful to provide only the amount of fresh air required and no more unless an enthalpy control system is in place.

It can also help to redesign registers and branch ducts to output the designed face velocity of the vent for better air mixing in the room. In extreme cases, we may add supplementary dehumidification to stop the issue once and for all.


Duct condensation 

Ventilation ducts will often condense moisture on the inside when they are routed through spaces that are cooler than the air contained inside. That is why it's a good practice to insulate ventilation ducts in most climates unless the duct is run completely in the conditioned space.

Supply air ducts will also condensate at times on the outside when one of the following situations occur:

  • The air in the duct is colder than designed
  • Insulation of the duct is insufficient
  • Insulation of the duct is compressed
  • Ventilation around the duct is poor (some ducts are designed to be buried in insulation, and others are not)
  • The moisture content around the duct is high

Usually, when you find condensation on ductwork, it is a combination of two or more of these issues.

All of the issues discussed above can usually be prevented by:

  • Proper ventilation of moisture-laden air (bathrooms, kitchens)
  • Better sealing of conditioned spaces
  • Better insulation of conditioned spaces and “cold” objects
  • Proper duct design and system airflow output
  • Keeping ducts and air handlers inside the conditioned envelope when possible
  • Placing vapor barriers on the “warm” side of structures to prevent moisture intrusion
  • Use of ERVs to positively pressurize the space (in very warm climates) and neutral in multi-season and cold climates
  • Installation of supplementary dehumidification when required
  • Keep the space no cooler than it must be for comfort
  • Size cooling equipment properly to extend run times and reduce space humidity
  • Do not bring in excessive fresh air during humid outdoor conditions

I also taught a class on how to stop sweaty ducts, vents, and systems. You can watch that class HERE.


P.S. – Jesse Claerbout and I teamed up to make a video about sweating ducts, units, and vents. I cover the science, and Jesse shows you how to address sweating ducts and systems in the field. You can watch it HERE. I also made a short podcast on that same topic, which you can listen to HERE.




Joe Reinhard
Joe Reinhard
7/14/17 at 05:10 PM

Bryan, Excellent delving into the details case and being honest w/ initial knowledge & attempts to understand & solve the 2004 mold growth case. That has to be a CARING tech’s worst nightmare & have cost hours of sleep!
Thanks for your time and being secure in what you know as proven truth – via scientific or experience – w/ courage to speak out. Great summary!

Loren Bartling
Loren Bartling
7/15/17 at 10:16 AM

While I don’t agree with you on some things, this article is right on the money. Understanding how the grains of moisture within the air we condition is such an important lesson.

7/15/17 at 11:43 AM

Thank you for the information on this problem. I’m glad you decided to share it bcuz I have this occasionally happen in my bathroom

Ron VanDyk
Ron VanDyk
7/15/17 at 02:07 PM

I believe over sizing your unit also will also cause humidity problems because of short cycling!

Richard Nottage
Richard Nottage
7/15/17 at 07:13 PM

Well written Bryan

Christopher Rendon
Christopher Rendon
7/16/17 at 11:19 AM

Great job man! Love the detail information! Of course like all Technicians we all form are own opinions but you can’t argue with results!

8/17/17 at 04:17 PM

Well written piece, usually when I see such I’m checking first the boot/register that is sweating for leakage or if others are sweating it may be whole space related. I’ve found some to just have a/c charge not correct aside from a system oversized and not dehumidifying properly.

Jerry Beach
Jerry Beach
6/28/22 at 06:02 AM

How to tell if a squirrel cage blower wheel is out-of-round when removed from shipping box before it is installed in blower assembly-residential applications and light commercial units. thanks.


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