Condensation – It Isn’t Where Hot Meets Cold

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Photo by Stephen Rardon

Whenever someone brings up undesired condensation on an air handler cabinet, or on a supply air duct or in a ventilation duct or on a vent like the one above, someone will inevitably say “condensation occurs where hot meets cold”.

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

The trouble was, the problem almost never went away  just by trying to keep hot from cold

Then in 2003 – 2005 we had some of the most active hurricane seasons in Florida on record with numerous land strikes and tons of power outages as well as 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.

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, 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 participate in installing 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, and 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 dewpoint. Period.

Dew point is simply the temperature at and below which air containing a particular amount of moisture can no longer contain that moisture and will begin to give up water in the form of condensation. Saying dew point is the same as saying the 100% humidity point.

Air can achieve dew point without coming into contact with a surface at all (see clouds), but often we observe that air hits dewpoint 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 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. This is not a great design right off the bat, and add in the fact that we ALSO have high latent (humidity) load so we run the blowers at low CFM output for 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 when a vent sweats it should be sealed. This is true, because it is just a good practice, but also because it prevents unconditioned, moist air from entering in around the boot or can and condensing moisture around the vent and on the ceiling. In my experience sweating registers are more often caused by 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 by increasing system airflow. While increasing air flow will increase the register 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. This 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 slight positive pressure.

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

It is also helpful when designing fresh air systems in humid climates to only provide 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, supplementary dehumidification can be added 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 will often be cooler than the air contained inside. This is why it is 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
  • The insulation of the duct is insufficient
  • The insulation of the duct is compressed
  • The 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

— Bryan





  1. Avatar Joe Reinhard says:

    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!

  2. Avatar Loren Bartling says:

    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.

  3. Avatar Robert says:

    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

  4. Avatar Ron VanDyk says:

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

  5. Avatar Richard Nottage says:

    Well written Bryan

  6. Avatar Christopher Rendon says:

    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!

  7. Avatar Matt says:

    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.

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