Condensation underneath Evaporator

I’m in Houston, Texas. Why is it that sometimes I see excessive condensate underneath the evaporator in the attic dripping into the overflow pan? I’ve had hotter but dryer attics where there is no condensation underneath the evaporator, and cooler more humid attics with condensation underneath the evaporator. Both attics seem to have the same ventilation set up. Could it involve the operation of the HVAC unit? The only explanation I can give my clients is that the attic needs better ventilation, but I feel there is something I’m missing here.

This evaporator coil ‘is designed’ to operate below the dew point. It’s like having an ice-cold glass of water sitting on a table in summertime. The ice water will create /condensate and pool water.

Dew point is the temperature at which air is saturated with water vapor, which is the ‘gaseous state’ of water.

I bring a Psychrometer / Thermo-Hygrometer to inspections. Dew point can also occur inside a wall assembly or attic space if/when moisture ‘from humidity inside the building’ moves out through the walls ‘and becomes cool enough.’ It can/well eventually reach ‘a point below the dew point’ where water will condense inside wall assemblies or attic spaces leading to moisture damage and rot.

Hope that helps.

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Lack of proper insulation of the unit and or leaking conditioned air. No amount of attic ventilation is going to correct it. Conventional, unconditioned Texas attic is a worst place for an air handler and duct work :slight_smile:


The enthalpy of fusion of a substance, also known as ( latent ) heat of fusion is the change in its enthalpy resulting from providing energy, typically heat , to a specific quantity of the substance to change its state from a solid to a liquid, at constant pressure.

The removal of this heat changes the state from vapor to a liquid. That is what an A/C does.

When you guys can’t find the hole in the roof causing moisture in a house, this is the primary source of that moisture your looking for. I’d recommend you fully understand this stuff to properly inspect houses.

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I was about to say the same thing but that is not true. Ventilation (more often than not) is the associated cause of this moisture.

  1. In Texas, there is more moisture in the outdoor air than inside the house (A/C is removing it). When you ventilate, you bring the moisture in when you ventilate the attic air out.

  2. In winter, ventilation brings in colder air which removes the latent heat of fusion and causes condensation of the more humid air inside the house.

You know the blue stuff you see in a thermal scan, when you test it indicates moisture and you call it water intrusion rather than air infiltration.

Good job Robert!

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I agree and that is what I said, read what I wrote again. I didn’t go into specifics as to why, like you have. That’s the reason more crawlspaces and attics are now becoming conditioned than not as opposed to in the past.

Wasn’t talking about your statement. I said I thought the same thing as you, before I read your post. That made me think about it. So I posted it. I’m agreeing with you but we were both wrong with that assumption.