Condensation on A/C ducts.

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I replaced A/C in a rent house after a fire. Climate is extreme south Louisiana. I completely rewired the house, reinsulated, didn't cut any corners. Also had to replace all ceiling tiles because of water damage from fire dept. (they did a great job saving the house).
The A/C and the electrical were the only things that my husband and I didn't tackle ourselves. Painting, reconstruction and everything else, we did. The house is a ranch style, 1800 sq. ft., fairly low pitch roof, with vents in soffit, but no ridge vent or turbines.
When the A/C guy came I requested a duct with a higher insulation rating, I asked for at least an R6 and I was told that the industry standard was an R8.
Tenant shows me water spots in the ceiling tiles. Upon inspection, it is where the duct tubing cross. Where they touch and cross, condensation collects and then drips down to the insulation to the ceiling tiles. Our temps lately have been hot, but mostly humid. Any ideas? Also, outside the compressor is producing more condensate than the previous unit..the slab that the unit sits on has a permanent green slime. The tenant changes the filter on a monthly basis, the thermostat is new, digital, the house is comfortable to them at 76 degrees and they said the bill hasn't changed or reflected an increase. The house doesn't feel muggy.
Perry
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Increased condensation outside is a Good Thing. The new unit is taking more water out of the air. This lowers the humidity inside and makes it seem cooler.
I don't get a picture of "where it crosses". What crosses what? If condensation is in some spots only, look for breaks in insulation. Seams that are un sealed for instance.
TB
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As a suggestion only cause it is impossible to know what is really happening. Have the tenant put the fan to the ON mode. Run the fan 24 /7. Condensate happens for a plethora of reasons. The duct insulation seams could be exposed, usually not a big deal. Time of high humidity and temps in the mid 70s your asking the a/c to remove a lot of humidity. Which is exactly what I would do to be comfortable.
Show the problems to the contractor and ask for his help.
I suggest that you put some PVC on the drain out by the compressor and divert the water into the lawn instead of the concrete. The slime might want lunch some day. ( Feed ME,,, Little Shop of Horrors )
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Setting the fan to will raise the relative humidity in the house 10 to 15%. Not a good idea. This is because the water on the indoor coil and in the drain pan re-evaporates when the compressor shuts off.
The problem is that the surface of the duct is below the dew point of the air in the attic. This happens when the surface of the duct is shielded from the attic temperatures by another object, especially when two supply ducts are in close proximity. air trapped between the ducts is cooled below the dew point ans drips condensate. The best answer is to use wide straps to support the ducts so they do not touch each other or wood trusses or insulation in the attic. I have a formula to find the surface temperature of the duct somewhere.
Stretch
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The formula for the outside surface Temp of a duct is:
T(surface)= OAT- (OAT-IAT) * OSF/( ISF+DI+OSF)
T(surface) Is the outside surface temperature of the duct OAT is the Outside Air Temperature, that is the air temperature the outside of the duct is exposed to IAT is the Inside Air Temperature, that is the supply air temperature inside the duct OSF is the Outside Surface Film (R-factor), that is the insulation factor for a nearly still air film at the outside surface of the duct, (usually about1.67 for shiny new duct) ISF is the Inside Surface Film (R-factor, that is the insulation factor for moving air at the inside surface of the duct (About R-0.2) DI is the duct insulation R-factor, (about R-8 for your ducts), if the contractor is being truthful
Run the formula to find the surface temperature of the duct. Note that if the air is trapped in a pocket formed around the duct by contact with other ducts or structure, yhe OAT can approach supply air temperatures very closely. That is usually where the problems start.
The dew point of the attic air will be the same as for the outdoor air, which you can get from the weather man or a Sling Psychrometer.
Stretch
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OAT = 90 F and IAT = 40 F makes
T(surface) = 90-(90-40)*1.67/(0.2+8+1.67) = 81.54 F...
Like this, viewed in a fixed font:
T(surface) | ISF DI | OSF IAT ---www---www------www--- OAT
<--------------------- I
Heatflow I = (OAT-IAT)/(ISF+DI+OSF) = (90-40)/(0.2+8+1.67) = 5.066 Btu/h, and T(surface) = OAT-I*OSF = 90-5.066x1.67 = 81.54 F.
Wrapping it all with R19 fiberglass insulation and poly film duct might decrease I to 1.732 and raise T(surface) to 90-0.67x1.963 = 88.87, while reducing the rate of condensation (if any) and saving energy. Exposing ducts to more attic air ~~~wastes~~~ energy.
T(surface) | 0.2 8 1.67 19 | 0.67 40 ---www---www---www---www------www--- 90
<---------------------------------
I = (90-40)/(0.2+8+1.67+19+0.67) = 1.693 Btu/h

Or Td = (460+OAT)/(1+(460+OAT)ln(RH/100)/9621)-460. For instance, 90 F air with RH = 50% has Td = 550/(1+550ln(0.5)/9621)-460 = 69 F.
Nick
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Nick,
I agree that exposing more surface of the duct to attic air will waste some energy. But if you read the original post, he is having trouble with ducts sweating, not a high electric bill. So I addressed HIS problem.
You show a supply air temperature of 40 degrees. I have been measuring supply duct temperatures for 30 years, and I have never seen one that low. Normally 50 to 60 degrees, depending on air flow and return air temperature. Most often around 55 degrees.
If you ran a supply air temperature that low, you would need special controls to prevent evaporator coil freezing. That low of a supply air temperature would definately increase heat gain through the ducts and waste energy. Also, the capacity and efficiency of the AC would go down.
We once modified a 25 ton chiller with a factory installed cold water kit to run at 25 degrees chilled water temperature instead of 42 degrees chilled water temperature. We added about 50% glycol to the chilled water. The capacity dropped from 25 tons to 10 tons. The efficiency dropped also.
Stretch
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Adding more duct insulation seems like a better way to solve his problem.
Nick
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Adding more duct insulation seems like a better way to solve his problem.
Nick
If the duct is not dried out first, moisture will be trapped inside between the vapor barriers, reducing the effectiveness of the insulation and degrading it over time.
Stretch
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So make a pinhole in the duct.
Nick
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That can create more condensation..

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Really? :-) How?
Nick
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1-depends on your definition of a pinhole:) 2-cold air, hitting hot metalic duct liner= condensation
Ask me how I knwo this...lol....just had a call last week that I would have bet my bottom dollar on looking it all over before gettin into the attic space above the business' eating and serving area would have been a roof leak after all the rain we had... It was nothing more than where someone previously had inserted a thermometer probe into the air duct (flex of course) and it was creating one hell of a condensation issue....the duct of course was laying on the dropped ceiling supports and when we finally located the hole (I know..sounds simple..had to see the installation that was left) the owners could not believe it either..

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Well, I'm thinking the air inside the AC supply duct has a lot less moisture than the air in the attic, so venting a tiny amount into the surrounding space containing more insulation and a vapor barrier around that would dry out any lingering trapped moisture that Stretch might find a concern. It is a serious concern. Any leak in the final vapor barrier could allow warm moist attic air to enter and condense inside the new arrangement. One way to prevent that is to allow a bit of conditioned air to leak out into the space surrounding the duct. This is similar to a house with the vapor barrier on the inside and more water vapor permeability outside of that.

How would this scenario apply inside the new insulation and vapor barrier?

Sounds like a different scenario...
Nick
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Nick, if you make a hole in the duct so air cold leaks out, you will be wasting energy! Also the cold leaking air will chill the inside of the vapor barrier Because the insulation will not stop the air flow, only act like a filter, which will chill the outside of the vapor barrier, causing it to sweat. You are back to the original problem again. Now the duct is sweating again, ruining the ceiling again. Seems like you are going in circles.
In addition, I can see you have never actually worked on ducts in a tight attic. Insulating with R-6 uses about 1.5 inch wrap. R-19 is about another 4.5 inches. So if the duct is six inch, the outside of the original duct with R-6 is 6" + 1.5" + 1.5" = 9" diameter. Now add R-19 to that which is 10" more. (5" on the left side and 5" on the right side.) Now the duct is 19" in diameter. Hope you have a BIG attic. Especially when the duct gets to the register near the outside wall where the roof slopes down to mneet the wall. I can just visualize you laying in that rock wool insultion in the attic trying to wrap that duct and getting all itchy. Then I visualize you falling through the drywall ceiling. I hope you land on a nice soft couch instead of the floor. If you had ever actually worked in a tight, hot attic, laying in the insulation, You would not propose such a thing.
By the way, you are using duct wrap, are you not? Standard insulation would not meet code when used to wrap a duct. Code also says to follow manufacturer's instructions. If your R-19 bat is not listed for use on ducts, you could get in trouble with the building inspector. The thickest duct wrap I have seen is about 3", or R-8. 2.25" is rated R-6. So R-19 DUCT WRAP would be more like 6.75".
This is rapidly getting out of hand. If you still think it is a good idea, go up in a tight attic tomprrow with some R-6 duct wrap tomorrow and put 3 layers on. Then you will see what hapens when theory meets practice.
Stretch
In short
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Just a pinhole...
Nick
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That pinhole, can indeed cause you to fail a blower door test.... yes....wasting energy...why do you think that ducts, installed correctly, are sealed from start to finish?

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No way.
Nick
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Nick, If you put in a hole so small it would not cause problems, it would not do anything either. A pinhole will do nothing. Stretch
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Then why are we led to believe it will fail a blower door test? :-)
Nick
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