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Condensation on A/C ducts. Perry Templeton 08-01-2005
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Posted by Perry Templeton on August 1, 2005, 6:19 am


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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Posted by on August 1, 2005, 4:36 am


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



Posted by SQLit on August 1, 2005, 5:33 am



> 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

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 )




Posted by Stretch on August 1, 2005, 6:24 am


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



Posted by Stretch on August 1, 2005, 6:42 am


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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