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

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

Perry Templeton wrote:

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

If there is not too much water dripping inside I have seen large cheap aluminum
broiler pans used to catch the water, which eventually evaporates. Outside, see
if you can make a catch pan to divert and drain water away. BTW is it possible
the A/C installer can be convinced he should fix the problems?

Lou

"BTW is it possible
the A/C installer can be convinced he should fix the problems?"

I don't know where the problem lies. That's why I posted.

I was referring to where the ducts cross. Big tubes in the ceiling.
Perry
wrote in message ...
Perry Templeton wrote:

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

If there is not too much water dripping inside I have seen large cheap
aluminum
broiler pans used to catch the water, which eventually evaporates.
Outside, see
if you can make a catch pan to divert and drain water away. BTW is it
possible
the A/C installer can be convinced he should fix the problems?

Lou

"Perry Templeton" wrote in message
...
"BTW is it possible
the A/C installer can be convinced he should fix the problems?"

I don't know where the problem lies. That's why I posted.

I was referring to where the ducts cross. Big tubes in the ceiling.
Perry


Again, Where the ducts cross what? Each other? More ducts? Collar ties of
the rafters? WHAT?
Greg

Where the ducts cross each other.
The big silver tubes in the attic. One particular spot in question, the
duct tubing was criss crossed and where they made contact, it seemed to
create more condensate there.
Perry
"Greg O" wrote in message
...

"Perry Templeton" wrote in message
...
"BTW is it possible
the A/C installer can be convinced he should fix the problems?"

I don't know where the problem lies. That's why I posted.

I was referring to where the ducts cross. Big tubes in the ceiling.
Perry


Again, Where the ducts cross what? Each other? More ducts? Collar ties of
the rafters? WHAT?
Greg

"Perry Templeton" wrote in message
...
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 )

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

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

Stretch wrote:

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...
IAT is the Inside Air Temperature...
OSF is the Outside Surface Film (R-factor)... about 1.67 for shiny new duct
ISF is the Inside Surface Film (R-factor... About R-0.2
DI is the duct insulation R-factor, (about R-8 for your ducts)

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

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.

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

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

Stretch wrote:

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.

Adding more duct insulation seems like a better way to solve his problem.

Nick