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Duct condensation after new system installed

Posted by Larryd999 (My Page) on
Wed, Jul 25, 12 at 11:55

Had a new Carrier Infinity system (Heat Pump - split system - crawlspace)in a 1962 vintage tri-level 3000 sq ft brick home in Huntsville, Alabama. We are experiencing very hot/humid conditions (95+ temp and 105 heat index). House has 3 yr old double-pane windows, upgraded insulation in attic and crawlspace - walls too difficult to breach/repair, so they have whatever they put in back in '62, probably next to nothing. Replaced a 10 yr old York with the highly recommended Carrier. Never had duct dripping with York or its' predecessor, an old Bard. Carrier system is great - quiet and pumps out very cold air, variable speed seems to be efficient. All ducts (rectangular and round sheet metal) are now sweating profusely. Original fiberglas insulation is soaked. I know that is ruined and must come off. Crawlspace ducts are easy access for new insulation. Ducts that feed upper level are trickier - they are under floor between joists in overhead of a finished basement with suspended ceiling. I have removed ceiling tiles to allow better air circulation, and bought a 45 pint/day dehumidifier for this area, which is extracting a lot of water. Crawlspace has 2 powered vent fans exhausting to outside. This fall when I have time off, I plan to take off all existing insulation, apply duct seal mastic and tape on all joints, and re-insulate with new insulation. I have not yet called my HVAC installer until i use up my DIY skills.
Questions: 1) Any interim solutions to minimize dripping until i can re-insulate? 2) Is insulation the only or best solution? 3) What is best new insulation? I have read closed-cell, fiberglas, even bubblewrap - it's all about air gap and sealing leaks...
All opinions and suggestions gladly accepted.


Follow-Up Postings:

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RE: Duct condensation after new system installed

You must have a high humidity level in the basement to cause all this condensation. Do the powered vent fans run continuously? If not then this would be an entry point for humid air to enter. If they do run all the time, then they could be causing a negative air pressure situation in the basement. The outside humid air is entering through any leakage points. I would think about getting rid of the fans and sealing the exhaust.

There is no harm in insulating metal duct work. In my opinion the right solution is to address the humidity problem. This humidity will enter your house which will cause your Infinity system to work harder to remove it.


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RE: Duct condensation after new system installed

The variable-speed system is probably producing colder air as it approaches the set point. That is a product of the program to control humidity. The byproduct is colder ducts which leads to the condensation.

It seems like dehumidifier in the basement is the right idea. The fans in the crawlspace might be increasing the condensation if there is more water available from the air than from the ground. Your best solution might be to enclose the crawl. Is there plastic on the ground now?


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RE: Duct condensation after new system installed

Thanks for the 2 posts offering advice. I have a good vapor barrier in the crawl space, but I am going to go back over it this weekend to check for tears or gaps, especially in back corners and around load bearing CMU columns. I may try turning off the 2 crawl space vent fans and covering the openings with plastic for a few days. The condensation/dripping has slowed with just a 24 hour run of the dehumidifier in the family room adjacent to the crawl space. We have emptied the container 4 times since initially turning it on around 8 PM last night. Obviously the fiberglas duct wrap is still soaked and will likely take a while to dry out. As I said, I will replace that this fall - maybe with something like ArmaFlex / K-Flex. Anyone have recommendations on the duct insulation products? I was reading about a product that seems like a double wall aluminum that is used as a radiant heat barrier in sheet form under roof decking, and as a duct wrap product. I know they use very expensive double wall stainless steel pipe insulation in the power plants where I work, and the air gap between the walls cuts heat loss tremendously on high temp steam lines. Anyway, I am pleased to date with the Carrier system with regard to cooling and quiet operation. Thanks again for the support!


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RE: Duct condensation after new system installed

not the foil bubble foil wrap!

instead when you undertake this job in the
crawlspace (not basement..crawlspace..right?)
go armed with
buckets of mastic and paint brushes
to apply
rolls of hardcast brand 1420 mastic tape
R-8 ductwrap (fsk foil scrim kraft paper with fg insulation)
fsk tape same material as ductwrap but tape fastner
duct stapler & staples
cordless drill & sheet metal screws
duct strap

switch unit to from fan on auto to on position
to keep ducts dry so that materials will stick
& dry. then when you finish the job..go back
to auto fan setting.

unwrap duct
remove duct tape
screw sections of duct to fittings
90 degree ells, supply boxes
duct take offs to plenum
mastic seal all joints & fittings
mastic seal plenum connections
to equipment
use duct strap to support ducts.
try to keep ducts 6" off ground.
re-wrap with R-8 ductwrap
staple seams with double row of staples
use fsk tape on top of seams. staple tape.

use duct wrap to externally insulate
supply & return plenums.

it will take several days, depending on size
of house & amount of ducts.

adding external insulation to supply plenum
will solve condensation issues that arise
from putting all this in humid crawlspace.

take plastic to help you slide around in there
and good lights to see to work by.

foil bubble foil for ductwork is easy..but
it doesn't work.

best of luck.


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