Now that we are in cooling season, our air ducts are sweating in the first 10 feet of the air handler.  Brand new Carrier Infinity system - 2 stage w/ dehumidifier mode.  I changed the setting in the Infinity controller so in dehumidify the airflow is increased, but the sweating seems to take place in low and high stage as well as dehumidify stage.  The air handler is in the basement (mostly finished) laundry room and the ducts are in the same location.  The ducts are mostly sheet metal - some of the older sections are aluminum.

I could set up my dehumidifer right next to the air handler, but that thing really sucks power so I'd prefer to avoid it.

On the other hand, the Infinity system does a fabulous job dehumidifying the house (and cooling, although it hasn't been handed a big test yet).

Any advice?

Thanks.

Insulate the ducts. Use rolls of ~R2 closed-cell stick-on foam directly on the ducts if your only goal is to stop the sweating.

If you care about the overall system efficiency, seal all seams & joints with duct mastic, then insulate to at least R6 on all as much of the supply ducts that you can get to. Be careful to air-seal any fiber insulation, otherwise you'll end up with condensation that you can't see now that it's hidden by duct-insulation causing corrosion on the ducts. Spray foam insulation does a great job on both, but may be difficult or impossible to retrofit cleanly.

Are these ducts & air handler in an unconditioned basement, attic, or somewhere else?

The ducts were sealed from the inside at the same time we had our system installed - aeroseal (also owned by carrier). We knew the ducts were bad - 60 year old aluminum ducts - according to their pressure sensors, went from 45% leaky at the furthest duct from the air handler to 2% leaky. It does seem to have helped a lot (although not sure what is attributable to the ducts vs. adding new registers vs. the new system).

The ducts and air handler are in a conditioned basement (partially finished laundry room - no supply or return registers in there but the rest of the basement has them and a large air return just outside the door to the back, which is normally open, but also has grills between the two areas for proper combustion air). We don't have a sump or ground water issue - it's only partially subterranian.

When there no chronic bulk-water issues, basement humidity issues generally track with outdoor dew points. In laundry rooms leakage from the dryer exhaust venting can be an issue, and the vent itself can be a major infiltration point. The better exterior vent caps for infiltration prevention look like this:

http://heartlandnatural.com/images/Dventdetail.gif

http://www.energyfederation.org/common/images/productfamilies/small/s_110.jpg

http://www.amazon.com/HEARTLAND-21000-Dryer-Vent-Closure/dp/B00009W3I4

This may be enough to reduce (but not necessarily eliminate) the sweating if it's a localized humidity issue.

If the foundation sill & band joist are accessible, a 1-2" closed-cell spray foam insulation & sealing job around the entire basement can also be a significant infiltration reduction, lowering the summertime humidity, as can weatherstripping any windows & exterior doors/hatches.

But the dew point of 75F 60% relative humidity air is 60F, and in dehumidify mode the AC air is likely to be well below 50F, so some amount of duct insulation is useful. If the ducts are rectangular in cross section & easy to access cut'n'cobble with several sheets of 1/2-1" foil-faced iso is cheaper than the elastomeric peel & stick stuff (K-Flex, etc.). But it's more work- you need to seal the seams & corners with FSK-tape (2" aluminum), and interfaces joists & floor/wall penetrations with 1-part SPF foam (like Dow Great Stuff, or similar). The key is to prevent room air from convecting in to deposit moisture behind the insulation to corrode the duct over time. Half-inch rigid board is less half the price self-adhesive 1/8" foam per square foot and 1.5-2x the R value. 1" rigid board is slightly more than half the price and 2-3x the R value. If you paint the ducts with an appropriate paint to protect it from occasional minor condensation leaks in your insulation air-sealing are less critical. R2 is good enough to avoid surface sweating, but R5-6 is recommended for system efficiency for ducts within conditioned space. (In unconditioned hot attics above the ceiling insulation you'd need to go R10 or higher.)

For somewhat more money (but far less work) you can insulate the ducts with ~1" (R6) of closed cell spray foam using one of the 2-part foam kits (FrothPak, Fomo Foam, Tiger Foam, etc.), which would be completely impervious to air, with no possibility of condensation against the metal. (The ducts will be quieter too.) Going any thinner than 1" with SPF isn't recommended, since thin spots & adhesion issues are more likely, and it can be difficult to get consistent results, but 2" would be overkill for R-value.

Good work on the duct-sealing! Your 45% duct leakage starting point is an all too common reality- reducing that to 2% is GREAT!