10:27AM | 01/20/04
Member Since: 03/04/03
44 lifetime posts
Last year I had a new forced hot air furnace and central AC system installed.I already had the original duct work in for the heat(square sheetmetal ducts)
so I figured now was time to add central AC

While the system works good, in very hot weather when the ac runs all day I notice the sheetmetal ducts close to the furance sweat and drip cold water onto the ground in two spots.And there are all wet beads of water on the bottom of the duct work Mostly at the widest portion of the duct work, before it splits up and goes up the ceiling.I can take a towel and wipe it dry but it returns fairly quickly.

For all I know it could be sweating inside the walls also. This concerns me, I am worried about damaging the walls.

Why does this happen and can I insulate with anything?? IS something under or oversized onmy system?? If so, what kind of insulation can I use for the metal ductworks?? Also, was this old duct work made to take cold air from the ac unit?? Iwas told it would be ok but I could have just been fed that line so he got the job.

Any help is greatly appreciated,

Thanks in advance,


02:41AM | 01/21/04
Member Since: 03/04/03
44 lifetime posts

"Get an independent measurement of the air flow thorugh your ductwork."

I am a laymen when it comes to these terms, Can you explain what exactly this entails?

WOuld I have to hire someone to do this??

Thanks in advance,



09:38AM | 01/21/04
Member Since: 03/04/03
44 lifetime posts

How about the company who put the system in approx 1 year ago???

Do you think that is a bad idea??



11:16AM | 01/22/04
You may also have a very humid basement causing the duct to sweat. You need to find a contractor that is experienced in air flow measurements. Your typical contractors are not versed in airflow problems.

plumber Tom

03:23PM | 01/22/04
Member Since: 05/10/03
801 lifetime posts
Harold is right on the money on this one! You need the proper clearances for air flow ABOVE the coil in the plenum. Common rectangular ducts require the installation of a vane to prevent the evaporator coil from freezing.


05:18AM | 01/23/04
Member Since: 03/04/03
44 lifetime posts

Thanks for the advice guys. I will look into getting a measurement of airflow done.

In the meantime how can I determine if there is a "vain" installed on my system??

What does it look like and where would I look for it?

Again, the system was installed but the existing old metal duct work was not changed.

Thanks in advance,



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