[kcb203]

I had a new Carrier Infinity system installed last week in Northern Virginia. So far everything is running splendidly except for one problem--the supply duct in the finished basement just past the coil is sweating profusely. The temperature in the basement is 63 with 60% humidity while the rest of the house is at 76 with 40-45 percent humidity. Outside it has been 88-90 and very humid.

Neither supply register in the basement is open, though there is an open return. I've got the supplies closed because it is so cold anyway. I've got two proposed solutions:

1) Open the supplies. This may bring in dryer air that will not condense on the ducts, though I expect the basement would get even colder.

2) Seal the ducts. I assume the basement is cold because there is duct leakage even though no register is open. If the ducts are sealed, the basement won't be so cold and the ducts won't sweat. My previous system didn't do this even though it was the same size, which makes me think it may not be just duct leakage.

I've got a 3-ton system, and the airflow has mostly been at 1050 cfm. Would it help to change from "Comfort" to "Efficiency" to increase the airflow? Would this result in a smaller temperature drop and less condensation?


Thanks.

 

[t527ed]

could put a dehumidifier in basement

 

[kcb203]

I've thought about a dehumidifier, but I thought that the Infinity should take care of it. The humidity in the rest of the house is much less than previous summers with the old unit. Perhaps my old A/C wasn't anywhere near capacity and wasn't getting the temperature drop across the coil that the new one is. Could that be the difference?

 

[t527ed]

colder ducts would definately be more prone to sweat. i would wait to hear from the carrier guys before messing with blower speed,but raising that would probably help

 

[Freezeking2000]

open basement supply and see what happens, or how about insulating the ducts?

 

[hyper_racing]

With the supplies closed off and the return open, you are not putting air back in the basement when it is removed. Therefore you are probably pulling back in a little outside air, which you said is very humid. Open up the supplies and let it condition the basement and it will stop sweating. If you are wanting to keep the registers closed to keep it from getting to cold, you will need to insulate the ducts with ductwrap.

 

[arc8]

I've got a 3-ton system, and the airflow has mostly been at 1050 cfm. Would it help to change from "Comfort" to "Efficiency" to increase the airflow? Would this result in a smaller temperature drop and less condensation?

Do you know what CFM you should have? Did you get a Heat-load analysis sheet from your installation company?
Perhaps you need to speed up that blower. The outlet(supply air) temp seems a bit low.
I think CFm of 1050 is good for dehumidification only.
Perhaps insulating the plenum is all that is needed seeing you are in a very muggy location.

 

[teddy bear]

I am a dehumidifier guy but for now you need to raise your duct temperature above the dew point of the basemen air or lower the dew point of basement air below the duct temp. Close the return in the basement which sucks in outside humid air. Enough supply air to positively pressurize the basement. Insulating your supply ducts with exterior vapor sealed insultaiion will warm the basement and stop sweating. Regular insulation allows condensation on the metal of the duct.
Increasing the air flow will also increase your duct temperature but decrease the dehumidification from the a/c. Keeping a basement below outdoor dew points will cause condensation in the exterior walls of the baement. Mold will grow. In the end, you will need a good dehumidifier for the days that you have no cooling load. Only the best dehumidifiers can do much with a cold basement. Chech out the Santa Fe dehumidifiers.

 

[kcb203]

I think insulating ducts and following teddy bear's suggestions are my only way to control this. Increasing the supply as hyper_racing suggested won't work--the dewpoint of the basement air is 49, while the dewpoint of the rest of the house is 53, so the basement is actually less humid in absolute terms.