Water collecting in duct work

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AndrewM1974

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I noticed that I was getting signs of a leak in my drywall ceiling. Went up in the attic only to find that the water is dripping from an in-line ventilation fan ( Nutone ILF250). The nearby bathroom shower is vented by a Nutone QTXEN150FLT-B which leads up and over to the in-line fan via 6” hard, metal duct. Past the in-line fan is a 6ft run to the 4” vent hood or exhaust out side of house. The kicker is that the last 2-3ft of it goes from 6” down to 4”. Am I getting condensation due to back pressure? Thanks very much for any insight/information shared! God bless
 

Dana

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I assume that by "ventilation fan", you really mean an exhaust vent, not a supply of outdoor air to the indoors?

You're getting condensation because you have humid air going into an un-insulated metal pipe located in an unconditioned attic, where it's cold.

Dripping from a seam or joint in the duct means the duct isn't well sealed either. All joints and seams need to be sealed (with duct mastic, not tape.)

I'm not sure exactly what the duct configuration is. Are both fans feeding the same ducts, that wye or tee in together? If yes, are there at least butterfly backflow preventers on each fan? Or is the ILF250 in series, as a flow-booster for the other fan?

A 150cfm or 250cfm fan into a short section of 4" duct isn't a big problem, but the combined 400 cfm is never going be real. The total duct length (equivalent lengths of all ells & tees) as well as the diameter determines the total amount of back pressure, and the actual vs. rated cfm the fans are delivering. The cfm rating of the fans is rated at 0.10 water inches, but even if they're only hitting half their rated flow, that's not the reason for the condensation. The duct walls are still WAY below the dew point of the humid air that's flowing in them.

Sealed, insulated ducts are the solution. With an insulated duct the duct wall temperatures rise quickly to the dew point of the exhaust air, condensing only a little in the beginning minutes of flow, but then re-evaporating the initial condensate as more air moves through, bringing the duct wall temperature above the dew point of the air. Without the insulation there will be times when the attic is cold enough that the duct walls can never rise above the dew point temperature. Sealing the ducts well is important, since the escaping air will leave it's moisture in the attic or attic-insulation, creating mold and/or ceiling dripping problems.
 

Dana

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Seal the ducts with duct mastic before insulating, or you'll end up with sopping wet insulation at every leak point. Even if a seam or joint looks tight, give it the mastic treatment. Hit every slip joint of adjustable ells, and don't ignore the lengthwise seams of straight pipe either.

If using tape to seal the connections to the fan unit, use a temperature rated high-quality aluminum tape (eg Nashua 324a, found at most box stores), and burnish it in- no bubbles under the tape.

hvac-cleaners-sealers.jpg


Nashua-324A.jpg
 
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Leo

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I assume that by "ventilation fan", you really mean an exhaust vent, not a supply of outdoor air to the indoors?

You're getting condensation because you have humid air going into an un-insulated metal pipe located in an unconditioned attic, where it's cold.

Dripping from a seam or joint in the duct means the duct isn't well sealed either. All joints and seams need to be sealed (with duct mastic, not tape.)

I'm not sure exactly what the duct configuration is. Are both fans feeding the same ducts, that wye or tee in together? If yes, are there at least butterfly backflow preventers on each fan? Or is the ILF250 in series, as a flow-booster for the other fan?

A 150cfm or 250cfm fan into a short section of 4" duct isn't a big problem, but the combined 400 cfm is never going be real. The total duct length (equivalent lengths of all ells & tees) as well as the diameter determines the total amount of back pressure, and the actual vs. rated cfm the fans are delivering. The cfm rating of the fans is rated at 0.10 water inches, but even if they're only hitting half their rated flow, that's not the reason for the condensation. The duct walls are still WAY below the dew point of the humid air that's flowing in them.

Sealed, insulated ducts are the solution. With an insulated duct the duct wall temperatures rise quickly to the dew point of the exhaust air, condensing only a little in the beginning minutes of flow, but then re-evaporating the initial condensate as more air moves through, bringing the duct wall temperature above the dew point of the air. Without the insulation there will be times when the attic is cold enough that the duct walls can never rise above the dew point temperature. Sealing the ducts well is important, since the escaping air will leave it's moisture in the attic or attic-insulation, creating mold and/or ceiling dripping problems.
I have water stains in my bathroom 1st floor ceiling, (ducting is in a heated floor) then 25’ horizontal run to hip roof outside. 110 cfm Broan new construction 2006. Bathroom use increased 500% by 2 kids moving in full time when stains began. They actually flow down interior wall and pool in crawlspace. Is this condensation? Other?
 
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