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Unread 07-27-2008, 12:20 PM
 
Location: DC Area, for now
3,517 posts, read 7,971,272 times
Reputation: 1969
Question Dryer Vent Condensation Prevention

I have a 2nd floor dryer located in the center of the house. When I bought the place, I found the dryer vent was propped up near the ridge vent and of course there was lint all over the attic.

I was loathe to punch a hole in the roof so I ran the vent across half the attic and vented out the soffit. I wrapped the whole vent in reflective insulation that was promoted as being good for that purpose.

I just got a new roof put on and had the roofer put in a dryer vent. In my activities of connecting the dryer vent to the new vent opening, I missed the joist and put my foot next to the point where the duct comes up thru the ceiling and came right thru. Many curses later, I saw why - it was crumbling with mold disintegrating the drywall, so at least I didn't destroy good drywall and it needs replacing anyway. Obviously I have had some serious condensation from the dryer duct. I know the duct was long and I know the soffit is not a good place to vent - it's why I wanted the vent put in the roof.

So, my question is, how do I make sure the redone duct won't continue to condense water and drip on the ceiling? From what I can find, insulating the duct is supposed to prevent this, yet the insulation obviously did not.

The new duct now does a ~45 degree bend to the new vent instead of the 90 degree turn across the attic.

Do I just need more (thicker) insulation?
Was the relflactive stuff the wrong thing and made it worse?
Any sure fire ideas for what I should do?
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Unread 07-27-2008, 07:07 PM
 
Location: Knoxville
3,182 posts, read 8,962,018 times
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First of all, you want the vent run to be the shortest you can. Every 90 degree turn adds five effective feet to the length of the run. So if the duct starts with a 90 at the wall, goes up 8 feet, has a 90 and goes 10 feet and another 90, you have an effective length of 33 feet. Depending on your dryer, this may be too long of a run.

Second, you want to make sure the duct material is metal ducting and not flexible plastic crap (or even flexible foil looking stuff) that will trap lint.

Going straight up is probably the shortest route. You just may have to clean it once in a while.

You can always add a booster fan if the run is too long, or the clothes don't seem to dry well.

To the insulation....you really only have to worry about this in the cool months, but the thicker and tighter the insulation the better.

Also, do not use screws to put the sections of duct together, they will catch lint on the inside and create problems.

Good luck.
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Unread 07-27-2008, 08:31 PM
 
Location: DC Area, for now
3,517 posts, read 7,971,272 times
Reputation: 1969
Thanks. Straight up wouldn't work as that would put the vent exactly at the peak of the roof. The new run goes up about 10 ft then a 45 degree turn for another 5 feet. No screws, just the aluminumized duct tape to seal and hold it and those screw clamps. i used all smooth duct too - no accordion pipe.

Any advice on the kind of insulation?
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Unread 07-27-2008, 08:44 PM
 
Location: Knoxville
3,182 posts, read 8,962,018 times
Reputation: 3075
A 45 degree angle really doesn't add effective length to the run.
To be honest, I have never seen a dryer vent insulted in any home I have ever inspected. I guess it "could" be an issue in the winter when the warm duct would be in a very cold attic, but in the summer, it's kind of a non issue, the attic will likely be warmer than the duct.

I guess if I was going to insulate the duct, I would use a flexible HVAC insulated duct material and tape it in place. If it was running horizontal in the attic, you could just used un-faced batt insulation and cover it, or get a couple bags of blown material to cover the duct if the insulation is the blown in type.
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Unread 07-27-2008, 11:18 PM
 
26,731 posts, read 17,890,658 times
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I think 25 TEL feet is the maximum allowed.

NM re read where your venting it through the roof.
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Unread 07-28-2008, 05:31 AM
 
Location: Knoxville
3,182 posts, read 8,962,018 times
Reputation: 3075
Default correction

Sorry, I made an error with the 45 degree elbow, it DOES add 2.5 feet to the effective length. There is an special elbow made that adds nothing to the length.

While the code calls out 25 feet, there is an exception that defers to the dryer manf specs. Many dryers have an effective length much longer than 25 feet - there is one that has a maximum of 90 feet. It must feel light a jet engine blast.
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Unread 01-25-2011, 09:05 AM
 
2 posts, read 24,691 times
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I have the same problem long dryer vent in attic - Hate to make new hole in roof. I cannot get the connections in the smooth duct (18ft) to be dry. Now I'm getting lots of moisture at the exit of house. Really hard to get the moisture to get out of the house. Im in CT we have 1+ foot of sno on roof and icycles off frozen gutters. In an attempt to cool off attic I want to insulate dryer vent but I cannot get it to stop leakin. Each connection has the metal duct tape. Its just a tough situation - any suggestions would be appreciated!!
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Unread 01-25-2011, 09:39 AM
 
Location: NE CT
1,496 posts, read 1,293,398 times
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Veting dryers throuh soffets and roofs seems to me to be the problem.

First the dryer's hot air is being forced upwards instead downward into a cellar and along a parallel plane with the floor joists and out through the exterior wall.

I have a laundry center on the first level and vent the dryer through the floor and then it turns at a 15 degree angle as it stretches and runs about 16' below the floor joist to the exterior wall in the cellar. I vent it out next to a cellar window. Never have had a condensation problem and I live in CT as well.

If your dryer is on a second floor, then I would vent directly out the exterior wall, with a vent cover that has a flap that closes when the dryer is not in use. Just know where you are drillling with the four inch drill cutter before you do it so you don't drill throuh any pipes or wires.

It is absolutely prudent to cut the dry wall out in a 8 to 12 inch square hole so you can look and see whats behind the wall before you drill the exterior wall. You can always repair the drywall around the vent pipe. Also be certain to stay between your studs that are 16" on center from the corner of the area in which the dryer is located. I think this is the most simple fix. You can always cover the attic vent area from inside the attic.

If at all possible, when veting a dryer, run the 4" venting pipe straight out through the wall and exterior wall, or down and out of the building, not up and out. Dryers don't have the motor capacity to force the air up and out long distances. It is easier on the dryer to vent straight out or down and out. You end up with back pressure when you vent up and out and this is not a good situation for your dryer. Takes longer to dry your material and forces the dryer to work harder than if the venting is correct.

Good luck
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Unread 01-25-2011, 03:19 PM
 
Location: Knoxville
3,182 posts, read 8,962,018 times
Reputation: 3075
You can always add a dryer booster fan to the duct. They work very well for those long runs.
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Unread 02-11-2011, 08:17 AM
 
2 posts, read 24,691 times
Reputation: 10
Default dryer venting

My dryer is in the center of the house so there is no going out a wall on that level. My house is a ranch and the existing dryer vent is in attic. I have elevated it in the attic to the level of the "flap" exiting the house, about 18" off attic floor. I have straight pipe not "flexible dryer venting pipe". I have recently insulated the length of piping in the attic for 2 reasons. 1. to keep the steam hopefully steam. and 2. to keep attic cooler to hopefully not encourage ice dams. I haven't been up to the attic to check of course so these are hopes today. Im in central CT.
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