Dryer venting through roof


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Old 11-24-22, 06:10 AM
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Dryer venting through roof

I am replacing a busted elbow at the roof vent for a dryer. Looks like there are two options - 1) I can put a new elbow at the roof so the riser duct remains plumb, 2) or, attach the riser duct directly to the roof vent at an angle (I think the roof pitch is between 30-45 degrees) and adjust the elbow at the bottom of the riser. The second option will eliminate one elbow and make the bottom elbow angle more than 90 (better for flow). I am also thinking of securing that riser with metal pipe strap. Riser is about 6-1/2 ft long.

Are there any disadvantages/cons to having long angled riser ducts vs. vertical? (like more lint sticking inside angled vs vertical ducts)
 
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Old 11-24-22, 06:32 AM
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The item that needs to be considered is flow restriction which increases with the addition of bends and lengths of pipe so technically the elimination of one elbow will improve (reduce restriction) air flow.

But, horizontal runs, with slight pitch to the exit are more desirable, it allows any condensate to flow away vs a vertical application where you know the condensate will end up.

Either way be sure to insulate the run to reduce condensation
 
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Old 11-24-22, 06:39 AM
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It is rarely advisable to vent a dryer vertically through a roof. Gravity causes lint to fall back down upon itself, creating a fire hazard.

Another incorrect method is when a dryer vent inside an attic is directed "at" or "toward" a roof vent that is only designed for ventilation. (Rather than using a louver designed for a dryer, such as a dryer jack) The dryer vent then improperly terminates inside the roof without reaching the exterior of the roof, which does not meet code. Dryer lint accumulates inside the roof and/or can clog the end of the duct, and it cannot be cleaned and maintained from the exterior side of the roof.

It is better if vertical runs are limited in height, (such as until they just reach through the top plate and into the attic which keeps them as low as possible, where it is insulated) then they should run horizontally to their termination outside the building. However you also do not want to exceed the maximum distance for your duct, so each location should be planned to connect 2 points that are the shortest distance apart.

An angled duct would only be used in favor of a horizontal duct when the maximum total distance of the duct would be exceeded. 45 degree elbows would generally be favored over 90s where possible.
 
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Old 11-24-22, 08:23 AM
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Thank you Marq1 and XSleeper. Yes, I do understand the problems with vertical venting. This is how the dryer vents were installed in these town homes many years ago. We have regular vent cleaning done. It sounded like if there is no other option but roof venting, the angle of the riser duct itself doesn't matter, right?

I have another question related to top plate connection. The duct from the dryer terminates about 1" above the top plate as shown in the picture. I removed the original elbow that was busted also. The joint is about 1" inside the top plate and there is no space to reach around the joint to wrap aluminum foil tape. I thought of applying a bead of mastic on the inside of the new elbow and push it down the crimp. Does this work to seal the joint? Are there other solutions? Thanks again.


 
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Old 11-24-22, 09:05 AM
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Pretty hard to give specifics sight unseen. Your best bet is to adhere exactly to the published mechanical code. See section 504 and your dryer mfg instruction manual.

https://codes.iccsafe.org/content/IM...xhaust-systems

Sounds like mastic is all you can do on a concealed seam like that.
 
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Old 11-24-22, 12:05 PM
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Would be easier to add an extension to that pipe, approx 6"-1' just to get the joint higher to work with.

Looks high enough where small/short self taping screws could be inserted then taped so you would not have to deal with that again.
 
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Old 11-24-22, 12:25 PM
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504.8.2: screws or similar fasteners that protrude more than 1/8" into the duct shall not be used.

Don't think they make "short self tapping screws" that would meet that requirement.
 
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Old 11-25-22, 06:13 AM
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Kind of was expecting that but in the real world when people have a 1/2", or more, of lint lining the duct and a tiny pinch of lint on the tip of that screw I'd say there is not much of an issue. Now for those who use inch long screws, I would not recommend that!
 
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Old 11-25-22, 07:03 AM
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Advice given here needs to conform to building codes, so yeah, you should have expected it.
 
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Old 11-25-22, 08:21 AM
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Any recommendations on mastic brands to use on the dryer ducting joints and seams?

The code says to use UL181B-M rated mastic. I asked two big box brands that sell UL181B-M rated duct sealant. One (Red Devil) said their sealant can;t be used on dryer venting ductwork. The other (Masterflow) said the product cannot be used if temps are more than 180F. I am still waiting to hear from others such as Ductmate, Designer Polymerics etc. But if anyone successfully used a specific mastic on dryer ducts it would help to know.
 
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Old Yesterday, 07:24 AM
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Drain hole for vertical venting?

I have been reading about condensation inside the dryer vent pipe running through an attic. I have a vertical pipe that's inside an attic. I thought about drilling a small hole in the elbow at the bottom of the vertical pipe, as shown int he picture). Ir would be taped up with aluminum foil tape, but opened periodically to drain any condensate sitting there. Will there be any issues with this approach?



 
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Old Yesterday, 07:33 AM
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A hole is only going to let part of the condensation out, the rest will keep running downward. I typically see a tee used instead of a 90, where the bottom of the tee has a cap on the bottom of it as a cleanout.

Condensation is the other reason why horizontal runs are preferred wherever possible. Your duct should be insulated in pipe wrap all the way to the louver.
 
 

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