Best way to vent a clothes dryer through an attic?

April 2, 2008 10:10 AM

What is the best way to vent a clothes dryer through an attic? Is Thermoflex unsafe?

I know venting a dryer through your roof isn't ideal, but that's how our 2-year-old house was built.

The previous owner used a Thermoflex tube to connect a metal pipe from the dryer to the plastic roof vent. The tube (photo) has fallen off the roof connection, so the dryer is now venting into our attic.

Thermoflex is flexible, insulated and not metal as far as I can tell. The Thermoflex web site says they don't sell any products suitable for dryers, so I'm wondering if I shouldn't just remove the Thermoflex altogether.

If so, what's the best replacement?

If the tube is okay, how do I keep it attached to the roof vent? As you can see in the photo, duct tape and dryer tape are not working.

Skill level: I am an uninformed klutz when it comes to home repairs.

posted by Yogurt to home & garden (13 comments total)

Small correction: it's Thermaflex with an "A."
posted by Yogurt at 10:11 AM on April 2

I would keep what you have and get a large "hose clamp" to cinch around the flexible duct where it attaches to the vent.

Use foil duct tape around the joint and the clamp after you have secured it, making sure the insulation is pulled up as close to the roof as it can go.

The insulation is there to prevent condensation from occuring - if it wasn't there, the warm dryer air would meet the cold pipe in the attic and form moisture, which would then run back down the pipe and into your dryer.
posted by davey_darling at 10:20 AM on April 2

Replace the Thermo flex with aluminum dryer vent pipe.
One of the biggest causes of vent hose fires is when this accumulated lint inside the vent hose ignites. Lint gets caught in the folds and creases and sticks there because of the humidity. Over time, the lint builds up to such a degree that the dryer cannot exhaust properly. This results in increased drying times initially and, ultimately, in a fire. Once a fire starts in a vinyl vent hose, the hose itself ignites and burns vigorously creating a fire that is very difficult to extinguish.
posted by hortense at 10:23 AM on April 2

We had a thread on this not long ago.

The answer is that you should install a rigid sheet metal duct, round. You can buy this in any decent hardware store along with elbows adjustable to any angle up to 90 degrees. Duct tape will hold the pieces together and will attach the duct to the roof vent. For insurance, you can connect sections with a sheet metal screw. Technically, this could snag lint and cause clogging, but it's not likely. You'll find a metal duct will make your dryer run more efficiently than anything else. You really don't need insulated material, since you're just venting the heat outside anyway. Thermoflex probably says it's not suitable for dryers because of the temperature of dryer exhaust -- there just too much heat buildup in there to be safe.
posted by beagle at 10:26 AM on April 2

Sorry I missed the earlier thread. I did search for "dryer".

The Thermaflex isn't vinyl -- it's some sort of unfamiliar smooth fabric inside, but the fire risk is what I was worried about. For what it's worth, there's no sign of lint buildup right now, and we've used it for six months. Not sure how long the previous owners used it.

So I seem to have two worries here -- the condensation and fire risk. Would it make sense to put the insulation I have around a new rigid duct (assuming it fits) or otherwise wrap the new duct? Beagle suggests it's not necessary, and the attic has been quite warm every time I've been there, but I did see another site recommending insulation.
posted by Yogurt at 10:46 AM on April 2

Even though it's the way your house was done, your dryer installation might not be to code, which requires that the outlet duct for the dryer have a run of no longer than 14 linear feet with no more than two 90 degree bends. At least that's what I remember from plancheck corrections (in California, which may have a different code than yours).
posted by LionIndex at 10:55 AM on April 2

LionIndex is right. Check your local code and do whatever it says. It will probably be overkill, but doing anything else risks insurance not paying out in the event of a fire. I don't know how it works in Canada, but in Texas that kind of stuff is generally covered in the city plumbing or mechanical code.
posted by Uncle Jimmy at 11:04 AM on April 2

Proper dryer venting
posted by caddis at 11:07 AM on April 2

Use a metal rigid duct if at all possible, flexible metal duct if not. Only use the flex where absolutely nessecary, connect it to rigid where straight runs are possible. Rigid ducting provides a cleaner air path for the exhaust (which allows your dryer to work faster) and it reduces the lint load and fire hazard over flexible ducting.

For sure do not use any flexible product that isn't specifically rated for dryer use. If at all possible use a pair of 45s instead of a single 90 when making bends and keep the total amount of bend to a minimum. Also you should verify the roof vent isn't blocked.

Yogurt writes "Would it make sense to put the insulation I have around a new rigid duct (assuming it fits) or otherwise wrap the new duct?"

You need to insulate the metal duct in a heating climate, the current black insulating sleeve will work fine if it's in good shape. The metal duct should be sealed with tape at every joint. I even run a strip up the linear joint where the round duct connects to itself. Do not use cloth duct tape for this purpose. The correct tape for this job is a self adhesive aluminum tape. You can buy it at most home improvement Borgs. It is expensive compared to cloth tapes.

PS: you want to get this fixed ASAP. The lint that makes it past the dryers lint screen will plug up the soffit and gable/ridge vents in your attic. If it's been like this a while you should check those vents to make sure they aren't plugged.
posted by Mitheral at 11:18 AM on April 2

Thanks everyone. I'll replace the flexible tube with a rigid metal duct and insulate the duct.
posted by Yogurt at 11:29 AM on April 2

(I'll check the B.C. building code too, but it's not freely available online, so that means a trip to the library.)
posted by Yogurt at 11:37 AM on April 2

(I'll check the B.C. building code too, but it's not freely available online, so that means a trip to the library.)
Your local permitting agency (town/city/county) almost certainly has a phone number for homeowners to ask code questions of officials -- it is in their best interest for things to be built correctly the first time, and they should be happy to give you the info you need.
posted by misterbrandt at 6:25 PM on April 2

While I don't think that the existing setup would cause a problem, I retract my earlier advice in favour of the (better) plan from Mitheral.
posted by davey_darling at 10:14 AM on April 7