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2 Part Question: Clothes Dryer Vent & Floor Joists

2 Part Question: Clothes Dryer Vent & Floor Joists


Old 11-08-04, 05:08 AM
Big Buddy
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Clothes Dryer Vent

I recently moved into a townhouse condominium. When it was built the clothes dryer was installed on the 1st floor, where it still is. The venting for the dryer goes through the floor into the basement where it has to travel about 40 feet before it gets vented to the outside. I cannot change the located of where the duct gets vented to the outside. A previous owned of the townhouse had installed a blower about half-way down the venting to help pull the air from the dryer to be vented to the outside. A dropped ceiling has also been installed in the basement by a previous owner. There is 2-3 inches between the dropped ceiling in the basement and the joists for the 1st floor. The clothes dryer vent now travels through the floor on the first into the basement and travels along between the suspended ceiling in hte basement and the floor joists. There is one point along its travels where the vent becomes compressed because it has to travel between the suspended ceiling and the floor joists. I would like to relieve that compression. Can I either cut a notch on the bottom side of the floor joist to relieve the constriction or would it be better to cut a hole through the floor joist to let the vent travel through that?
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Old 11-08-04, 05:49 AM
Sharp Advice's Avatar
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Location: The Shake and Bake State USA
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Hello: Big Buddy. Welcome to the Electric Appliances forum.

Your question contains two seperate parts. One dealing with the vent length and the other dealing with *flooring joists. Both valid questions. However, the flooring joists portion is not an appliance service area.

I'll provide you with some help on the ducting of the venting system, than move the question into another, more appropriate forum topic.

The venting is much to long. See chart below.

Added in a power assisted venting fan may provide some help and than again may be counter productive. The cfm of the assist fan has to equal that of the dryers internal fan or one fan will overpower the other. Which than effects normal dryer operation.

The questions now are:
Is dryer operation effected?
If so, specifically describe how?

If dryer operation is effected by long dry times or incompleted drying, disconnect the machine from the venting system, at the back of the dryer.

Test by drying a normal load and note any differences. If drying times improve or are faster and or completed in one cycle, the vent is either and or restricted, too long and or the outside flapper door inside the exhaust is not working properly.

If no difference is noted in drying times, or no problems exist with the machine or dry times and or final results upon complishion of the cycle, etc, correct only the restriction at the joist.

With the excessively long run of the venting, cleaning of the ducting should be done yearly, if not more so. Or based upon needed. Alos check the assist fan. May or is likely to become linted up and effect the air flow and or ducting system, etc.

Maximum Exhaust Vent Lengths:
4 Inch Rigid Metal Ducting with 4 Inch Vent Hood.
#of elbows:
0 elbows...44 feet
1 elbow....34 feet
2 elbows..26 feet
3 elbows..20 feet

Above With 2 Inch Vent Hood:
0 elbows...34 feet
1 elbow....26 feet
2 elbows..20 feet
3 elbows..14 feet

Maximum Exhaust Vent Lengths:
4 Inch Flexible Metal Ducting with 4 Inch Vent Hood.
0 elbows...24 feet
1 elbow....20 feet
2 elbows..16 feet
3 elbows..12 feet

Above With 2 Inch Vent Hood:
0 elbows...20 feet
1 elbow....16 feet
2 elbows..12 feet
3 elbows....8 feet

*The professionals in this forum topic will address the portion of the question pertaining to the flooring joist(s).

Use the reply button to add additional information or ask additional questions. Doing so will automatically move your question to the top of the forums list of questions.

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Old 11-09-04, 12:13 PM
Banned. Rule And/Or Policy Violation
Join Date: Jul 2004
Posts: 463
Sharp Advice's comments are correct insofar as length, direction, and exhaust vents go.

Under no cirumstance should joists be cut or bored or notched to accomodate a dryer vent. To do so would be a building code violation in most instances.

Sounds as if your previous owner did what he could and what was within 'code' to make the overly long exhaust vent work.

A 'power vent' is needed since codes limit dryer vent lengths to 25' and less depending upon elbows and bends.
Old 11-14-04, 08:28 AM
Join Date: Mar 2003
Posts: 109
Since it sounds like the ducting is accessable, you can also change it from the flex hose to the smooth wall ductwork. The smooth wall comes in different sizes so you should be able to piece together the sections. Just use duct tape at each joint.

The advantage here is since its smooth wall, it has much less drag on the airflow and is also much less prone to lint build up. It also can compress easier than the corregated stuff to get past the joist.
Old 11-14-04, 08:52 AM
Doug Aleshire's Avatar
Join Date: Feb 2002
Location: United States
Posts: 4,680

If I am not mistaken, isn't that what Sharp Advice just mentioned? He gave Rigid and Flexible data.

Big Buddy,

Homebild and I agree that there are limits to notching or drilling into floor joists. This can cause structural deficiencies. I try avoid any notching or holes if at all possible. Sometimes you just have no choice but should ensure that you don't destroy the total strength intended of what you are altering.

R502.8.1 - Sawn Lumber

Notches in solid lumber joists, rafters and BEAMS shall not exceed one-sixth of the depth of the member, shall not be longer than one-third the depth of the member and shall not be located in the middle one-third of the span. Notches at the end shall not exceed one-fourth the depth of the member. The tension side of members 4 inches or greater in nominal thickness shall not be notched except at the ends of the members.

R502.8.2 - Engineered wood products

Cut's, notches and holes bored in trusses, laminated veneered lumber, glued laminated members or I joists are not permitted unless the effect of such penetrations are specifically considered in the design of the member.

Just some added thoughts

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