3" Rigid Duct - Thoughts?

Dryer venting

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3" Rigid Duct - Thoughts?
« on: January 09, 2017, 08:37:29 PM »
I've recently moved into a house built in 1978, and just had a 2016 Samsung electric dryer installed a few weeks ago.

Features include indicator sensors/lights alerting the user to both a clogged lint screen, as well as compromised ducting.

The 4" inlet on the wall was mounted near the floor, however the existing flexible ducting had to make 2 x 90 degree turns plus a 180.

Accordingly- the indicator light for "compromised ducting" triggered after installation, which ultimately led me to the purchase of a dryerbox.

However, when opening up the wall- I discovered the 4" inlet was actually a 4" to 3" adapter. The actual 3" rigid metal ducting runs a total of 12' straight up and exits the roof with no 90 degree bends. It has a newly installed weatherguard with no screen on the roof.

Since this point, I have cleaned the full length of the ducting using a Gardus Linteater and I have opened up the wall and cut the rigid duct to the point where I was ready to install the dryerbox- then use a new 4" to 3" adapter to connect my flex hose up to. BUT:

I've just performed a test before installing the dryer box using new semi-rigid flexible aluminum duct.
(I have a Gardus Lintgard on order.)
The "compromised ducting" alert has not triggered with at least one 45 minute test run of the dryer.

Rigid venting reads approximately 110 degrees Fahrenheit behind where the dryerbox would be installed.

So, assuming the monometer reading is within specs for the given dryer- any concerns to raise here? Safety? Code compliance?

(Note: rigid vent replacement/upgrade will not be anything approaching a trivial installation)
« Last Edit: January 09, 2017, 08:43:35 PM by Oldhousenewproblems »


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Re: 3" Rigid Duct - Thoughts?
« Reply #1 on: May 15, 2017, 08:38:01 AM »
Wow, super sorry I missed this post.  Great question and you likely have moved ahead with something, but here is my response. 

Two important factors: one, is that the building code clearly indicates that the in wall ducting has to be 4" ID galvanized metal pipe.  so, if you were to sell the house one day, and the buyers inspector was pretty sharp, then they may have you replace it then.  The other issue is that 4" pipe provides 12.5 sq inches of area, 3" pipe provides 7.  that is almost 50% reduction (huge). 

New dryers create a lot of air flow and are expecting a 4" conduit to the outside.  You are hindering the units efficiency and wasting energy.  I bet the LintGard is indicating 1.2 wci of pressure or more.  ideally your pressure should be around .4 wci, measured with a Magnehelic Gauge. 

One more point: if you have a warranty issue in the future and the reduced conduit was discovered, you may not be covered.  Hope all this helps.  Thanks for asking. 
Rick Harpenau
In-O-Vate Technologies, Inc
Jupiter, FL  33477