# Under-slab dryer vent



## kyhowey

Builder with a slab-on-grade floor.  Someone decided to run 4" PVC under the slab for the dryer vent.  HVAC inspector called me to see what I thought.  Code says it must be constructed with "rigid metal duct, having smooth interior with joints running in the direction of air flow."

What do you think of PVC since it's under a concrete slab?  As I type this, it looks like the HVAC inspector will let it go this time with a "don't do it again" warning.  He is going to require a window well on the exterior to maintain clearance from the finished grade.


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## Rider Rick

I don't see a problem with PVC what is the length?


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## Francis Vineyard

Why should it be okay just because in encased in concrete? It still has the potential to ignite. How do inspectors keep up with exceptions should more than one contractor that make the same mistakes over and over?

This is not only a violation of the code but the manufacturer's standards which are usually less restrictive than the code.

Apparently he's asking for forgiveness than permission.

Francis


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## Rider Rick

We use plastic from the dryer to exhaust vent.


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## Gregg Harris

kyhowey said:
			
		

> Builder with a slab-on-grade floor.  Someone decided to run 4" PVC under the slab for the dryer vent.  HVAC inspector called me to see what I thought.  Code says it must be constructed with "rigid metal duct, having smooth interior with joints running in the direction of air flow."  What do you think of PVC since it's under a concrete slab?  As I type this, it looks like the HVAC inspector will let it go this time with a "don't do it again" warning.  He is going to require a window well on the exterior to maintain clearance from the finished grade.


2009 IRC 1502.4.1 is very specific in stating the material requirement, but if you look at the intent for the requirement to prevent fire and build up of lint inside of the exhaust duck it makes sense. Now look at M1503.2 for Domestic kitchen Exhaust under exceptions for running schedule 40 PVC under concrete slab on grade.

I do not believe that this scenario was thought of in the code determination for material for dryers.

My contention is that if the instillation requirements in 1503.2 are followed I would approve it.


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## Francis Vineyard

Rider Rick, I believe you but that is not permission or an exception to allow the violation and risk life and property.

Gregg good point but one thing that immediately comes to mind is the kitchen exhaust is opened at both ends and cooking is usually attended to whereas the dryer connection is concealed and should blockage occur the potential of fire to spread is high with the dryer unattended.

Francis


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## codeworks

it's a wet location, just like an underground conduit for conductors. imo, i don't like it. who's going to maintain it? it will clog, due to lint and moisture, thats a given.what kind of measures are being taken to prevent this from backing up with water in the event of high water ?    screens are not permitted by section m1502.2, 2006 irc.  what is the maximum length? i'm a big fan of UP for vent's , just because they work best that way. just because you can, doesn't mean you should.  also, will the dryer manufacturer allow this in their application/installation instructions ? and, a dryer isn't a down draft range


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## Rider Rick

I would have a problem with a screen at the end of a dryer vent but not with PVC.

I allowed PVC for ducting for bathroom exhaust fans.


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## Gregg Harris

Rider Rick said:
			
		

> We use plastic from the dryer to exhaust vent.


 Rick this would be a direct code violation of IRC M1501.1IFGC M614.6

IMC 504.6.1


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## Yankee

Have him run the metal inside the PVC, then he's all set


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## fatboy

Metal duct required.


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## Gregg Harris

Francis technically they are both open to atmosphere on both ends.

Consider the exception to IFGC 408.4 excluding sediment traps on illuminating appliances, ranges, cloths dryers and outdoor grills because they are manually operated rather than automatic.

If a fire is going to start in a dryer it is going to be at a point of ignition not down stream in the exhaust. The reason for requiring the metal transition and metal duct is to prevent the fire from spreading. If the exhaust where to begin to fill with lint it will happen irregardless of which smooth ducting is used. If it where to begin to build up inside of the exhaust piping the internal limits on the dryer either gas or electric will trip.


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## Rider Rick

Greg,

Would you quote IRC M150.1 for me.

M150.1 says you can't use plastic venting from dryer to wall?


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## globe trekker

kyhowey,

A good meomory aid would be for the contractor to jackhammer up the

slab and replace the underground pvc piping and install above slab.

Also, the window wells typically get filled up with landscaping debris

over time, plus the well itself will function like a cystern during rain

/ snow events. What does Aaaaaahnold say, _" ..bad idea!"   _ 

_._


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## Gregg Harris

Rider Rick said:
			
		

> Greg,Would you quote IRC M150.1 for me.
> 
> M150.1 says you can't use plastic venting from dryer to wall?


Sorry I should have used a year before the section If you are looking at 2009 it would be M 1502.4.3 Transition Ducts the verbiage is the same

Transition ducts used to connect the dryer to exhaust shall be a single length that is listed and labeled in accordance with UL 2158 A. Transition ducts shall not be concealed within construction.


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## Rider Rick

Greg,

In the code is there anywhere it says you can't use plastic ducting from the dryer to the wall?


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## Gregg Harris

Rider Rick said:
			
		

> Greg,In the code is there anywhere it says you can't use pastic ducting from the dryer to the wall?


In each of the sections that I sited, IFGC verbiage is" cloths dryer transition ducts used to connect duct system SHALL be metal.

The others use listed and labeled UL 2158 A


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## north star

*= = =*

Yankee,

Can you please explain how a compliant metal duct can be installed

inside the pvc ducting!......I have never seen that done and want

to learn..........Thank you!

*= = =*


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## Rider Rick

Greg,

So I understand you correctly in the International Fire Code a plastic flex dryer duct that is sold at Home Depot to hook up to your dryer is not to code?


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## Gregg Harris

Rider Rick said:
			
		

> Greg,So I understand you correctly in the International Fire Code a plastic flex dryer duct that is sold at Home Depot to hook up to your dryer is not to code?


Rick I did not quote the International Fire Code. They were IMC= Mechanical, IRC = Residential, and IFG= Fuel Gas.

Now to answer your question more than likely the white flex that you have found at HD does not have a UL 2158A /label/listing. 430 degrees F would be one of the factors and I would bet that if you put the vinyl tubing in an oven I doubt that it would sustain 200 degrees aside from being flamable


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## Rider Rick

Well Thank You Greg,

I had no idea that the dryer vent plastic/vinyl flex duct are not to code and were a fire danger.


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## Francis Vineyard

Gregg I agree with you, but dryer fires continue as late of 2006 statistics; albiet they are probably cause by older dryers.  Wonder if ductless dryers will fare better.












http://www.iccsafe.org/cs/codes/documents/2009-10cycle/proposedchanges/imc.pdf

http://www.iccsafe.org/cs/codes/documents/2009-10cycle/roh/imc.pdf

Dryer testing for the die hards: http://www.cpsc.gov/library/foia/foia03/os/dryer.pdf



Francis


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## Francis Vineyard

Rider Rick said:
			
		

> Greg,So I understand you correctly in the International Fire Code a plastic flex dryer duct that is sold at Home Depot to hook up to your dryer is not to code?


RR I presume what you're seeing is the "flexible air connector" that is advertised as fire resistant UL 181 on the same shelf next to the dryer transition ducts. This is a common mistake made with air ducts and dryer ducts and I do wonder sometimes if it's done for the vender's convenience or a product marketing strategy to group the flexible ducts together in one place.

Francis


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## Rider Rick

Francis,

I'm going to stop by Home Depot and take a better look but I didn't know there were two difference ones.

I'm sure the air ducts are less money and I'll bet some people are hooking them to there dryers without knowing.

Rick


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## Gregg Harris

Thank you Francis

I guess I was not the only one with the idea.    Although I disagree with the action
statements for denial Ad I will run tests on both gas and electric. There is always 2015


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## Rider Rick

I went to Home Depot and you can't buy any plastic/vinyl flex ducting the dryer vent flex duct is rated at 450 degrees but the end of the line hood on the out side of the wall does have a screen.


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## Francis Vineyard

How would you know the difference between flex dryer and air connector installed on the job?
















Francis


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## Gregg Harris

Francis Vineyard said:
			
		

> How would you know the difference between flex dryer and air connector installed on the job?
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Instead of having the UL 181 label it should have UL 2158 A label on the package. I am going to check a couple of suppliers in the next couple of days. The one I am using on my dryer does not have labeling on the ducting itself. I am trying to find someone with a gas dryer to run temperature tests on to compare them with my electric.

Home Depot has on there web that the ones they supply are rated for electric but not gas. I am contacting MFG's to see if I can get more info.


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## Doug Ables

Rider Rick said:


> I don't see a problem with PVC what is the length?


Codes state metal. As a dryer vent cleaner in the slab PVC vents are a terror to clean. The PVC creates static electricity as air moves over it, making lint stick to it more readily thereby creating a hazardous situation quickly.  We rarely clean a slab vent that we do not get 5-10 gallons or more of lint out of it. 
Also the vents, due to their configuration create a trough for water to collect in. A normal load of clothes drying creates over  a gallon of water in condensate. That water must be expelled from the vent. A slab vent will collect it until it has to be suctioned out. I have removed 17 gallons of water in one vent after a couple years use. The vents ruin the equipment that is made to clean dryer vents due to the 4-90 degree turns. Builders who install these useless vents should be made to clean them and see how much of a pain they are to us and the homeowner when we tell them they need to be rerouted. Also your comment about why are the plastic ducts sold at home depot if they are not meant to be used as a dryer vent. The answer is this....plastic vents are ok for air vents such as bathroom exhaust fans but not dryer vents. The only reason they are in the dryer vent area is the workers at HD and Lowes do not know any better. They are the same size and vents-so why not? When you add heat to plastic it becomes brittle and develops a hole. I have a picture of such an occurrence but don't know how to post it. But common sense tells me heat and thin plastic do not go together.


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## Paul Sweet

Rider Rick said:


> So I understand you correctly in the International Fire Code a plastic flex dryer duct that is sold at Home Depot to hook up to your dryer is not to code?



Big boxes sell lots of stuff that doesn't meet code.


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## Francis Vineyard

Mr. Doug Ables, welcome to the forum and thanks for the additional information and sharing your experience.


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## north star

*$ : $ : $*


Doug Ables,

Also, ...Welcome to The Building Codes Forum !

To be able to post pictures \ images on to this Forum,
one would need to become a Paid Subscription
Sawhorse !........I am encouraging you and others
to join and become Sawhorses.

FWIW, ...the term Sawhorse was selected by the
Forum owner and accepted by the membership.....It
is a definition that; IMO, ...seems appropriate and fits
very well.   

*$ : $ : $*


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## fatboy

Welcome to the forum, and way to come out swinging with a great post!


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