A dryer vent should extend no more than ___ feet.
Generally speaking, but there are always exceptions, for longer and shorter runs.
irc and imc have a 10’ difference
bottom of this page
25 based on IRC. Reduce 5ft for each 90 and 2 1/2ft for each 45 bend.
too bad 35’ is not a choice smarty pants…
Hey Barry. 
Follow the manfacture specifications …
Sometimes it is.
M1501.3 IRC
Where the make and model of the clothes dryer to be installed is known and the manufacturer’s installation are provided, the maximum length of the exhaust duct, shall be in accordance with the dryer manufacrturer instructions.
More and more newer dryers are capable of more than the 25’.
And here is a dryer box Barry is talking about.
I believe Barry was referring to this at the bottom of the page. :D:mrgreen:
I know, I read it.
I was showing what a dryer box was and why at occasions, the dryer vent can exceed 25 ft.
Manufactures instructions dictate that.
Now for a scenario, Maine has mostly cellars, or basements, and that is where you have the dryers most of the time.
Now using the 25 foot rule, that means if the dryer is not installed against the exterior foundation wall at all times with its back to, it will not meet that requirement.
25’ is the rule of thumb or code per IRC, so now deduct two 90’s, that is 10’, and deduct another 90, cause the flex is crumbled and pinched in the back of the dryer hook-up, and that is another 5’.
So you are left with a 10’ straight section of duct that you can use.
I will tell you right now that over 75% of dryers in this area do not meet that criteria.
The dryer box saves on the lineal footage and eliminates pinching.
Manufacturer’s instructions will prevail on the installation and I am careful on calling out this as a defect, since we can not site Code or should not. 
Check this out here, might think different about the IRC requirement part that we are limited to 25’.
http://www.appliance411.com/faq/dryer-vent-length.shtml#chart:)
IRC Code says says 25’ or Manufactures Instructions, so calling out Manufacutrers Instructions is Citing Code in a round about way.
M1502.4.4 Duct length. The maximum allowable exhaust
duct length shall be determined by one of the methods
specified in Section M1502.4.4.1 or M1502.4.4.2.
Never new that installing something per Manufacturers installation manual was a round about Code.
That is a new one on me Brian.
Good discussion, gets me thinking…I do not have an 09 IRC copy yet myself and will have to get one.
I do not recall seeing this requirement before either…
It is great to see so much discussion over what I thought was such a simple question.
Thanks to all of you that participated.
It was a great question Greg, thank you for posting it.
That’s how inventions are made Greg, talking about it. :mrgreen:
Big difference on slab living and Basements. Utilities all end up in a basement, and conditions are not allways conducive to the IRC, so other means of adaption are necessary like design around Manufactures specifications and sometimes adding inline fans made for dryer exhaust.
The last condo I inspected the exhaust duct was concealed and the run was about 40’ and was disconnected in the wall behind the dryer.
The client was the Assistant Chief of the Fire Department for my Town. :mrgreen:
All he said was “Wow”.
On a recent commercial job I saw a 50 foot dryer vent approved on the plans with a certain residential dryer listed. I don’t remember the manuf. but it was a residential unit.
I just want to point out that even though there are other options you have to choose what would be the correct answer from the ones available. Many tests given for certification have similar questions that have several answers that are not listed on the test.
I am a licensed contractor in the state of California. There is the IRC and the IMC. However we are sup post to go by cities codes and the city where i am from goes by CMC. Which is a different company. I have checked other cities and they go by the same. Which is 14 feet Not counting two elbows.Only two elbows are allowed. However when i spoke to the city inspector.He said there is exceptions. The inspector will not pass anything different. Unless You have an engineer sign’s off on it. If the engineer signs saying it will work. The inspectors are forced to sign it two. The problem with this is it really does not work for most dryers. Some dryers have really strong air flow and some do not. If you install a venting system for a Maytag (just examples).But that person sells the house and someone uses a roper(just example).That is where there becomes a problem. Where the maintenance costs are higher to maintain that system. Are you have to do a reroute. I find some systems that are signed off.That uses 6 inch and goes 35 to 40 feet. And no dryer will work very well even brand new. I have customers tell me they bought the house brand new. And it took and hour to dry the cloths. That is just wrong. They leave the customer stuck with a problem on a new house. Something needs to be done to stop this from continuing.Thank you and i hope this help.If you have any questions just let me know. I would be glad to help. I work with inspectors in my area to help.