International Residential Code (IRC) says the maximum exhaust duct length is 35 feet. However, it also says that this length can be overruled by the manufacturers instructions, if the instructions are provided to the inspector.
2012 International Residential Code (IRC)
Chapter 15 Exhaust Systems
Section 1502 Clothes Dryer Exhaust
M1502.4.4.1 Specified length. The maximum length of the exhaust duct shall be 35 feet (10 668 mm) from the connection to the transition duct from the dryer to the outlet terminal. Where fittings are used, the maximum length of the exhaust duct shall be reduced in accordance with Table M1502.4.4.1. The maximum length of the exhaust duct does not include the transition duct.
![Table M1502.4.4.1](https://i.stack.imgur.com/kADRS.png)
M1502.4.4.2 Manufacturer’s instructions. The size and maximum length of the exhaust duct shall be determined by the dryer manufacturer’s installation instructions. The code official shall be provided with a copy of the installation instructions for the make and model of the dryer at the concealment inspection. In the absence of fitting equivalent length calculations from the clothes dryer manufacturer, Table M1502.4.4.1 shall be used.
I couldn't find anything that says the termination has to be a specific height above the ground, just that it must be 3 feet in any direction from any openings into the building. And that it should have a backdraft damper, and no screens
2012 International Residential Code (IRC)
Chapter 15 Exhaust Systems
Section 1502 Clothes Dryer Exhaust
M1502.3 Duct termination. Exhaust ducts shall terminate on the outside of the building. Exhaust duct terminations shall be in accordance with the dryer manufacturer’s installation instructions. If the manufacturer’s instructions do not specify a termination location, the exhaust duct shall terminate not less than 3 feet (914 mm) in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.
The code also says that the ducts must be 4" smooth walled ducts, and sections can't be attached using fasteners that protrude into the duct more than 1/8".
2012 International Residential Code (IRC)
Chapter 15 Exhaust Systems
Section 1502 Clothes Dryer Exhaust
M1502.4.1 Material and size. Exhaust ducts shall have a smooth interior finish and be constructed of metal having a minimum thickness of 0.0157 inches (0.3950 mm) (No. 28 gage). The duct shall be 4 inches (102 mm) nominal in diameter.
M1502.4.2 Duct installation. Exhaust ducts shall be supported at intervals not to exceed 12 feet (3658 mm) and shall be secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Exhaust duct joints shall be sealed in accordance with Section M1601.4.1 and shall be mechanically fastened. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch (3.2 mm) into the inside of the duct.
NOTE: Always check with your local code enforcement agency before begining any project, as codes vary from location to location.
So here I go Answering my own question. I was really hoping to get some objective feedback.
I chose to go out the gable end. Ultimately, of all of the choices, I went with what would be the straightest possible run, even though it added some length.
Basically, here's the thing, for any of us doing a dryer vent run, and we are NOT exceeding the manufacturers length for run, we aren't breaking any code, and while time will tell whether the efficiency is better or worse, I truly suspect the added bend (even just the one if I went soffit) would have been "5 extra feet" and physically forces the air to move directions.
The Gable run basically was 4x5' in the ceiling of the attic and a downward slope with taped joints, so there is one 90 off the highest end which goes DIRECTLY down.
I moved the pipe so it dropped directly into the mud room and didn't touch the garage space at all, thus, the only other 90 is the one that comes off the back of the dryer.
I suspect this will be the easiest run to clean. It only penetrates the ceiling. Other than moving the first column of air (straight up 12') to the 90, it then slants downward at a little more than 1/4" per foot, right to the gable. I used aluminum hobby wire and hung it from the rafters (existing 2x4's) across them gave me a nice spot to work with. I just need to insulate it next.
Indeed the previous flex tubing was clogged, a fire waiting to happen. Makes me wonder where the moisture was going. No water in it, but it sure was full of lint.
Now when it snows I'm confident snow will not block the duct exit on the gable, any condensate that becomes water will still make it outside thank you gravity, and it will be easy to clean out.
next I'll probably buy on of those sensors just to be a perfect reminder of when to clean it out. (time to do some research).
Best Answer
I've had good luck with the brushes that come with the Lint Eater type products you mentioned... but tied on to a piece of rope.
First step is to get the rope through the vent. I hook up a shop vac to one end and feed the rope in from the other. The shop vac will suck the rope through the vent. Sometimes tying a cloth rag to the end of the rope increases the force of the suction on the rope. When the rope arrives at the other end, tie the Lint Eater brush to the rope.
Now comes the fun part: pull the rope back out the other way, dragging the brush behind it.
Additionally, you can tie a rope to the other end of the brush before you pull it back. This lets you pull the brush back and forth a couple of times without having to repeat the shop vac trick.