Dryer vent in crawl space

[Bill Taylor]

My dryer vent is into crawl space right now, but I am going to run it about 15 feet with two elbows and out through the foundation wall into a carport. what could I do to make sure it works well, and to contain lint in carport? Other option would be to go straight across the crawl space about 30 feet and out through the foundation wall there. Is distance a problem, as compared to the two ells going back to carport option. I may someday enclose carport with garage door as well.
:) I created this mess by adding a room and moving the laundry, so I'm the source of the problem.

[James Duffin]

Here is what the NC Code says...



SECTION 504
CLOTHES DRYER EXHAUST

504.1 Installation.

Clothes dryers shall be exhausted in accordance

with the manufacturer's instructions. Dryer exhaust systems
shall be independent of all other systems and shall convey
the moisture and any products of combustion to the outside of
the building.
Exception:


This section shall not apply to listed and labeled

condensing (ductless) clothes dryers.
504.2 Exhaust penetrations.


Ducts that exhaust clothes dryers

shall not penetrate or be located within any fireblocking,
draftstopping or any wall, floor/ceiling or other assembly
required by the


International Building Code to be fire-resistance

rated, unless such duct is constructed of galvanized steel or aluminum
of the thickness specified in Section 603.4 and the
fire-resistance rating is maintained in accordance with the


International

Building Code.


Fire dampers, combination fire/smoke

dampers and any similar devices that will obstruct the exhaust
flow, shall be prohibited in clothes dryer exhaust ducts.
504.3 Cleanout.


Each vertical riser shall be provided with a

means for cleanout.
504.4 Exhaust installation.


Dryer exhaust ducts for clothes

dryers shall terminate on the outside of the building and shall
be equipped with a backdraft damper. Screens shall not be
installed at the duct termination. Ducts shall not be connected
or installed with sheet metal screws or other fasteners that will
obstruct the exhaust flow. Clothes dryer exhaust ducts shall not
be connected to a vent connector, vent or chimney. Clothes
dryer exhaust ducts shall not extend into or through ducts or
plenums.
504.5 Makeup air.


Installations exhausting more than 200 cfm

(0.09 m


3/s) shall be provided with makeup air. Where a closet is

designed for the installation of a clothes dryer, an opening having
an area of not less than 100 square inches (0.0645 m


2) shall

be provided in the closet enclosure.
504.6 Domestic clothes dryer ducts.


Exhaust ducts for

domestic clothes dryers shall be constructed of metal and shall
have a smooth interior finish. With the exception of the transition
duct flexible ducts are prohibited The exhaust duct shall
be a minimum nominal size of 4 inches (102 mm) in diameter.
The entire exhaust system shall be supported and secured in
place and shall terminate not less than 12 inches (305 mm)
above finished grade. The male end of the duct at overlapped
ductjoints shall extend in the direction of airflow. Clothes dryer
transition ducts used to connect the appliance to the exhaust
duct system shall be limited to single lengths not to exceed 8
feet (2438 mm) and shall be listed and labeled for the application.
Transition ducts shall not be concealed within construction
and must remain entirely within the room in which the
appliance is installed.
Exception:


Where the duct termination is less than 12

inches (305 mm) above finished grade an areaway shall be
provided with a cross-sectional area not less than 200 square
feet (186m.£.) The bottom of the duct termination shall be
no less than 12 inches (305 mm) above the areaway bottom
504.6.1 Maximum length.


The maximum length of a

clothes dryer exhaust duct shall not exceed 45 feet (13 716
mm) from the dryer location to the outlet terminal. The maximum
length of the duct shall be reduced 5 feet (1524 mm)
for each 45 degree (0.79 rad) bend and 10 feet (3048 mm)
for each 90 degree (1.6 rad) bend. The maximum length of
the exhaust duct does not include the transition duct.


&e..

Table 603 4 for gage thickness
Exception:


Where the make and model of the clothes

dryer to be installed is known and the manufacturer's
installation instructions for such dryer are provided to the
code official, the maximum length of the exhaust duct,
including any transition duct, shall be permitted to be in
accordance with the dryer manufacturer's installation
instructions. Where exhaust ducts are installed in concealed
locations the developed length of the exhaust
duct system shall be indicated by permanent labels or
tags installed in an observable location.
504.6.2 Rough-in required.


Where a compartment or

space for a domestic clothes dryer is provided, an exhaust
duct system shall be installed in accordance with Sections
504.6 and 504.6.1.
504.7 Commercial clothes dryers.


The installation of dryer

exhaust ducts serving Type 2 clothes dryers shall comply with
the appliance manufacturer's installation instructions. Exhaust
fan motors installed in exhaust systems shall be located outside
of the airstream. In multiple installations, the fan shall operate
continuously or be interlocked to operate when any individual
unit is operating. Ducts shall have a minimum clearance of 6
inches (152 mm) to combustible materials. Clothes dryer transition
ducts used to connect the appliance to the exhaust duct
system shall be limited to single lengths not to exceed 8 feet
(2438 mm) in length and shall be listed and labeled for the
application. Transition ducts shall not be concealed within con struction

[Jerry Peck]

Most clothes dryers are rated to push at least 50 feet (although there are some which will only push 35 feet or so), and some are rated to push up to 90 feet, so you really need to get the manufacturer and model number of your dryer and the installation instructions and find out what your dryer is rated for.

The two 90 ells equals 10 feet, unless you can use long radius ells, in which case the long radius ells are factored in at 18 inches each, or 3 feet for two long radius ells ... 3 feet versus 20 feet makes a big difference in the effective length of the dryer duct.

Another item which affect allowed dryer duct length is the exhaust hood - this is addressed in the installation instructions instead of in the code: typically the louver type and the hood type are rated the same and have less restriction, thus more duct length is allowed, than the angled/sloped exhaust hood.

The short answer, then, is to read your installation instructions.

[John Kogel]

A long straight run should be alright. Elbows tend to trap moisture. The carpot is not an ideal destination. Use metal foil tape, not screws, to secure the pipes together.

[Jerry Peck]

A long straight run should be alright.

But "how long"? Even long straight runs can be too long.


Elbows tend to trap moisture.

Typically, it is hard to have less than two elbows (unless one of the newer dryer boxes is recessed into the wall as they all a straight up (or down) connection instead of a 90 elbow, then you still have one at the top or the bottom where it turn across the attic or crawlspace.