Dryer Vent Pipe

Yesterday’s frame inspection revealed a dryer vent pipe that was partially installed. The run was going to easily be greater than 30’. Now I realize that booster fans are a manufacturer’s decision and there is no set rule anymore. The builder did mention that the 4" pipe was going to be connected to a 5" pipe that would terminate to the exterior and that would eliminate the need for a booster fan. Have any of you heard that?

If the contractor is building to IRC, he is doing it wrong. To me, rerouting the ducting would be an easier fix than to add a booster fan. All he has to be is within 25 feet of the outside.

Technically, most dryers vent out a rate of 200-230 cfm and they cannot have too much back pressure, so the duct cannot be too undersized. At least 150 CFM is require to keep lint from settling inside the ducting, so the duct cannot be overly big. A person would need to do a lot of math to figure it all out. The back pressure rate is what I could not figure out with out doing a lot of research.

I would say that if the builder is building to IRC, you now have to determine to which edition of it…
The 25 feet rule is one method of determining the maximum allowable exhaust duct length for the 2006-2009 edition. The 2012 edition bumps it up to 35 feet…

Then the builder can also use a second method which is included in the 2006 edition as an exception, then is changed in to articles in the 2009-2012 editions under rule M1502.4.4.2 Manufacturer’s instructions by allowing the max length to be determined by the dryer manufacturer’s installation instruction, if the builder provides the code official with a copy of the installation instructions for the make and model of the dryer.

It will be about eight years in my area before they adopt the 2012 codes.

I wonder what happens if the dryer quits in a few years?

No kidding! Or the owner sales the house and takes his dryer with him, then the new buyer puts in an older unit…
That being said, most new dryers are more efficient.
Certain cities in my area has “already” adopted the 2009 edition. I’m suspecting it’s the same for Richard as he is just a little North of me.

Thanks for your input…however I was asking about the transition from a 4" dryer vent pipe to a 5" dryer vent pipe about 12 feet along the run. Perry Homes (a very large Houston area production builder) is saying that with this increase in vent pipe diameter that it negates the need for a booster fan. Has anyone heard of this?

Most builders do not include dryers as part of their new construction package since it is not a built in appliance…Thus the builder has no idea what the dryer brand will be that the buyer installs.

This is out in Cross Creek Ranch Will. It’s in Fulshear, just west of Katy, if you are familiar with West Houston.

I know exactly where that is. Did a few inspection around there.
I did not find anything about 5" pipes, however the IRC does give a chart for deduction lengths depending on the diameter of the elbow. The 2 sizes are 4" and 6". So, I guess it would be up to AHJ to accept the 5" Φ.

Here is the IRC length article. The info in the commentary is what I was reading.
IRC 2012
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.

Commentary: The maximum exhaust duct length of 35 feet (7620
mm) is the requirement for domestic clothes dryers.
The 35-foot (7620 mm) limit is based on the worst
case scenario where the dryer is rated for a maximum
duct length of 35 feet (7620 mm). The intent
was to ensure that the least capable dryer available
would be compatible with a 35-foot (7620 mm) duct
system. If the exhaust duct system is designed for
worst case, it should work properly in all cases. Note
that there may be some dryers on the market that
specify a duct length of less than 35 feet (10 668
mm), and the manufacturer’s instructions would prevail
in such case. It should be noted that the code
does not recognize the use of booster fans with
clothes dryer exhaust systems. A UL standard for
dryer exhaust duct power ventilators has been developed.
The use of a booster fan would have to be
approved by the code official under the provisions of
Section R104.11.
Because the maximum length is based on equivalent
length, all fittings must be accounted for in accordance
with Table M1502.4.4.1. The length is
measured from the point where the transition duct (if
used) connects to the rigid exhaust duct system to
the terminal outdoors (see Table M1502.4.4.1). The
transition duct is excluded from the calculation of
equivalent length of the duct system.
Table M1502.4.4.1 assigns an equivalent length to
the various types of elbow fittings that are available.
The first two rows of the table cover the traditional
elbows that the code and dryer manufacturer’s installation
instructions have always addressed. The other
rows in the table cover newer, high-performance
elbows that have longer turning radii and smoother
walls (i.e., no mitered joints); thus, the equivalent
length is substantially reduced. When an installer discovers
that the length of an exhaust duct will exceed
the limits of the code, he or she may find that using
higher performance elbows will keep the length within
the allowable limits.

As there is insufficient data collected at the time of inspection and reported on this post. I will decline to make a recommendation on this condition.

If you wish to collect the appropriate data, I will be more than happy to respond.
You are not a code inspector, you are a home inspector. That means that you find significant deficiencies (which means that because of the condition and there is a hazardous potential).

If you want it changed, you need to prove the requirement for it.

JMWAO: just my wild *** opinion

Thanks Will. I am contacting the AHJ since there was no reference to a 5" pipe in the 2003 IRC which Fulshear is still using.