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Old 09-30-2010, 11:03 AM   #1
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re: hvac ducting


I have a simple question about HVAC ducting.

I have a bathroom fan that will be vented through the roof. I have been told by some that flexible pipe is not as good as the ridgid, but it is easier to install.

I think I will go with the ridgid pipe. I like things done correctly. But what I want to know is, which is the proper type to use in this case, galvanized steel, or aluminum??

what are aluminum ducts generally used for??, and why

what are galvanized ducts generally used for?? and why

thanks

-g


Last edited by gramps416; 09-30-2010 at 11:07 AM.
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Old 09-30-2010, 11:53 AM   #2
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re: hvac ducting


Don't waste your time and money, run it in insulated flex.

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Old 09-30-2010, 02:37 PM   #3
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re: hvac ducting


okay, but what about the other questions regarding proper material use

thanks
-gramps

p.s. how much longer would the ridgid take than the flex?
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Old 09-30-2010, 02:49 PM   #4
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re: hvac ducting


How much longer depends on if you need to use any ells or not.

Aluminum is used a lot for dryer vents.
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Old 10-01-2010, 10:08 AM   #5
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re: hvac ducting


no aluminum for dryer vents.
But I want to know for future use,

what makes someone choose aluminum over steel? or vice versa? (advantages and disadvantages)

Last edited by gramps416; 10-01-2010 at 10:11 AM.
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Old 10-01-2010, 05:33 PM   #6
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re: hvac ducting


Aluminum is cheaper then galvanized steel/sheet metal.

Gal, hold ups better to abuse.
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Old 10-01-2010, 06:41 PM   #7
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re: hvac ducting


Quote:
Originally Posted by gena View Post
Don't waste your time and money, run it in insulated flex.
I don't think insulated flex is rated for the amount of moisture coming from a bathroom exhaust.

At least I would not be comfortable using it.
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Old 10-01-2010, 08:27 PM   #8
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re: hvac ducting


As far as I know the manufacturers don't say you can't. They only specify max temperature and pressure. I've never been turned down for using it. I like the plastic uninsulated stuff but you can only run it 15 ft, per the manufacturer.
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Old 10-01-2010, 08:40 PM   #9
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re: hvac ducting


Quote:
Originally Posted by gena View Post
run it in insulated flex.

Ideally no rodents will ever get into anybody's house, but since that does happen I try to stay away from insulated flex in places where I won't be able to see it regularly.

Also, in my prior city the code officer did not allow ANY plastic anywhere in bath vent line. His reason - this is just what he said, I'm not sure I buy it - was that sometimes fires start in the vent fan housing itself and all metal helps keep the fire from the house itself. When I expressed doubt, he said that condensation always happens in those lines, and when the water drips back into the fan housing things tend to corrode over time, and that can lead to electrical fires in the fan unit. I suppose that makes some theoretical sense, but how often do such fires really threaten a structure? Beats me.

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Old 10-01-2010, 08:55 PM   #10
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re: hvac ducting


Some inspectors will ask/make you do a lot of things the code doesn't require.
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Old 10-01-2010, 09:01 PM   #11
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re: hvac ducting


Airflow is affected using flex rather than straight smooth wall pipe: “Friction loss in straight duct is dependent on the relationships of duct diameter, air velocity in the duct, and duct roughness as major components, and to a much lesser degree on air density. As one can imagine, flex duct with its helical corrugations is going to be much “rougher” or less smooth than galvanized duct. This is especially true if it is not stretched out to the extent possible during installation. Slack duct allows the coils of reinforcing wire to relax, which bunches up the polyester and pushes it into the interior of the core, adding more resistance to airflow.” From: http://rockwallcontrols.com/Resident...ag=air-ducting

And from that article: “A handy rule that is very effective and reliable is to increase the size of flex duct one diameter to neutralize the added friction loss compared to that of galvanized duct for the same CFM.”


More moisture will be deposited on the colder pipe walls through the attic because of the turbulent airflow and more surface area of flex pipe. You need to insulate the pipe to help temper it against the outside attic air. A termination hood back-draft damper will help stop outside air into the duct reducing its temperature even more if cold out. Be sure to tape the joints of the ducting and individual elbows before wrapping with vapor barrier and insulation when using rigid pipe rather than contribute to frost in the attic or ice dams on the roof from an unknown disconnected pipe there.

Gary
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Old 10-02-2010, 10:02 AM   #12
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re: hvac ducting


I read an article on "ask the builder" a while ago, stating, that ridgid pipe is all that should be used to prevent that moisture from building up at all.
http://www.askthebuilder.com/546_Bat...tilation.shtml

I have little experience with HVAC, and by far not a professional in this area, but what he says makes sense to me about using a ridgid pipe, like GBR said. I have been pressured by many people to use the aluminum flex because "it is easier" and "you'll save time and money".

I don't know what to believe at this point with this topic that seems divided in the approach. I want to do things right. The two bathroom fans used are more expensive panisonic models.
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Old 10-02-2010, 10:11 AM   #13
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re: hvac ducting


You can use Aluminum ridged pipe. Looks almost the same as galvanized pipe.
It actually has a better flow rate the galvanized does.
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Old 10-02-2010, 12:35 PM   #14
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Quote:
Originally Posted by brandonmcginnis View Post
Some inspectors will ask/make you do a lot of things the code doesn't require.
There are some code requirements that do not work for certain localities. The code official can then alter or make the requirements tougher to ensure safety.

I f an inspector cites you for non code violation you can fight them.

I will go to court and have done so if I thought the violation was malicious.

Once I told an inspector he was wrong about a violation. He told me I was not an inspector my self so I could not possibly be able to interpret the code book.

i guess when an hvac guy graduates Code Inspections and Enforcement school he drinks a magic potion that gives him power to understand the same code book better than the guys whose work he has jurisdiction over.

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