FHA main trunk replacement: Follow-up

Posted by Jack on October 2, 2009, 8:13 am

Took your advice from your responses to yesterdays question.

22"x8" is a standard rectangular size I could get at a supplier or on-line.
I've currently got 6 flex-ducts going to the room registers. It's a
one-zone system. No branches. I think I could do the gig myself, but I am
unsure of sizing the duct.

Currently, the trunk size becomes smaller as you travel the length of the
trunk. There are three places where a reducer fitting is in place. The
quote from the sheet metal company only stated 22" x 8". Not knowing
anything about duct sizing, how would I know what size to get and where to
put the reducer fittings? Otherwise, it looks like a job I could do using
standard prefabricated parts.

Thanks again for your consideration.

Posted by DT on October 2, 2009, 9:56 am

>Took your advice from your responses to yesterdays question.
>22"x8" is a standard rectangular size I could get at a supplier or on-line.
>I've currently got 6 flex-ducts going to the room registers. It's a
>one-zone system. No branches. I think I could do the gig myself, but I am
>unsure of sizing the duct.
>Currently, the trunk size becomes smaller as you travel the length of the
>trunk. There are three places where a reducer fitting is in place. The
>quote from the sheet metal company only stated 22" x 8". Not knowing
>anything about duct sizing, how would I know what size to get and where to
>put the reducer fittings? Otherwise, it looks like a job I could do using
>standard prefabricated parts.

Just copy the layout of the existing duct. The replacement duct should have
the same area as the existing duct at each point. If the calculation falls
in between avilable duct sizes, go with the next larger one, as there is
slightly less flow in a 'flatter' rectangular duct than a square one. If it
starts out as 8x22 then at some point it will neck down to 8x20 or whatever
and then 8x16, etc. The reducers are available where you buy the ducts.

So if the existing duct is 12x12 at some point, an 8x18 would have the same
area. Ducts are made in 2" increments, but the supply house may only stock
the more common sizes and you will need to order them.

--
Dennis

Posted by RicodJour on October 2, 2009, 11:19 am

> Took your advice from your responses to yesterdays question.
> 22"x8" is a standard rectangular size I could get at a supplier or on-lin=

> I've currently got 6 flex-ducts going to the room registers. =A0It's a
> one-zone system. =A0No branches. =A0I think I could do the gig myself, bu=

t I am

> unsure of sizing the duct.
> Currently, the trunk size becomes smaller as you travel the length of the
> trunk. =A0There are three places where a reducer fitting is in place. =A0=

The

> quote from the sheet metal company only stated 22" x 8". =A0Not knowing
> anything about duct sizing, how would I know what size to get and where t=

> put the reducer fittings? =A0Otherwise, it looks like a job I could do us=

ing

> standard prefabricated parts.
> Thanks again for your consideration.

Use graph paper to draw the ductwork to scale, making note of all of
the sizes, reducers, etc., then fax it over to a local HVAC supply
house and ask for a quote on the materials as is, and another quote
for reducing the height of the ductwork to the desired height. When
estimating for a DIYer project start with the materials cost as that
is relatively fixed. If the materials cost is somewhere between 1/3
and 1/2 of the contractor's quote, then the price you got is probably
a reasonable one.

Here's a thread from another forum that might be of interest to you:
http://www.thesheetmetalshop.com/PNphpBB2-printview-t-352-start-0.html

R

Posted by hr(bob) hofmann@att.net on October 2, 2009, 3:12 pm